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Chickenpox

Infectious MONDO:0005700 Viral exanthem Herpesvirus infection

Chickenpox (varicella) is a highly contagious acute primary infection caused by varicella-zoster virus (VZV, human herpesvirus 3). It is characterized by a generalized pruritic vesicular rash, fever, and malaise. Transmission occurs via respiratory droplets and direct contact with vesicle fluid. After primary infection, VZV establishes lifelong latency in sensory ganglia and may reactivate as herpes zoster (shingles). While typically self-limited in immunocompetent children, chickenpox can cause serious complications including pneumonia, encephalitis, and bacterial superinfection, particularly in neonates, adults, pregnant women, and immunocompromised individuals. Universal childhood vaccination has dramatically reduced incidence, morbidity, and mortality in countries with established vaccination programs.

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Mappings
1
Definitions
0
Inheritance
4
Pathophysiology
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Histopathology
4
Phenotypes
0
Genes
3
Treatments
0
Subtypes
4
Differentials
3
Datasets
3
Trials
🏷

Classifications

Harrison's Chapter
infectious disease viral infectious disease
📘

Definitions

1
CDC varicella case classification
The CDC case classification for varicella surveillance distinguishes confirmed, probable, and suspect cases based on clinical presentation and laboratory evidence. A confirmed case requires either laboratory confirmation (VZV PCR, DFA, or culture) or an epidemiologic link to a confirmed or probable case with compatible clinical illness.
CASE_DEFINITION all ages
Inclusion criteria
  • Acute generalized maculopapulovesicular rash Illness with acute onset of diffuse maculopapulovesicular rash without other apparent cause.
  • Epidemiologic linkage or laboratory confirmation Epidemiologic linkage to a confirmed or probable varicella case, or laboratory confirmation of VZV infection by PCR, DFA, or culture.
Exclusion criteria
  • Vaccination-associated rash Rash attributable to recent varicella vaccination (vaccine-strain VZV) rather than wild-type infection.
Show evidence (2 references)
PMID:36265857 SUPPORT Human Clinical
"The prompt recognition and diagnosis of varicella is important because when confirmed, clinical and public health measures need to be taken swiftly."
CDC review emphasizing the importance of standardized case classification for varicella surveillance and public health response.
PMID:27188665 SUPPORT Human Clinical
"VZV is the only human herpesvirus for which highly effective vaccines are available."
VZV primer covering diagnostic and surveillance approaches including clinical and laboratory criteria for case confirmation.
⚙

Pathophysiology

4
Primary VZV infection via respiratory mucosa
VZV enters the host through the upper respiratory tract via inhalation of infectious aerosols or direct contact with vesicle fluid. The virus initially replicates in the nasopharyngeal lymphoid tissue (including tonsils) and regional lymph nodes, infecting CD4+ T lymphocytes with activation/memory and skin-homing features that serve as vehicles for subsequent systemic dissemination.
Pharyngeal mucosa link
Show evidence (2 references)
PMID:37632006 SUPPORT Human Clinical
"Varicella-Zoster virus (VZV) is a pathogenic human alpha herpes virus that causes varicella (chicken pox) as a primary infection and, following a variable period of latency in different ganglionic neurons, it reactivates to produce herpes zoster (shingles)."
Review establishing VZV's primary infection route and lifecycle.
PMID:16188994 SUPPORT In Vitro
"The pathogenesis of varicella-zoster virus (VZV) involves a cell-associated viremia during which infectious virus is carried from sites of respiratory mucosal inoculation to the skin."
Study directly establishing the respiratory mucosal inoculation site as the origin of VZV pathogenesis in the SCID-hu xenograft model.
T-cell-associated viremia and systemic dissemination
Following initial replication in lymphoid tissue, VZV disseminates via cell-associated viremia in T lymphocytes. VZV DNA is detectable in T-lymphocytes as early as 8-10 days prior to rash onset. The secondary viremia seeds the virus to the skin and mucous membranes, producing the characteristic vesicular exanthem. Concurrent innate immune evasion through IRF3 antagonism (by ORF61p, ORF47p, and IE-62) and suppression of type I interferon signaling facilitates systemic spread.
T cell link
Defense response to virus link ↓ DECREASED
Show evidence (4 references)
PMID:25453570 SUPPORT Human Clinical
"VZV DNA, primarily in T-lymphocytes, is detected as early as 8-10 days prior to rash and can persist for a week."
Key study demonstrating T-lymphocyte-associated VZV viremia preceding the rash.
PMID:39051773 SUPPORT In Vitro
"VZV sEVs are non-infectious yet transcriptionally altered primary human cells, suppressing the antiviral type 1 interferon response and promoting neuroinvasion of a secondary pathogen in vivo."
Study demonstrating VZV immune evasion through suppression of type I interferon responses via extracellular vesicles.
PMID:21835786 SUPPORT In Vitro
"ORF61 down-modulates the IRF3-mediated IFN-β pathway by degradation of activated IRF3 via direct interaction, which may contribute to the pathogenesis of VZV infection."
Mechanistic study demonstrating that VZV ORF61 protein degrades activated IRF3 via ubiquitin-proteasome pathway, directly supporting the IRF3 antagonism described in the viremia entry.
+ 1 more reference
Cutaneous vesicle formation
VZV infects epidermal keratinocytes, causing ballooning degeneration, multinucleated giant cell formation, and intraepidermal vesiculation. Viral envelope glycoproteins (gB/gE, gH/gL) promote cell-to-cell fusion and syncytium formation, supporting spread through skin without requiring extracellular virion release. The characteristic rash progresses through macules, papules, vesicles, and crusts, with lesions in multiple stages simultaneously (centripetal distribution).
Keratinocyte link
Viral genome replication link ↑ INCREASED
Skin epidermis link
Show evidence (3 references)
PMID:39110717 SUPPORT In Vitro
"we report a virus-mediated immune-evasion strategy that disarms MAIT cell responses."
Study showing VZV disrupts MAIT cell polyfunctional effector responses, which are important at barrier sites like skin, supporting the mechanism of cutaneous immune evasion during vesicle formation.
PMID:14726820 SUPPORT Human Clinical
"HLA-DR and ICAM-1 expressions were not observed in VZV- and HSV-infected keratinocytes, contrasting with their upregulation in the surrounding epidermis and inside nonviral blisters."
Immunohistochemistry study of human herpes zoster (not primary varicella) lesions demonstrating MHC-II and ICAM-1 downregulation in VZV-infected keratinocytes. Since the same virus infects the same cell type, these immune evasion mechanisms are biologically relevant to primary varicella cutaneous pathology as well.
PMID:38184594 SUPPORT In Vitro
"VZV gE inhibited MAVS oligomerization and STING translocation to disrupt MAVS- and STING-mediated interferon (IFN) responses, and PINK1/Parkin-mediated mitophagy was required for VZV gE-mediated inhibition of IFN production."
Study demonstrating that VZV glycoprotein E promotes mitophagy to evade innate antiviral signaling, tested in a 3D human skin organ culture model, directly relevant to immune evasion during cutaneous infection.
Establishment of latency in sensory ganglia
During primary infection, VZV gains access to peripheral nerves and sensory ganglia, establishing lifelong latency in cranial nerve and dorsal root ganglia neurons. Latency is characterized by highly restricted viral transcription dominated by VZV latency-associated transcripts (VLTs) and VLT-ORF63 fusion transcripts, while most lytic genes are silenced. The virus may reactivate decades later as herpes zoster, particularly with aging or immunosuppression.
Sensory neuron of dorsal root ganglion link
Establishment of viral latency link
Dorsal root ganglion link
Show evidence (3 references)
PMID:38858365 SUPPORT In Vitro
"we discover numerous VZV latency-associated transcripts (VLTs)-like circRNAs (circVLTslytic), which contain multiple exons and different isoforms within the same back-splicing breakpoint."
Study characterizing VLT-related transcripts and their roles during VZV infection, supporting the latency transcription model.
PMID:37632006 SUPPORT Human Clinical
"following a variable period of latency in different ganglionic neurons, it reactivates to produce herpes zoster (shingles)."
Review confirming lifelong ganglionic latency with subsequent reactivation as herpes zoster.
PMID:20874010 SUPPORT Human Clinical
"It is now established beyond doubt that latent VZV is predominantly located in human ganglionic neurons. Virus gene transcription during latency is epigenetically regulated, and appears to be restricted to expression of at least six genes, with expression of gene 63 being the hallmark of latency."
Authoritative review on VZV ganglionic latency establishing neuronal localization and the epigenetically regulated, restricted transcription program dominated by gene 63.
⬡

