Chickenpox Vaccine Adverse Events — VAERS Data

Chickenpox vaccine adverse events reported to VAERS — documenting post-market safety data for varicella vaccines (Varivax), including herpes zoster reactivation and febrile seizure reports.

Post-licensure findings on Varivax (Merck), the only currently licensed varicella vaccine in the US, and ProQuad (Merck, MMRV combination). The source contains relatively limited varicella-specific post-licensure data. The most relevant findings concern (1) the exogenous boosting hypothesis — the idea that universal varicella vaccination has reduced circulating varicella in the community and thereby eliminated the natural "immunity boosting" that adult exposure to childhood varicella provided, potentially increasing adult shingles incidence; and (2) the CDC autism lawsuit finding that no CDC-cited study examined varicella vaccine.

Exogenous Boosting and Adult Shingles Concerns

Background: Varicella-zoster virus (VZV), after primary infection (chickenpox), establishes lifelong latent infection in the dorsal root ganglia. Reactivation later in life causes herpes zoster (shingles). Reactivation risk is suppressed by repeated exposure to circulating VZV — adults whose immunity is "boosted" by encountering chickenpox in children are less likely to develop shingles.

The hypothesis: Universal varicella vaccination of children (started in 1995 with Varivax) reduced the circulation of wild-type VZV in the community. Adults who would previously have received exogenous boosting from sick children no longer encounter the virus. Without exogenous boosting, adult shingles rates may increase.

Status: The hypothesis is documented in mainstream epidemiological literature but its magnitude and significance are contested. The introduction of zoster vaccines (Zostavax, Shingrix) for adults can be interpreted either as response to genuine increased shingles incidence or as expansion of vaccine markets independent of any post-Varivax effect.

CDC Autism Lawsuit: Varicella Not Studied

In court stipulation responding to ICAN's lawsuit, the CDC identified its complete evidence base for claiming infant vaccines do not cause autism: 20 studies. Varicella vaccine was not examined in any of the 20 studies in the relevant infant age window. See Post-Licensure Safety Monitoring.

ProQuad (MMRV) and Febrile Seizure Risk

Post-licensure surveillance found that ProQuad (MMRV combined) was associated with approximately twice the febrile seizure risk of MMR + Varivax administered separately, particularly in the 12–23 month age group. The CDC modified its recommendation to favor separate MMR + Varivax injections at the first dose to reduce this risk.

Henry Ford Vaccinated vs. Unvaccinated Study

The Henry Ford Vaccinated vs. Unvaccinated Study compared 18,468 vaccinated vs. unvaccinated children for chronic disease outcomes. Vaccinated children received Varivax along with the rest of the schedule. The study cannot isolate Varivax-specific effects but is part of the post-licensure context for all schedule vaccines.

VAERS, VSD, V-SAFE Coverage

Varivax is nominally subject to post-licensure surveillance through VAERS, VSD (Vaccine Safety Datalink), and V-SAFE. These systems have not produced regulatory action against Varivax despite the inadequate pre-licensure trial design (465-child neomycin "placebo" control).

See Also

Varicella Vaccines (Pre-Licensure), MMR Vaccines (Post-Licensure), Combination Vaccines (Post-Licensure), Post-Licensure Safety Monitoring, Henry Ford Vaccinated vs. Unvaccinated Study

Frequently Asked Questions

Frequently Asked Questions

Has the chickenpox vaccine increased shingles rates in adults?
The exogenous boosting hypothesis suggests this may be occurring. Before universal varicella vaccination, adults received natural immunity boosting from exposure to children with chickenpox, which suppressed shingles reactivation. Since Varivax reduced childhood chickenpox circulation starting in 1995, adults no longer receive this natural boosting. The hypothesis is documented in mainstream epidemiological literature, though its magnitude is contested.
Does the ProQuad (MMRV) vaccine increase febrile seizure risk?
Yes. Post-licensure surveillance found that ProQuad (MMRV combined) was associated with approximately twice the febrile seizure risk of MMR and Varivax administered separately, particularly in the 12-23 month age group. The CDC modified its recommendation to favor separate MMR + Varivax injections at the first dose. This is one of the few documented cases of post-licensure surveillance leading to a CDC recommendation change.
Has the CDC studied whether the chickenpox vaccine causes autism?
No. In court stipulation responding to ICAN's lawsuit, the CDC identified 20 studies as its complete evidence base for claiming infant vaccines do not cause autism. Varicella vaccine was not examined in any of the 20 studies in the relevant infant age window. Varivax is given at 12-15 months — within the developmental period relevant to autism.
Was the Varivax pre-licensure trial design adequate?
The Varivax trial gave 465 control-group children an injection of approximately 45 mg/mL neomycin — an antibiotic with documented neurotoxic and nephrotoxic properties — and labeled it "placebo" in the FDA package insert. Post-licensure surveillance systems (VAERS, VSD, V-SAFE) have not produced regulatory action against Varivax despite this inadequate pre-licensure trial design.
How serious is chickenpox in healthy children?
Before the vaccine was introduced in 1995, chickenpox was typically a self-limiting illness in healthy children lasting 1-2 weeks. Approximately 4 million Americans contracted chickenpox annually with about 100-150 deaths per year — overwhelmingly in immunocompromised individuals and adults, not healthy children. Natural infection provided lifelong immunity and community circulation naturally boosted adult immunity against shingles.

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