1. Basic Information
Disease Protected Against
Varicella (Chickenpox) is caused by varicella-zoster virus (VZV). Before vaccination, varicella caused ~4 million cases, ~10,500–13,500 hospitalizations, and ~100–150 deaths annually in the U.S. Complications include bacterial superinfection (primarily Group A Strep), pneumonia, encephalitis, and congenital varicella syndrome. VZV establishes latency in dorsal root ganglia; reactivation later in life causes herpes zoster (shingles).
CDC Schedule (U.S., 2025)
| Dose | Age | Product |
|---|---|---|
| Dose 1 | 12–15 months | Varivax (standalone) or ProQuad (MMRV) |
| Dose 2 | 4–6 years | Varivax or ProQuad. Minimum interval: 3 months between doses. |
Source: CDC ACIP, 2025 schedule. ACIP recommends MMR + varicella separately for dose 1 in children 12–47 months due to MMRV febrile seizure risk; MMRV may be used for dose 2 or for dose 1 in children ≥48 months.
2. Pre-Licensure Clinical Trial Data
Varivax was licensed in 1995 based on clinical trials in ~11,000 children and adults. The pivotal efficacy trial enrolled ~1,000 children. ProQuad (MMRV) was licensed in 2005 based on trials in ~6,000 children.
| Metric | Data |
|---|---|
| Pre-licensure safety database (Varivax) | ~11,000 individuals |
| Efficacy (1 dose) | ~85% (all varicella); ~97% (severe varicella) |
| Efficacy (2 doses) | ~98% (all varicella); near-100% (severe varicella) |
| Most common reactions | Injection site reactions (~20–30%), fever (~10–15%), varicella-like rash (~3–5%, may be infectious) |
MMRV Febrile Seizure Finding
The MMRV pre-licensure trials observed a higher rate of fever (38–40%) and a numerically higher rate of febrile seizures compared to MMR + varicella given separately. The febrile seizure risk was confirmed post-licensure in VSD studies: ~1 additional febrile seizure per 1,250 MMRV doses compared to MMR + varicella given separately, in the 5–12 day post-vaccination window. This led to the ACIP preference for separate MMR + varicella for dose 1.
3. Post-Licensure Safety Data
Varicella vaccine has >30 years of post-licensure safety data. Key VSD findings:
- MMRV febrile seizures: ~1 per 1,250 doses vs. ~1 per 2,500 for MMR + varicella separately (5–12 day window). ACIP preference for separate administration for dose 1 under age 4.
- Herpes zoster (shingles) from vaccine strain: Documented in vaccinated children and adults; rate is lower than after wild-type VZV infection. Estimated at ~18–26 per 100,000 person-years in vaccinated children vs. ~74 per 100,000 after wild-type infection.
- Vaccine-strain transmission: Occasional transmission of vaccine-strain VZV from a vaccinated person who develops a rash to susceptible contacts. Rare; mostly from immunocompromised vaccinees. No documented transmission from vaccinated persons without a rash.
- ITP: MMRV and MMR + varicella both associated with a small ITP risk (attributable primarily to the MMR component — see MMR page).
⚠ Critical Caveat
VAERS data represent unverified reports. A report to VAERS does not mean the vaccine caused the event.
VAERS Reporting Data — Halma & Varon (2025), DARE-SAFE
The DARE-SAFE paper (Halma & Varon, Pharmacoepidemiology 2025, CC BY 4.0) analyzed VAERS reports for vaccines administered in the United States from 2006–2022. The following data are extracted from Table 1 of that paper for this vaccine (Varicella standalone (Varivax); excludes MMRV (MMR-Varicella combination, which had 15,668 reports / 20 deaths / 42.9M doses / 0.0466 death rate)):
| Metric | Value |
|---|---|
| U.S. doses administered (2006–2022) | 143,906,028 |
| Total VAERS AE reports | 48,863 |
| AE reporting rate (per 100,000 doses) | 34.0 |
| Total death reports | 84 |
| Death reporting rate (per 100,000 doses) | 0.0584 |
| AE-to-Death ratio | 582:1 |
Source: Halma MT, Varon J. DARE-SAFE. Pharmacoepidemiology. 2025. DOI: 10.3390/pharma4010005. CC BY 4.0. Data from Table 1.