Causal Graph

graph LR
    T_cell_associated_viremia_and_systemic_dissemination["T-cell-associated viremia and systemic dissemination"]
    Establishment_of_latency_in_sensory_ganglia["Establishment of latency in sensory ganglia"]
    Primary_VZV_infection_via_respiratory_mucosa["Primary VZV infection via respiratory mucosa"]
    Cutaneous_vesicle_formation["Cutaneous vesicle formation"]

    Primary_VZV_infection_via_respiratory_mucosa --> T_cell_associated_viremia_and_systemic_dissemination
    T_cell_associated_viremia_and_systemic_dissemination --> Cutaneous_vesicle_formation
    T_cell_associated_viremia_and_systemic_dissemination --> Establishment_of_latency_in_sensory_ganglia

    style T_cell_associated_viremia_and_systemic_dissemination fill:#dbeafe
    style Establishment_of_latency_in_sensory_ganglia fill:#dbeafe
    style Primary_VZV_infection_via_respiratory_mucosa fill:#dbeafe
    style Cutaneous_vesicle_formation fill:#dbeafe
●

Phenotypes

4
Integument(1) Metabolism(1) Other(2)
Integument 1
Pruritus VERY_FREQUENT Pruritus (HP:0000989)
Show evidence (2 references)
PMID:29219259 SUPPORT Human Clinical
"It is a common childhood illness characterised by an itchy vesicular rash and fever"
Clinical review explicitly identifying pruritus (itchy rash) as a characteristic feature of childhood chickenpox.
PMID:28846365 SUPPORT Human Clinical
"Chickenpox results in a skin rash forming small, itchy blisters that scab over"
StatPearls reference confirming itchy blisters as the hallmark presentation of chickenpox.
Metabolism 1
Fever VERY_FREQUENT Fever (HP:0001945)
Show evidence (2 references)
PMID:29219259 SUPPORT Human Clinical
"It is a common childhood illness characterised by an itchy vesicular rash and fever, which usually resolves spontaneously without medical intervention."
Clinical review explicitly identifying fever as a characteristic feature of childhood chickenpox.
PMID:28846365 SUPPORT Human Clinical
"The rash typically starts on the chest, back, and face and then spreads, accompanied by fever, fatigue, pharyngitis, and headaches, usually lasting 5 to 7 days."
StatPearls reference documenting fever as a core accompanying symptom of the varicella rash lasting 5-7 days.
Other 2
Vesicular exanthem VERY_FREQUENT Vesicular eruption (HP:0033697)
Show evidence (2 references)
PMID:37632006 SUPPORT Human Clinical
"Varicella-Zoster virus (VZV) is a pathogenic human alpha herpes virus that causes varicella (chicken pox) as a primary infection"
Review confirming vesicular exanthem as the defining clinical feature of primary VZV infection (chickenpox).
PMID:25453570 SUPPORT Human Clinical
"Varicella-zoster virus (VZV) can be detected in the blood from approximately 5 days before to 4 days after varicella."
Study establishing temporal relationship between viremia and the characteristic varicella rash.
Malaise FREQUENT Malaise (HP:0033834)
Show evidence (1 reference)
PMID:37632006 SUPPORT Human Clinical
"Varicella-Zoster virus (VZV) is a pathogenic human alpha herpes virus that causes varicella (chicken pox) as a primary infection"
Review documenting varicella as a systemic infection with constitutional symptoms including malaise.
💊

Treatments

3
Varicella vaccination MAXO:0001017
Live attenuated varicella vaccine (Oka strain) is the primary preventive measure. Two-dose childhood vaccination achieves greater than 90% effectiveness against any disease and nearly 100% against severe disease.
Show evidence (1 reference)
PMID:38416279 SUPPORT Human Clinical
"As HZ is preventable by vaccination, national HZ vaccination recommendations should include the need for and timing of vaccination"
Review confirming vaccination as the primary prevention strategy for VZV disease, applicable to both varicella and herpes zoster.
Antiviral therapy MAXO:0000058
Drug: acyclovir ↗
Oral acyclovir is recommended for adolescents, adults, and immunocompromised patients. Intravenous acyclovir is used for severe disease or immunocompromised hosts. Treatment is most effective when initiated within 24 hours of rash onset.
Show evidence (1 reference)
PMID:25453570 SUPPORT Human Clinical
"The duration and magnitude of VZV DNAemia correlates with immune status and the efficacy of antiviral therapy."
Study demonstrating that antiviral therapy modulates VZV viremia, supporting acyclovir as standard treatment.
Supportive care MAXO:0000950
Symptomatic management includes antipyretics (avoiding aspirin due to Reye syndrome risk), antihistamines for pruritus, and calamine lotion. Good hygiene prevents bacterial superinfection of lesions.
Show evidence (1 reference)
PMID:38416279 SUPPORT Human Clinical
"debilitating symptoms including neuropathic pain, and complications such as post-herpetic neuralgia, vision loss, and rarely, stroke, and increased severity in immunocompromised individuals."
Review documenting the symptomatic burden of VZV disease that necessitates supportive care.
🌍

Environmental Factors

3
Seasonal variation
Varicella incidence shows significant seasonal variation, but timing and amplitude of peaks vary by geography and climate context. Regional notification data support recurrent seasonal patterns rather than a single universal peak window.
Show evidence (2 references)
PMID:33733653 SUPPORT Human Clinical
"Here, we examined chickenpox and shingles notifications from Thailand and found strong seasonal incidence in both diseases, with a 3-month lag between peak chickenpox transmission season and peak shingles reactivation."
Time-series analysis demonstrates robust seasonal structure in chickenpox transmission.
PMID:26963841 SUPPORT Human Clinical
"We established significant seasonality of varicella notifications"
Independent surveillance analysis confirms statistically significant varicella seasonality.
Immunosuppression
Immunocompromised individuals (organ transplant recipients, patients receiving chemotherapy or high-dose corticosteroids, HIV/AIDS) are at greatly increased risk of severe or disseminated varicella with visceral involvement (pneumonia, hepatitis, encephalitis) and higher mortality.
Show evidence (2 references)
PMID:8827537 SUPPORT Human Clinical
"In immunocompromised patients with altered cell mediated immunity, and rarely in healthy individuals, varicella results in a life-threatening infection."
Review establishing that altered cell-mediated immunity is the primary risk factor for severe varicella.
PMID:27188665 SUPPORT Human Clinical
"Infection with varicella zoster virus (VZV) causes varicella (chickenpox), which can be severe in immunocompromised individuals, infants and adults."
VZV primer confirming increased severity in immunocompromised individuals.
Overcrowded living conditions
Close-contact settings such as schools, daycare centers, military barracks, and shelters facilitate rapid VZV transmission due to the airborne nature of the virus and high R0.
Show evidence (1 reference)
PMID:38814821 SUPPORT Human Clinical
"Varicella Outbreak Among Recent Arrivals to New York City, 2022-2024."
MMWR report documenting a varicella outbreak in congregate shelter settings among recent arrivals, demonstrating the role of overcrowding in transmission.
🔬

Biochemical Markers

2
VZV-specific IgM (Positive)
Show evidence (1 reference)
PMID:3016110 SUPPORT Human Clinical
"The early production of VZV-specific IgG or IgM antibodies did not correlate with the severity of the clinical infection"
Study of 64 healthy subjects and 21 immunocompromised patients demonstrating VZV-specific IgM production during acute primary varicella infection.
VZV-specific IgG (Positive)
Show evidence (1 reference)
PMID:19449259 SUPPORT Human Clinical
"Specific antibodies in the sera during enrollment and at the end of the longest incubation periods were determined."
Study using VZV serology to determine infection status and secondary attack rates in household contacts.
🔀

Differential Diagnoses

4

Conditions with similar clinical presentations that must be differentiated from Chickenpox:

Mpox (monkeypox) MONDO:0002594
Overlapping Features Mpox presents with a vesiculopustular rash that can closely mimic varicella. Both diseases feature fever and progressive skin lesions that evolve through macules, papules, vesicles, and crusts. The 2022 global outbreak increased clinical overlap concerns.
Distinguishing Features
  • Mpox lesions are typically deeper, firmer, and progress synchronously (same stage)
  • Prominent lymphadenopathy is characteristic of mpox but not varicella
  • Mpox has a longer incubation period (5-21 days vs 10-21 days)
  • Varicella lesions appear in successive crops with multiple stages simultaneously
Show evidence (2 references)
PMID:39503781 SUPPORT Human Clinical
"Although Mpox may mimic other diseases such as chickenpox or syphilis, lymphadenopathy is a distinguishing feature."
Review explicitly identifying chickenpox as a key differential for mpox, with lymphadenopathy as the primary distinguishing feature.
PMID:36552964 SUPPORT Human Clinical
"monkeypox, an emerging infectious disease caused by the monkeypox virus (MPXV), has clinical manifestations that are similar to those of VZV infections."
Study confirming clinical similarity between VZV and MPXV infections and the need for laboratory differentiation.
Herpes simplex virus infection Not Yet Curated MONDO:0004609
Overlapping Features Disseminated or primary herpes simplex virus (HSV) infection can present with vesicular lesions resembling varicella, particularly in neonates and immunocompromised patients. Eczema herpeticum in patients with atopic dermatitis can also mimic varicella.
Distinguishing Features
  • HSV lesions tend to be grouped and localized rather than generalized
  • HSV vesicles often recur at the same site
  • Tzanck smear cannot distinguish HSV from VZV; PCR is required
  • HSV more commonly involves oral or genital mucosa
Show evidence (1 reference)
PMID:36265857 SUPPORT Human Clinical
"healthcare providers should keep varicella in the differential diagnosis of a maculopapular or vesicular rash"
CDC review noting the importance of differentiating varicella from other vesicular rash illnesses.
Impetigo Not Yet Curated MONDO:0004592
Overlapping Features Impetigo is a superficial bacterial skin infection that can present with vesicles and crusted lesions, potentially confused with the crusting phase of varicella. Bullous impetigo in particular can produce vesiculobullous lesions.
Distinguishing Features
  • Impetigo produces honey-colored crusts without the centripetal distribution
  • No systemic symptoms (fever, malaise) unless complicated
  • Lesions are typically localized rather than generalized
  • Responds to topical or systemic antibiotics
Show evidence (1 reference)
PMID:29219259 SUPPORT Human Clinical
"Serious, and rarely fatal, complications can occur, including pneumonia, central nervous system infection, overwhelming secondary bacterial infections, especially with Group A streptococcus, and necrotising fasciitis."
Review noting secondary bacterial infection (including impetigo-like superinfection) as a complication of varicella, highlighting the clinical overlap.
Hand, foot and mouth disease Not Yet Curated MONDO:0005779
Overlapping Features Hand, foot and mouth disease (HFMD) is an enteroviral infection that can present with vesicular lesions, particularly in young children. The age group overlap with varicella can cause diagnostic confusion.
Distinguishing Features
  • HFMD lesions are concentrated on palms, soles, and oral mucosa (acral distribution)
  • Varicella has centripetal distribution (trunk predominant)
  • HFMD vesicles are typically smaller and more uniform
  • Oral ulcers are prominent in HFMD but not typical in varicella
Show evidence (1 reference)
PMID:36265857 SUPPORT Human Clinical
"The prompt recognition and diagnosis of varicella is important because when confirmed, clinical and public health measures need to be taken swiftly."
CDC review emphasizing the need for accurate differential diagnosis of vesicular rash illnesses to enable appropriate public health responses.
📊

Related Datasets

3
Varicella-zoster virus hijacks the type I interferon response and antigen presentation pathways in matured hiPSC-derived neurospheroids geo:GSE273529
NanoString nCounter immune profiling (targeted hybridization-based digital counting; currently mapped to MICROARRAY because schema data_type lacks a targeted expression/NanoString category) of VZV-infected vs Sendai virus-infected hiPSC-derived neurospheroids. Demonstrates that VZV actively suppresses type I interferon signaling and antigen presentation pathways in a CNS-like environment, while Sendai virus triggers robust antiviral responses.
human MICROARRAY n=12 NanoString nCounter (585 immune transcripts)
neuron-enriched hiPSC-derived neurospheroids link
Conditions: VZV-infected neurospheroids Sendai virus-infected neurospheroids mock-infected controls
PMID:39351233
Directly relevant to VZV immune evasion pathophysiology. Shows VZV suppresses innate immunity in neurons, supporting the latency establishment mechanism. cell_type_term uses neuron to represent the predominant infected cell population within the neurospheroid model.
Lack of strong innate immune reactivity renders macrophages alone unable to control productive Varicella-Zoster Virus infection in an isogenic human iPSC-derived neuronal co-culture model. geo:GSE228390
Bulk RNA-seq of iPSC-derived neuron-macrophage co-cultures infected with VZV. Despite macrophage immune competence in isolation, co-cultured macrophages failed to suppress VZV replication in neurons, suggesting additional immune cell types are required for effective antiviral control.
human BULK RNA SEQ n=80
iPSC-derived macrophages and neurons link
Conditions: VZV-infected neuron-macrophage co-cultures mock-infected controls macrophages alone neurons alone
PMID:37287975
Large sample study modeling VZV neuroimmune interactions. Relevant to understanding why VZV establishes latency despite innate immune surveillance.
RNA-Sequencing data of Varicella Zoster Virus (VZV)-infected human dermal fibroblasts (HDF) geo:GSE121385
RNA-seq comparing host transcriptomic responses in human dermal fibroblasts infected with clinical versus vaccine (Oka) VZV strains. Identifies differential immune pathway activation between wild-type and attenuated virus.
human BULK RNA SEQ n=6 Illumina HiSeq 2500
human dermal fibroblasts link
Conditions: clinical VZV strain-infected vaccine (Oka) VZV strain-infected mock-infected controls
PMID:31658769
Relevant to understanding differential host responses to wild-type vs vaccine VZV strains, informing vaccine mechanism of action.
🔬

Clinical Trials

3
NCT06806137 PHASE_III RECRUITING
Phase 3a, observer-blind, randomized, controlled study evaluating the immunogenicity and safety of GSK's investigational varicella vaccine (VNS Vaccine) compared with Varivax when given as a second dose to healthy children 3 months after the first dose at 12-15 months of age.
Target Phenotypes: Vesicular eruption ↗
Show evidence (1 reference)
clinicaltrials:NCT06806137 SUPPORT
"The purpose of this study is to evaluate the immune response and safety of GSKs investigational varicella vaccine (VNS Vaccine) compared to an already approved varicella vaccine, Varivax (VV), when administered as second dose to healthy children."
Phase III trial evaluating a next-generation varicella vaccine in children, directly relevant to chickenpox prevention.
NCT06855160 PHASE_III RECRUITING
Phase 3a, open-label, randomized, controlled study evaluating the immunogenicity and safety of intramuscular administration of GSK's investigational varicella vaccine co-administered with Priorix (MMR), compared with subcutaneous Varivax and Priorix, as a first dose in healthy children 12-15 months of age.
Target Phenotypes: Vesicular eruption ↗
Show evidence (1 reference)
clinicaltrials:NCT06855160 SUPPORT
"This study aims to assess the immune response and safety of GSK's candidate chickenpox and marketed MMR vaccines when given to children 12 to 15 months of age via a muscle injection."
Phase III trial evaluating intramuscular route for varicella vaccination, potentially improving ease of administration.
NCT07415252 PHASE_III NOT_RECRUITING
Phase III, randomized, double-blinded, active-controlled, multinational study assessing the safety and immunogenicity of a two-dose regimen of SKYVaricella (NBP608) compared with licensed varicella vaccines in children aged 12 months to 12 years. Approximately 780 participants.
Target Phenotypes: Vesicular eruption ↗
Show evidence (1 reference)
clinicaltrials:NCT07415252 SUPPORT
"The goal of this study is to evaluate the safety and immunogenicity of an investigational varicella vaccine in children. Researchers will compare the investigational vaccine, NBP608, with licensed varicella vaccines."
Phase III multinational trial of a novel varicella vaccine candidate (SKYVaricella) in children.
{ }