📚 Important Interpretive Caveats (from the paper itself)
- Reporting rate ≠ incidence rate. VAERS is a passive, unverified system. A report means someone submitted a claim of temporal association, not a confirmed causal event. The paper is explicit that causality cannot be inferred from these numbers alone.
- Reporting behavior is not uniform. More serious, unusual, or media-salient events are reported at much higher rates than mild ones. Products receiving more public, media, legal, and clinical attention (particularly COVID-19 vaccines, which also benefited from V-safe active-surveillance prompts and CICP compensation pathways) generate more reports per dose regardless of true risk.
- Age and comorbidity confounding is not adjusted. COVID-19 vaccines were disproportionately administered to elderly and comorbid populations (nursing homes, 65+, high-risk groups in early 2021) with much higher background all-cause mortality than the general child/working-age population. Some fraction of temporally-associated deaths would occur regardless of vaccination, and the paper does not perform a background-rate comparison.
- Stimulated reporting is a known, documented phenomenon. Media coverage, plaintiff attorney solicitation, and advocacy campaigns — all independently inflate VAERS reporting propensity. The paper cites this literature but does not correct for it.
- Small-denominator rows are unreliable. Rates computed from small denominators (e.g., monovalent measles, DT, mumps, rubella) have enormous statistical uncertainty and should not be compared to vaccines with hundreds of millions of administered doses without noting the wide confidence intervals.
Source: Halma MT, Varon J. DARE-SAFE: A data analysis and reporting engine for safety signal detection and pharmacovigilance. Pharmacoepidemiology. 2025;4(1):5. DOI: 10.3390/pharma4010005. CC BY 4.0.
4. Documented Adverse Events
▶ Strong Evidence
- Injection site reactions: 20–30%. Strong
- Fever: 10–15% (Varivax alone); 38–40% (MMRV). Strong
- Varicella-like rash: 3–5% (mild; may be infectious). Strong
- Febrile seizures (MMRV): ~1 per 1,250 doses in the 5–12 day window. Strong
- Herpes zoster (vaccine strain): 18–26 per 100,000 person-years; lower than after wild-type infection. Strong
▶ No Causal Association
- Autism: No association. MMRV contains the same MMR components studied extensively for autism (see MMR page). No Association
5. Disease Prevention Benefits
| Metric | Pre-Vaccine Era (~1990–1994) | Post-Vaccine Era (2-dose, ~2010+) |
|---|---|---|
| Varicella cases (annual) | ~4 million | >97% reduction; varicella is no longer endemic in the U.S. |
| Hospitalizations (annual) | ~10,500–13,500 | >90% reduction; almost exclusively unvaccinated or immunocompromised |
| Deaths (annual) | ~100–150 | <10/year |
| Herpes zoster in children | ~74 per 100,000 | ~18–26 per 100,000 (vaccine-strain); overall pediatric zoster has declined |
Source: CDC Pink Book; MMWR. The introduction of a second dose in 2006 effectively addressed breakthrough varicella observed with the 1-dose schedule. Herd immunity has been observed.
6. Evidence Summary
Varicella vaccine has >30 years of post-licensure data. The safety profile is well-characterized. The primary safety concern is the MMRV-associated febrile seizure risk, which is product-specific and mitigated by ACIP's preference for separate MMR + varicella administration for dose 1. Breakthrough varicella with the 1-dose schedule was addressed by the 2006 2-dose recommendation. Long-term data indicate that vaccine-strain zoster is less common than zoster after wild-type infection.
7. Key References
- IOM. Adverse Effects of Vaccines: Evidence and Causality. National Academies Press; 2012.
- Klein NP, et al. Measles-mumps-rubella-varicella combination vaccine and the risk of febrile seizures. Pediatrics. 2010;126(1):e1–e8.
- CDC. Pink Book — Varicella chapter. cdc.gov/pinkbook
- CDC. VSD. cdc.gov/vaccine-safety/about/vsd.html
- CDC/FDA. VAERS. vaers.hhs.gov