Source YAML

click to show 
name: Chickenpox
creation_date: "2026-02-18T19:08:31Z"
updated_date: "2026-02-19T17:50:34Z"
category: Infectious
synonyms:
- Varicella
- Primary VZV infection
description: >
  Chickenpox (varicella) is a highly contagious acute primary infection caused
  by varicella-zoster virus (VZV, human herpesvirus 3). It is characterized by
  a generalized pruritic vesicular rash, fever, and malaise. Transmission occurs
  via respiratory droplets and direct contact with vesicle fluid. After primary
  infection, VZV establishes lifelong latency in sensory ganglia and may
  reactivate as herpes zoster (shingles). While typically self-limited in
  immunocompetent children, chickenpox can cause serious complications including
  pneumonia, encephalitis, and bacterial superinfection, particularly in
  neonates, adults, pregnant women, and immunocompromised individuals. Universal
  childhood vaccination has dramatically reduced incidence, morbidity, and
  mortality in countries with established vaccination programs.
disease_term:
  preferred_term: chickenpox
  term:
    id: MONDO:0005700
    label: chickenpox
parents:
- Viral exanthem
- Herpesvirus infection
infectious_agent:
- name: Varicella-zoster virus
  infectious_agent_term:
    preferred_term: Human alphaherpesvirus 3
    term:
      id: NCBITaxon:10335
      label: Human alphaherpesvirus 3
  description: >
    VZV (human herpesvirus 3) is a neurotropic alphaherpesvirus with a biphasic
    lifecycle comprising lytic infection in permissive tissues and latent infection
    in sensory ganglia. VZV encodes immune evasion proteins (ORF61p, ORF47p,
    IE-62) that antagonize type I interferon signaling, and envelope glycoproteins
    (gB, gE, gH/gL, gI, gM) that mediate cell-to-cell spread and skin tropism.
    During latency, transcription is restricted to VZV latency-associated
    transcripts (VLTs) and VLT-ORF63 fusion transcripts.
  evidence:
  - reference: PMID:38858365
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Our large scanning analysis identifies and subsequent experiments confirm 200 VZV circRNAs."
    explanation: Study characterizing VZV transcript complexity during lytic infection, including identification of VLT-like circRNAs.
  - reference: PMID:37632006
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Varicella-Zoster virus (VZV) is a pathogenic human alpha herpes virus that causes varicella (chicken pox) as a primary infection and, following a variable period of latency in different ganglionic neurons, it reactivates to produce herpes zoster (shingles)."
    explanation: Comprehensive review establishing VZV as the causative agent of chickenpox with subsequent ganglionic latency and reactivation.
transmission:
- name: Airborne/respiratory droplet transmission
  description: >
    VZV is transmitted primarily via the airborne route through inhalation of
    infectious aerosols or respiratory droplets from an infected individual.
    The virus is well-recognized as an airborne pathogen alongside measles and
    tuberculosis, requiring airborne infection isolation precautions (N95
    respirators) in healthcare settings. The infectious period extends from
    1-2 days before rash onset until all lesions have crusted over.
  evidence:
  - reference: PMID:30704406
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "these terms are often used confusingly when discussing specific infection control interventions for individual pathogens that are accepted to be mostly transmitted by the airborne (aerosol) route (e.g. tuberculosis, measles and chickenpox)"
    explanation: Commentary establishing chickenpox as a well-recognized airborne-transmitted pathogen alongside measles and tuberculosis.
  - reference: PMID:29219259
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Chickenpox is an extremely contagious infectious disease caused by varicella zoster virus (VZV)."
    explanation: Clinical review confirming the highly contagious nature of chickenpox.
- name: Direct contact with vesicle fluid
  description: >
    Transmission also occurs through direct contact with fluid from vesicular
    lesions. Vesicle fluid contains high titers of infectious VZV. This route
    is particularly important in household and healthcare settings where close
    physical contact occurs.
  evidence:
  - reference: PMID:27188665
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Infection with varicella zoster virus (VZV) causes varicella (chickenpox), which can be severe in immunocompromised individuals, infants and adults."
    explanation: Comprehensive VZV primer documenting both respiratory and contact transmission routes.
- name: Vertical transmission
  description: >
    Maternal varicella infection during pregnancy can result in transplacental
    transmission to the fetus. Infection in the first or second trimester may
    cause congenital varicella syndrome, while maternal infection around
    delivery (5 days before to 2 days after birth) can result in severe
    neonatal varicella with mortality up to 30% without treatment.
  evidence:
  - reference: PMID:8827537
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Early commencement of acyclovir is recommended for children with both varicella and altered cell mediated immunity, newborns during the first 2 weeks of life, preterm infants in the neonatal nursery, and severe varicella or shingles (including ocular zoster) in any patient, as well as during pregnancy."
    explanation: Guidelines recommending treatment for neonatal and perinatal varicella, reflecting the risk of vertical transmission.
prevalence:
- population: Global
  notes: >
    Before vaccine availability in the United States, varicella caused
    approximately 4 million cases annually. In pre-universal-immunization
    Europe, seroprevalence data show most individuals are infected by
    adolescence. Routine childhood vaccination programs have markedly reduced
    incidence.
  evidence:
  - reference: PMID:27584717
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Before availability of varicella vaccine in the United States, an estimated 4 million varicella cases, 11,000-13,500 varicella-related hospitalizations, and 100-150 varicella-related deaths occurred annually."
    explanation: CDC surveillance report provides direct pre-vaccine U.S. burden estimates for primary varicella.
  - reference: PMID:28826422
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "By the age of 15 years, over 90% of the population has been infected by varicella in all countries except for Greece (86·6%) and Italy (85·3%)."
    explanation: Systematic pre-immunization seroprevalence review provides direct evidence for high adolescent cumulative infection prevalence in Europe.
epidemiology:
- name: Incubation period
  description: >
    Time from exposure to onset of rash, typically 14-16 days but ranging
    from 10-21 days.
  unit: days
  minimum_value: 10
  maximum_value: 21
  mean_range: "14-16"
  evidence:
  - reference: PMID:28846365
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Symptoms begin 10 to 21 days after exposure; the average incubation period is about 2 weeks."
    explanation: StatPearls varicella reference explicitly documenting the 10-21 day incubation period with a 14-day average.
- name: Secondary attack rate
  description: >
    The proportion of susceptible household contacts who develop varicella
    after exposure to an index case.
  unit: percentage
  minimum_value: 61
  maximum_value: 100
  mean_range: "75-90"
  notes: >
    Among the highest secondary attack rates of any infectious disease,
    reflecting VZV's extreme contagiousness in close-contact settings.
  evidence:
  - reference: PMID:19449259
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Secondary attack rate (SAR) was lower in hepatitis A (43.3%) than those for varicella (75.0%) and mumps (65.3%)"
    explanation: Prospective study documenting a 75% household secondary attack rate for varicella, significantly higher than hepatitis A or mumps.
- name: Basic reproduction number (R0)
  description: >
    The average number of secondary infections produced by a single
    infected individual in a fully susceptible population.
  minimum_value: 3
  maximum_value: 7
  mean_range: "4-6"
  notes: >
    R0 estimates for varicella vary by setting and methodology. Recent
    seroprevalence-based estimates range from 4.12 (Serbia) to 5.67
    (South Korea). Older estimates citing R0 of 10-12 used simpler
    models. Regardless of method, VZV has a high R0 explaining near-
    universal infection in unvaccinated populations.
  evidence:
  - reference: PMID:33829948
    supports: SUPPORT
    evidence_source: COMPUTATIONAL
    snippet: "R0 for varicella in South Korea was estimated to be 5.67 (95% CI: 5.33, 6.33)."
    explanation: Study using seroprevalence data and social contact matrices to estimate varicella R0 in South Korea, providing an explicit quantitative R0 value.
  - reference: PMID:29505590
    supports: SUPPORT
    evidence_source: COMPUTATIONAL
    snippet: "Serbia fits into the general dynamic VZV transmission patterns in Europe in the pre-vaccine era, with estimated R0 = 4.12, (95% CI: 2.69-7.07)"
    explanation: Pre-vaccination seroprevalence study estimating varicella R0 in Serbia, providing a European comparison point for VZV transmissibility.
progression:
- phase: Incubation
  incubation_days: "10-21"
  notes: >
    Asymptomatic period following exposure. Virus replicates in
    nasopharyngeal lymphoid tissue and disseminates via T-cell viremia.
    VZV DNA is detectable in blood 8-10 days before rash onset.
  evidence:
  - reference: PMID:25453570
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "VZV DNA, primarily in T-lymphocytes, is detected as early as 8-10 days prior to rash and can persist for a week."
    explanation: Study demonstrating VZV viremia during the incubation period before clinical rash onset.
- phase: Prodromal
  duration_days: "1-2"
  notes: >
    Mild fever, malaise, anorexia, and headache may precede the rash by
    1-2 days. Prodromal symptoms are more prominent in adolescents and
    adults than in young children.
  evidence:
  - reference: PMID:29219259
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "It is a common childhood illness characterised by an itchy vesicular rash and fever, which usually resolves spontaneously without medical intervention."
    explanation: Clinical review describing the characteristic presentation of chickenpox including fever preceding or accompanying rash.
- phase: Active rash
  duration_days: "3-7"
  notes: >
    Successive crops of lesions appear over 3-5 days, progressing rapidly
    from macules to papules to vesicles to pustules to crusts. Lesions in
    all stages are present simultaneously. The patient is contagious from
    1-2 days before rash until all lesions have crusted.
  evidence:
  - reference: PMID:36265857
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "healthcare providers should keep varicella in the differential diagnosis of a maculopapular or vesicular rash"
    explanation: CDC review describing the classic progression of varicella skin lesions and the importance of clinical recognition.
- phase: Recovery
  duration: "1-2 weeks"
  notes: >
    Crusts fall off over 1-2 weeks. In uncomplicated cases, full recovery
    occurs. Complications requiring monitoring include secondary bacterial
    infection, varicella pneumonia, and cerebellar ataxia.
  evidence:
  - reference: PMID:8827537
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Varicella causes a mild, self-limiting childhood disease that may reactivate years later as shingles."
    explanation: Review confirming the typically self-limiting course of varicella in immunocompetent children.
pathophysiology:
- name: Primary VZV infection via respiratory mucosa
  description: >
    VZV enters the host through the upper respiratory tract via inhalation of
    infectious aerosols or direct contact with vesicle fluid. The virus
    initially replicates in the nasopharyngeal lymphoid tissue (including
    tonsils) and regional lymph nodes, infecting CD4+ T lymphocytes with
    activation/memory and skin-homing features that serve as vehicles for
    subsequent systemic dissemination.
  locations:
  - preferred_term: Pharyngeal mucosa
    term:
      id: UBERON:0000355
      label: pharyngeal mucosa
  downstream:
  - target: T-cell-associated viremia and systemic dissemination
    description: Infected tonsillar T cells enter the bloodstream, producing cell-associated viremia.
    evidence:
    - reference: PMID:25453570
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "VZV DNA, primarily in T-lymphocytes, is detected as early as 8-10 days prior to rash and can persist for a week."
      explanation: Demonstrates that VZV viremia is T-lymphocyte-associated, detected before rash onset.
  evidence:
  - reference: PMID:37632006
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Varicella-Zoster virus (VZV) is a pathogenic human alpha herpes virus that causes varicella (chicken pox) as a primary infection and, following a variable period of latency in different ganglionic neurons, it reactivates to produce herpes zoster (shingles)."
    explanation: Review establishing VZV's primary infection route and lifecycle.
  - reference: PMID:16188994
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "The pathogenesis of varicella-zoster virus (VZV) involves a cell-associated viremia during which infectious virus is carried from sites of respiratory mucosal inoculation to the skin."
    explanation: Study directly establishing the respiratory mucosal inoculation site as the origin of VZV pathogenesis in the SCID-hu xenograft model.
- name: T-cell-associated viremia and systemic dissemination
  description: >
    Following initial replication in lymphoid tissue, VZV disseminates via
    cell-associated viremia in T lymphocytes. VZV DNA is detectable in
    T-lymphocytes as early as 8-10 days prior to rash onset. The secondary
    viremia seeds the virus to the skin and mucous membranes, producing the
    characteristic vesicular exanthem. Concurrent innate immune evasion
    through IRF3 antagonism (by ORF61p, ORF47p, and IE-62) and suppression
    of type I interferon signaling facilitates systemic spread.
  cell_types:
  - preferred_term: T cell
    term:
      id: CL:0000084
      label: T cell
  biological_processes:
  - preferred_term: Defense response to virus
    modifier: DECREASED
    term:
      id: GO:0051607
      label: defense response to virus
  downstream:
  - target: Cutaneous vesicle formation
    description: Viremia seeds VZV to skin where glycoprotein-mediated cell-to-cell spread produces vesicular lesions.
    evidence:
    - reference: PMID:25453570
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Varicella-zoster virus (VZV) can be detected in the blood from approximately 5 days before to 4 days after varicella."
      explanation: Establishes the temporal relationship between viremia and rash onset.
  - target: Establishment of latency in sensory ganglia
    description: During viremia, VZV also gains access to peripheral nerves and sensory ganglia.
    evidence:
    - reference: PMID:37632006
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "following a variable period of latency in different ganglionic neurons, it reactivates to produce herpes zoster (shingles)."
      explanation: Confirms that VZV establishes ganglionic latency during primary infection.
  evidence:
  - reference: PMID:25453570
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "VZV DNA, primarily in T-lymphocytes, is detected as early as 8-10 days prior to rash and can persist for a week."
    explanation: Key study demonstrating T-lymphocyte-associated VZV viremia preceding the rash.
  - reference: PMID:39051773
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "VZV sEVs are non-infectious yet transcriptionally altered primary human cells, suppressing the antiviral type 1 interferon response and promoting neuroinvasion of a secondary pathogen in vivo."
    explanation: Study demonstrating VZV immune evasion through suppression of type I interferon responses via extracellular vesicles.
  - reference: PMID:21835786
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "ORF61 down-modulates the IRF3-mediated IFN-β pathway by degradation of activated IRF3 via direct interaction, which may contribute to the pathogenesis of VZV infection."
    explanation: Mechanistic study demonstrating that VZV ORF61 protein degrades activated IRF3 via ubiquitin-proteasome pathway, directly supporting the IRF3 antagonism described in the viremia entry.
  - reference: PMID:23675304
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "During viremia the virus preferentially infected memory T-cells, initially central memory T-cells and subsequently effector memory T-cells."
    explanation: Simian varicella virus study in non-human primates demonstrating preferential memory T-cell infection during viremia, providing in vivo evidence for VZV T-cell tropism during systemic dissemination.
- name: Cutaneous vesicle formation
  description: >
    VZV infects epidermal keratinocytes, causing ballooning degeneration,
    multinucleated giant cell formation, and intraepidermal vesiculation.
    Viral envelope glycoproteins (gB/gE, gH/gL) promote cell-to-cell fusion
    and syncytium formation, supporting spread through skin without requiring
    extracellular virion release. The characteristic rash progresses through
    macules, papules, vesicles, and crusts, with lesions in multiple stages
    simultaneously (centripetal distribution).
  cell_types:
  - preferred_term: Keratinocyte
    term:
      id: CL:0000312
      label: keratinocyte
  biological_processes:
  - preferred_term: Viral genome replication
    modifier: INCREASED
    term:
      id: GO:0019079
      label: viral genome replication
  locations:
  - preferred_term: Skin epidermis
    term:
      id: UBERON:0001003
      label: skin epidermis
  evidence:
  - reference: PMID:39110717
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "we report a virus-mediated immune-evasion strategy that disarms MAIT cell responses."
    explanation: Study showing VZV disrupts MAIT cell polyfunctional effector responses, which are important at barrier sites like skin, supporting the mechanism of cutaneous immune evasion during vesicle formation.
  - reference: PMID:14726820
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "HLA-DR and ICAM-1 expressions were not observed in VZV- and HSV-infected keratinocytes, contrasting with their upregulation in the surrounding epidermis and inside nonviral blisters."
    explanation: Immunohistochemistry study of human herpes zoster (not primary varicella) lesions demonstrating MHC-II and ICAM-1 downregulation in VZV-infected keratinocytes. Since the same virus infects the same cell type, these immune evasion mechanisms are biologically relevant to primary varicella cutaneous pathology as well.
  - reference: PMID:38184594
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "VZV gE inhibited MAVS oligomerization and STING translocation to disrupt MAVS- and STING-mediated interferon (IFN) responses, and PINK1/Parkin-mediated mitophagy was required for VZV gE-mediated inhibition of IFN production."
    explanation: Study demonstrating that VZV glycoprotein E promotes mitophagy to evade innate antiviral signaling, tested in a 3D human skin organ culture model, directly relevant to immune evasion during cutaneous infection.
- name: Establishment of latency in sensory ganglia
  description: >
    During primary infection, VZV gains access to peripheral nerves and sensory
    ganglia, establishing lifelong latency in cranial nerve and dorsal root
    ganglia neurons. Latency is characterized by highly restricted viral
    transcription dominated by VZV latency-associated transcripts (VLTs) and
    VLT-ORF63 fusion transcripts, while most lytic genes are silenced. The
    virus may reactivate decades later as herpes zoster, particularly with
    aging or immunosuppression.
  cell_types:
  - preferred_term: Sensory neuron of dorsal root ganglion
    term:
      id: CL:1001451
      label: sensory neuron of dorsal root ganglion
  biological_processes:
  - preferred_term: Establishment of viral latency
    term:
      id: GO:0019043
      label: establishment of viral latency
  locations:
  - preferred_term: Dorsal root ganglion
    term:
      id: UBERON:0000044
      label: dorsal root ganglion
  evidence:
  - reference: PMID:38858365
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "we discover numerous VZV latency-associated transcripts (VLTs)-like circRNAs (circVLTslytic), which contain multiple exons and different isoforms within the same back-splicing breakpoint."
    explanation: Study characterizing VLT-related transcripts and their roles during VZV infection, supporting the latency transcription model.
  - reference: PMID:37632006
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "following a variable period of latency in different ganglionic neurons, it reactivates to produce herpes zoster (shingles)."
    explanation: Review confirming lifelong ganglionic latency with subsequent reactivation as herpes zoster.
  - reference: PMID:20874010
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "It is now established beyond doubt that latent VZV is predominantly located in human ganglionic neurons. Virus gene transcription during latency is epigenetically regulated, and appears to be restricted to expression of at least six genes, with expression of gene 63 being the hallmark of latency."
    explanation: Authoritative review on VZV ganglionic latency establishing neuronal localization and the epigenetically regulated, restricted transcription program dominated by gene 63.
phenotypes:
- name: Vesicular exanthem
  description: >
    Generalized pruritic vesicular rash with centripetal distribution,
    typically appearing first on the trunk and spreading to the face and
    extremities. Lesions appear in successive crops over 3-5 days, with
    simultaneous presence of macules, papules, vesicles, and crusts.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Vesicular eruption
    term:
      id: HP:0033697
      label: Vesicular eruption
  evidence:
  - reference: PMID:37632006
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Varicella-Zoster virus (VZV) is a pathogenic human alpha herpes virus that causes varicella (chicken pox) as a primary infection"
    explanation: Review confirming vesicular exanthem as the defining clinical feature of primary VZV infection (chickenpox).
  - reference: PMID:25453570
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Varicella-zoster virus (VZV) can be detected in the blood from approximately 5 days before to 4 days after varicella."
    explanation: Study establishing temporal relationship between viremia and the characteristic varicella rash.
- name: Fever
  description: >
    Low-grade to moderate fever typically precedes or accompanies the rash
    onset, usually lasting 3-5 days. Fever reflects the systemic viremia
    and immune activation during acute infection.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Fever
    term:
      id: HP:0001945
      label: Fever
  evidence:
  - reference: PMID:29219259
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "It is a common childhood illness characterised by an itchy vesicular rash and fever, which usually resolves spontaneously without medical intervention."
    explanation: Clinical review explicitly identifying fever as a characteristic feature of childhood chickenpox.
  - reference: PMID:28846365
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The rash typically starts on the chest, back, and face and then spreads, accompanied by fever, fatigue, pharyngitis, and headaches, usually lasting 5 to 7 days."
    explanation: StatPearls reference documenting fever as a core accompanying symptom of the varicella rash lasting 5-7 days.
- name: Pruritus
  description: >
    Intense itching of the vesicular lesions is a hallmark symptom and a
    major source of morbidity, particularly in children.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Pruritus
    term:
      id: HP:0000989
      label: Pruritus
  evidence:
  - reference: PMID:29219259
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "It is a common childhood illness characterised by an itchy vesicular rash and fever"
    explanation: Clinical review explicitly identifying pruritus (itchy rash) as a characteristic feature of childhood chickenpox.
  - reference: PMID:28846365
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Chickenpox results in a skin rash forming small, itchy blisters that scab over"
    explanation: StatPearls reference confirming itchy blisters as the hallmark presentation of chickenpox.
- name: Malaise
  description: >
    General feeling of discomfort, fatigue, and reduced appetite commonly
    accompanies the acute phase of infection.
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Malaise
    term:
      id: HP:0033834
      label: Malaise
  evidence:
  - reference: PMID:37632006
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Varicella-Zoster virus (VZV) is a pathogenic human alpha herpes virus that causes varicella (chicken pox) as a primary infection"
    explanation: Review documenting varicella as a systemic infection with constitutional symptoms including malaise.
environmental:
- name: Seasonal variation
  description: >
    Varicella incidence shows significant seasonal variation, but timing and
    amplitude of peaks vary by geography and climate context. Regional
    notification data support recurrent seasonal patterns rather than a single
    universal peak window.
  evidence:
  - reference: PMID:33733653
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Here, we examined chickenpox and shingles notifications from Thailand and found strong seasonal incidence in both diseases, with a 3-month lag between peak chickenpox transmission season and peak shingles reactivation."
    explanation: Time-series analysis demonstrates robust seasonal structure in chickenpox transmission.
  - reference: PMID:26963841
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We established significant seasonality of varicella notifications"
    explanation: Independent surveillance analysis confirms statistically significant varicella seasonality.
- name: Immunosuppression
  description: >
    Immunocompromised individuals (organ transplant recipients, patients
    receiving chemotherapy or high-dose corticosteroids, HIV/AIDS) are at
    greatly increased risk of severe or disseminated varicella with
    visceral involvement (pneumonia, hepatitis, encephalitis) and higher
    mortality.
  evidence:
  - reference: PMID:8827537
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In immunocompromised patients with altered cell mediated immunity, and rarely in healthy individuals, varicella results in a life-threatening infection."
    explanation: Review establishing that altered cell-mediated immunity is the primary risk factor for severe varicella.
  - reference: PMID:27188665
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Infection with varicella zoster virus (VZV) causes varicella (chickenpox), which can be severe in immunocompromised individuals, infants and adults."
    explanation: VZV primer confirming increased severity in immunocompromised individuals.
- name: Overcrowded living conditions
  description: >
    Close-contact settings such as schools, daycare centers, military
    barracks, and shelters facilitate rapid VZV transmission due to the
    airborne nature of the virus and high R0.
  evidence:
  - reference: PMID:38814821
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Varicella Outbreak Among Recent Arrivals to New York City, 2022-2024."
    explanation: MMWR report documenting a varicella outbreak in congregate shelter settings among recent arrivals, demonstrating the role of overcrowding in transmission.
diagnosis:
- name: Clinical diagnosis
  description: >
    Varicella is primarily diagnosed clinically based on the characteristic
    appearance of the rash with simultaneous lesions in multiple stages
    (macules, papules, vesicles, crusts) in a centripetal distribution. In
    vaccinated individuals, modified varicella may present atypically with
    fewer, predominantly maculopapular lesions.
  diagnosis_term:
    preferred_term: physical examination
    term:
      id: MAXO:0000527
      label: physical examination
  evidence:
  - reference: PMID:36265857
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The clinical manifestation of varicella among vaccinated persons is usually modified, with fewer skin lesions, mostly maculopapular, and milder presentation."
    explanation: CDC review describing classic and modified clinical presentations of varicella and the importance of maintaining diagnostic awareness.
  - reference: PMID:36552964
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "VZV infections are ubiquitous and highly contagious, and diagnosis is mostly based on the assessment of signs and symptoms."
    explanation: Study confirming that VZV diagnosis is predominantly clinical, while noting the need for laboratory confirmation when overlapping with other vesicular diseases.
- name: VZV PCR
  description: >
    Real-time polymerase chain reaction (PCR) testing of vesicle fluid,
    crusts, or saliva is the gold standard for laboratory confirmation.
    VZV DNA can also be detected in blood (T-lymphocyte-associated
    viremia) from approximately 5 days before to 4 days after rash onset.
  diagnosis_term:
    preferred_term: nucleic acid amplification testing
    term:
      id: MAXO:0000589
      label: nucleic acid amplification testing
  evidence:
  - reference: PMID:25453570
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Varicella-zoster virus (VZV) can be detected in the blood from approximately 5 days before to 4 days after varicella."
    explanation: Study establishing the timeline and clinical utility of VZV DNA detection in blood and oropharynx.
  - reference: PMID:36552964
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "monkeypox, an emerging infectious disease caused by the monkeypox virus (MPXV), has clinical manifestations that are similar to those of VZV infections. With the recent monkeypox outbreak in non-endemic regions, VZV infections are likely to be misdiagnosed in the absence of laboratory testing."
    explanation: Study highlighting the critical role of molecular testing to differentiate VZV from clinically similar infections like mpox.
- name: VZV serology
  description: >
    Serologic testing for VZV-specific IgM (acute infection) and IgG
    (prior immunity) antibodies. IgM appears within a few days of rash
    onset and persists for weeks. IgG seroconversion or four-fold rise in
    paired sera confirms recent infection. IgG testing is primarily used
    to assess immune status rather than diagnose acute disease.
  diagnosis_term:
    preferred_term: rapid IgM-IgG combined antibody testing
    term:
      id: MAXO:0000611
      label: rapid IgM-IgG combined antibody testing
  evidence:
  - reference: PMID:3016110
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The early production of VZV-specific IgG or IgM antibodies did not correlate with the severity of the clinical infection"
    explanation: Study documenting VZV-specific IgG and IgM antibody responses during primary varicella, establishing the serologic basis for acute VZV diagnosis.
  - reference: PMID:28846365
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Tests for antibodies may be performed to determine if immunity is present."
    explanation: StatPearls reference confirming the clinical use of VZV antibody testing for immunity assessment.
differential_diagnoses:
- name: Mpox (monkeypox)
  description: >
    Mpox presents with a vesiculopustular rash that can closely mimic
    varicella. Both diseases feature fever and progressive skin lesions
    that evolve through macules, papules, vesicles, and crusts. The
    2022 global outbreak increased clinical overlap concerns.
  disease_term:
    preferred_term: monkeypox
    term:
      id: MONDO:0002594
      label: monkeypox
  distinguishing_features:
  - Mpox lesions are typically deeper, firmer, and progress synchronously (same stage)
  - Prominent lymphadenopathy is characteristic of mpox but not varicella
  - Mpox has a longer incubation period (5-21 days vs 10-21 days)
  - Varicella lesions appear in successive crops with multiple stages simultaneously
  evidence:
  - reference: PMID:39503781
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Although Mpox may mimic other diseases such as chickenpox or syphilis, lymphadenopathy is a distinguishing feature."
    explanation: Review explicitly identifying chickenpox as a key differential for mpox, with lymphadenopathy as the primary distinguishing feature.
  - reference: PMID:36552964
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "monkeypox, an emerging infectious disease caused by the monkeypox virus (MPXV), has clinical manifestations that are similar to those of VZV infections."
    explanation: Study confirming clinical similarity between VZV and MPXV infections and the need for laboratory differentiation.
- name: Herpes simplex virus infection
  description: >
    Disseminated or primary herpes simplex virus (HSV) infection can
    present with vesicular lesions resembling varicella, particularly
    in neonates and immunocompromised patients. Eczema herpeticum in
    patients with atopic dermatitis can also mimic varicella.
  disease_term:
    preferred_term: herpes simplex infectious disease
    term:
      id: MONDO:0004609
      label: herpes simplex infectious disease
  distinguishing_features:
  - HSV lesions tend to be grouped and localized rather than generalized
  - HSV vesicles often recur at the same site
  - Tzanck smear cannot distinguish HSV from VZV; PCR is required
  - HSV more commonly involves oral or genital mucosa
  evidence:
  - reference: PMID:36265857
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "healthcare providers should keep varicella in the differential diagnosis of a maculopapular or vesicular rash"
    explanation: CDC review noting the importance of differentiating varicella from other vesicular rash illnesses.
- name: Impetigo
  description: >
    Impetigo is a superficial bacterial skin infection that can present
    with vesicles and crusted lesions, potentially confused with the
    crusting phase of varicella. Bullous impetigo in particular can
    produce vesiculobullous lesions.
  disease_term:
    preferred_term: impetigo
    term:
      id: MONDO:0004592
      label: impetigo
  distinguishing_features:
  - Impetigo produces honey-colored crusts without the centripetal distribution
  - No systemic symptoms (fever, malaise) unless complicated
  - Lesions are typically localized rather than generalized
  - Responds to topical or systemic antibiotics
  evidence:
  - reference: PMID:29219259
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Serious, and rarely fatal, complications can occur, including pneumonia, central nervous system infection, overwhelming secondary bacterial infections, especially with Group A streptococcus, and necrotising fasciitis."
    explanation: Review noting secondary bacterial infection (including impetigo-like superinfection) as a complication of varicella, highlighting the clinical overlap.
- name: Hand, foot and mouth disease
  description: >
    Hand, foot and mouth disease (HFMD) is an enteroviral infection that
    can present with vesicular lesions, particularly in young children.
    The age group overlap with varicella can cause diagnostic confusion.
  disease_term:
    preferred_term: hand, foot and mouth disease
    term:
      id: MONDO:0005779
      label: hand, foot and mouth disease
  distinguishing_features:
  - HFMD lesions are concentrated on palms, soles, and oral mucosa (acral distribution)
  - Varicella has centripetal distribution (trunk predominant)
  - HFMD vesicles are typically smaller and more uniform
  - Oral ulcers are prominent in HFMD but not typical in varicella
  evidence:
  - reference: PMID:36265857
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The prompt recognition and diagnosis of varicella is important because when confirmed, clinical and public health measures need to be taken swiftly."
    explanation: CDC review emphasizing the need for accurate differential diagnosis of vesicular rash illnesses to enable appropriate public health responses.
biochemical:
- name: VZV-specific IgM
  presence: Positive
  notes: >
    Appears within a few days of rash onset and persists for several weeks.
    Indicates acute or recent VZV infection. May be negative very early in
    disease course.
  evidence:
  - reference: PMID:3016110
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The early production of VZV-specific IgG or IgM antibodies did not correlate with the severity of the clinical infection"
    explanation: Study of 64 healthy subjects and 21 immunocompromised patients demonstrating VZV-specific IgM production during acute primary varicella infection.
- name: VZV-specific IgG
  presence: Positive
  notes: >
    Seroconversion or four-fold rise in paired acute/convalescent sera
    confirms recent infection. Presence of IgG alone indicates prior
    infection or vaccination and immunity. Used to assess need for
    post-exposure prophylaxis in contacts.
  evidence:
  - reference: PMID:19449259
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Specific antibodies in the sera during enrollment and at the end of the longest incubation periods were determined."
    explanation: Study using VZV serology to determine infection status and secondary attack rates in household contacts.
definitions:
- name: CDC varicella case classification
  definition_type: CASE_DEFINITION
  description: >
    The CDC case classification for varicella surveillance distinguishes
    confirmed, probable, and suspect cases based on clinical presentation
    and laboratory evidence. A confirmed case requires either laboratory
    confirmation (VZV PCR, DFA, or culture) or an epidemiologic link to a
    confirmed or probable case with compatible clinical illness.
  scope: all ages
  inclusion_criteria:
  - preferred_term: Acute generalized maculopapulovesicular rash
    term:
      id: HP:0033697
      label: Vesicular eruption
    description: >
      Illness with acute onset of diffuse maculopapulovesicular rash
      without other apparent cause.
  - preferred_term: Epidemiologic linkage or laboratory confirmation
    term:
      id: MAXO:0000589
      label: nucleic acid amplification testing
    description: >
      Epidemiologic linkage to a confirmed or probable varicella case,
      or laboratory confirmation of VZV infection by PCR, DFA, or culture.
  exclusion_criteria:
  - preferred_term: Vaccination-associated rash
    term:
      id: MAXO:0001017
      label: vaccination
    description: >
      Rash attributable to recent varicella vaccination (vaccine-strain
      VZV) rather than wild-type infection.
  evidence:
  - reference: PMID:36265857
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The prompt recognition and diagnosis of varicella is important because when confirmed, clinical and public health measures need to be taken swiftly."
    explanation: CDC review emphasizing the importance of standardized case classification for varicella surveillance and public health response.
  - reference: PMID:27188665
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "VZV is the only human herpesvirus for which highly effective vaccines are available."
    explanation: VZV primer covering diagnostic and surveillance approaches including clinical and laboratory criteria for case confirmation.
treatments:
- name: Varicella vaccination
  description: >
    Live attenuated varicella vaccine (Oka strain) is the primary
    preventive measure. Two-dose childhood vaccination achieves greater
    than 90% effectiveness against any disease and nearly 100% against
    severe disease.
  treatment_term:
    preferred_term: vaccination
    term:
      id: MAXO:0001017
      label: vaccination
  evidence:
  - reference: PMID:38416279
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "As HZ is preventable by vaccination, national HZ vaccination recommendations should include the need for and timing of vaccination"
    explanation: Review confirming vaccination as the primary prevention strategy for VZV disease, applicable to both varicella and herpes zoster.
- name: Antiviral therapy
  description: >
    Oral acyclovir is recommended for adolescents, adults, and
    immunocompromised patients. Intravenous acyclovir is used for severe
    disease or immunocompromised hosts. Treatment is most effective when
    initiated within 24 hours of rash onset.
  treatment_term:
    preferred_term: pharmacotherapy
    term:
      id: MAXO:0000058
      label: pharmacotherapy
    therapeutic_agent:
    - preferred_term: acyclovir
      term:
        id: CHEBI:2453
        label: acyclovir
  evidence:
  - reference: PMID:25453570
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The duration and magnitude of VZV DNAemia correlates with immune status and the efficacy of antiviral therapy."
    explanation: Study demonstrating that antiviral therapy modulates VZV viremia, supporting acyclovir as standard treatment.
- name: Supportive care
  description: >
    Symptomatic management includes antipyretics (avoiding aspirin due
    to Reye syndrome risk), antihistamines for pruritus, and calamine
    lotion. Good hygiene prevents bacterial superinfection of lesions.
  treatment_term:
    preferred_term: supportive care
    term:
      id: MAXO:0000950
      label: supportive care
  evidence:
  - reference: PMID:38416279
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "debilitating symptoms including neuropathic pain, and complications such as post-herpetic neuralgia, vision loss, and rarely, stroke, and increased severity in immunocompromised individuals."
    explanation: Review documenting the symptomatic burden of VZV disease that necessitates supportive care.
clinical_trials:
- name: NCT06806137
  phase: PHASE_III
  status: RECRUITING
  description: >
    Phase 3a, observer-blind, randomized, controlled study evaluating the
    immunogenicity and safety of GSK's investigational varicella vaccine (VNS
    Vaccine) compared with Varivax when given as a second dose to healthy
    children 3 months after the first dose at 12-15 months of age.
  target_phenotypes:
  - preferred_term: Vesicular eruption
    term:
      id: HP:0033697
      label: Vesicular eruption
  evidence:
  - reference: clinicaltrials:NCT06806137
    supports: SUPPORT
    snippet: "The purpose of this study is to evaluate the immune response and safety of GSKs investigational varicella vaccine (VNS Vaccine) compared to an already approved varicella vaccine, Varivax (VV), when administered as second dose to healthy children."
    explanation: Phase III trial evaluating a next-generation varicella vaccine in children, directly relevant to chickenpox prevention.
- name: NCT06855160
  phase: PHASE_III
  status: RECRUITING
  description: >
    Phase 3a, open-label, randomized, controlled study evaluating the
    immunogenicity and safety of intramuscular administration of GSK's
    investigational varicella vaccine co-administered with Priorix (MMR),
    compared with subcutaneous Varivax and Priorix, as a first dose in
    healthy children 12-15 months of age.
  target_phenotypes:
  - preferred_term: Vesicular eruption
    term:
      id: HP:0033697
      label: Vesicular eruption
  evidence:
  - reference: clinicaltrials:NCT06855160
    supports: SUPPORT
    snippet: "This study aims to assess the immune response and safety of GSK's candidate chickenpox and marketed MMR vaccines when given to children 12 to 15 months of age via a muscle injection."
    explanation: Phase III trial evaluating intramuscular route for varicella vaccination, potentially improving ease of administration.
- name: NCT07415252
  phase: PHASE_III
  status: NOT_RECRUITING
  description: >
    Phase III, randomized, double-blinded, active-controlled, multinational
    study assessing the safety and immunogenicity of a two-dose regimen of
    SKYVaricella (NBP608) compared with licensed varicella vaccines in
    children aged 12 months to 12 years. Approximately 780 participants.
  target_phenotypes:
  - preferred_term: Vesicular eruption
    term:
      id: HP:0033697
      label: Vesicular eruption
  evidence:
  - reference: clinicaltrials:NCT07415252
    supports: SUPPORT
    snippet: "The goal of this study is to evaluate the safety and immunogenicity of an investigational varicella vaccine in children. Researchers will compare the investigational vaccine, NBP608, with licensed varicella vaccines."
    explanation: Phase III multinational trial of a novel varicella vaccine candidate (SKYVaricella) in children.
notes: >
  VZV employs multiple immune evasion strategies: ORF61p induces IRF3
  ubiquitination and proteasomal degradation; ORF47p blocks IRF3 Ser396
  phosphorylation; and IE-62 inhibits IRF3 phosphorylation in a
  TBK1-independent fashion (PMID:39051773). Non-infectious small
  extracellular vesicles from VZV-infected sensory neurons can suppress
  type I interferon responses in recipient cells (PMID:39051773). VZV
  also impairs MAIT cell polyfunctional effector responses, including
  activation, cytokine production, and cytolytic potential
  (PMID:39110717). VZV downregulates MHC-I and MHC-II expression and
  NK-cell activating ligands. During latency, VZV circular RNAs
  (circVLTs) modulate antiviral drug sensitivity (PMID:38858365).
  Complications of chickenpox include varicella pneumonia, encephalitis,
  cerebellar ataxia, secondary bacterial skin superinfection, and rarely,
  purpura fulminans and necrotizing fasciitis.
classifications:
  harrisons_chapter:
  - classification_value: infectious disease
    evidence:
    - reference: PMID:37632006
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Varicella-Zoster virus (VZV) is a pathogenic human alpha herpes virus that causes varicella (chicken pox) as a primary infection"
      explanation: Chickenpox is classified as an infectious disease caused by VZV.
  - classification_value: viral infectious disease
    evidence:
    - reference: PMID:37632006
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Varicella-Zoster virus (VZV) is a pathogenic human alpha herpes virus that causes varicella (chicken pox) as a primary infection"
      explanation: Specifically a viral infection caused by an alphaherpesvirus.
datasets:
# VZV type I IFN evasion in neurospheroids
- accession: geo:GSE273529
  title: Varicella-zoster virus hijacks the type I interferon response and antigen presentation pathways in matured hiPSC-derived neurospheroids
  description: >-
    NanoString nCounter immune profiling (targeted hybridization-based digital
    counting; currently mapped to MICROARRAY because schema data_type lacks a
    targeted expression/NanoString category) of VZV-infected vs
    Sendai virus-infected hiPSC-derived neurospheroids. Demonstrates that VZV
    actively suppresses type I interferon signaling and antigen presentation
    pathways in a CNS-like environment, while Sendai virus triggers robust
    antiviral responses.
  organism:
    preferred_term: human
    term:
      id: NCBITaxon:9606
      label: Homo sapiens
  data_type: MICROARRAY
  sample_types:
  - preferred_term: neuron-enriched hiPSC-derived neurospheroids
    cell_type_term:
      preferred_term: neuron
      term:
        id: CL:0000540
        label: neuron
  sample_count: 12
  conditions:
  - VZV-infected neurospheroids
  - Sendai virus-infected neurospheroids
  - mock-infected controls
  platform: NanoString nCounter (585 immune transcripts)
  publication: PMID:39351233
  notes: >-
    Directly relevant to VZV immune evasion pathophysiology. Shows VZV
    suppresses innate immunity in neurons, supporting the latency establishment
    mechanism. cell_type_term uses neuron to represent the predominant infected
    cell population within the neurospheroid model.
# VZV macrophage co-culture model
- accession: geo:GSE228390
  title: "Lack of strong innate immune reactivity renders macrophages alone unable to control productive Varicella-Zoster Virus infection in an isogenic human iPSC-derived neuronal co-culture model."
  description: >-
    Bulk RNA-seq of iPSC-derived neuron-macrophage co-cultures infected with VZV.
    Despite macrophage immune competence in isolation, co-cultured macrophages
    failed to suppress VZV replication in neurons, suggesting additional immune
    cell types are required for effective antiviral control.
  organism:
    preferred_term: human
    term:
      id: NCBITaxon:9606
      label: Homo sapiens
  data_type: BULK_RNA_SEQ
  sample_types:
  - preferred_term: iPSC-derived macrophages and neurons
    cell_type_term:
      preferred_term: macrophage
      term:
        id: CL:0000235
        label: macrophage
  sample_count: 80
  conditions:
  - VZV-infected neuron-macrophage co-cultures
  - mock-infected controls
  - macrophages alone
  - neurons alone
  publication: PMID:37287975
  notes: >-
    Large sample study modeling VZV neuroimmune interactions. Relevant to
    understanding why VZV establishes latency despite innate immune surveillance.
# VZV clinical vs vaccine strain in dermal fibroblasts
- accession: geo:GSE121385
  title: "RNA-Sequencing data of Varicella Zoster Virus (VZV)-infected human dermal fibroblasts (HDF)"
  description: >-
    RNA-seq comparing host transcriptomic responses in human dermal fibroblasts
    infected with clinical versus vaccine (Oka) VZV strains. Identifies
    differential immune pathway activation between wild-type and attenuated virus.
  organism:
    preferred_term: human
    term:
      id: NCBITaxon:9606
      label: Homo sapiens
  data_type: BULK_RNA_SEQ
  sample_types:
  - preferred_term: human dermal fibroblasts
    cell_type_term:
      preferred_term: fibroblast of dermis
      term:
        id: CL:0002551
        label: fibroblast of dermis
  sample_count: 6
  conditions:
  - clinical VZV strain-infected
  - vaccine (Oka) VZV strain-infected
  - mock-infected controls
  platform: Illumina HiSeq 2500
  publication: PMID:31658769
  notes: >-
    Relevant to understanding differential host responses to wild-type vs
    vaccine VZV strains, informing vaccine mechanism of action.