Varivax is the brand-name vaccine that protects against chickenpox (varicella). Manufactured by Merck Sharp & Dohme and licensed by the FDA, it contains a live but weakened form of the varicella-zoster virus. Two doses are recommended for children, adolescents, and adults who have never had chickenpox or been vaccinated.
How Varivax Works
Because Varivax uses a live attenuated virus, it mimics a natural infection without causing full-blown disease. Once injected, the weakened virus prompts your immune system to mount two lines of defense. First, your body produces antibodies (IgG) that circulate in the blood and can neutralize the virus on future contact. Second, it activates specialized immune cells, both helper and killer T-cells, that remember the virus and respond quickly if you’re ever exposed again. This combination of antibody and cell-based immunity is what makes the vaccine highly effective over the long term.
Recommended Schedule
For children, the standard schedule is two doses: the first between 12 and 15 months of age, and the second between ages 4 and 6. At least three months should separate the two doses in children under 13.
Adolescents and adults aged 13 and older who lack evidence of immunity need two doses spaced 4 to 8 weeks apart. The current CDC immunization schedule also recommends two doses for any adult born in 1980 or later who hasn’t been vaccinated or had chickenpox. If you were born before 1980, you’re generally presumed to have been exposed, though certain groups (healthcare workers, for example) may still need documentation of immunity.
How Effective It Is
A single dose of Varivax is 82% effective at preventing any form of chickenpox and nearly 100% effective at preventing severe cases. Two doses raise protection significantly: in pre-licensure clinical trials, two doses were 98% effective against any chickenpox and 100% effective against severe disease. Real-world studies after the vaccine was widely adopted show two-dose effectiveness around 92%, with estimates ranging from 88% to 98% depending on the study.
The takeaway is clear: one dose offers strong protection, but two doses close nearly all of the remaining gap.
Breakthrough Chickenpox
A small number of vaccinated people do catch chickenpox, known as breakthrough varicella. It typically looks quite different from the disease in unvaccinated individuals. Most people with breakthrough infection have little or no fever, develop fewer than 50 skin lesions, and recover faster. The rash tends to appear as flat spots rather than the fluid-filled blisters associated with classic chickenpox.
That said, about 25% to 30% of people vaccinated with only one dose who get breakthrough chickenpox will have symptoms similar to an unvaccinated person. Two-dose recipients tend to have even milder cases, though data on this group are still limited. Because breakthrough cases can look so mild, they’re sometimes hard to diagnose based on appearance alone.
Common Side Effects
Varivax side effects are generally mild and short-lived. In clinical studies of children aged 12 months through 12 years:
- Injection-site reactions: About 1 in 5 children experienced soreness, swelling, or redness within three days of the first dose. After the second dose, roughly 1 in 4 had these reactions.
- Fever: Around 7 in 100 children developed a fever after the first dose, dropping to 4 in 100 after the second dose.
- Mild rash: About 3 in 100 children developed a chickenpox-like rash after the first dose, and 1 in 100 after the second dose.
The rash that occasionally follows vaccination is usually just a handful of spots and resolves on its own. Serious reactions are rare.
Who Should Not Get Varivax
Because Varivax is a live vaccine, certain groups should avoid it. Women who are pregnant or may be pregnant should not receive it. People with significant immune deficiency also need to be cautious. While the single-antigen Varivax vaccine can be given to some HIV-positive individuals depending on their immune status, the combination MMRV vaccine (ProQuad, which bundles measles, mumps, rubella, and varicella together) should not be used in people with HIV or other conditions that compromise immunity.
Anyone with a history of severe allergic reaction to a previous dose or to a component of the vaccine, such as gelatin or the antibiotic neomycin, should also avoid it.
Varivax vs. MMRV (ProQuad)
You may see varicella vaccine offered as either Varivax on its own or as part of the combination MMRV vaccine, ProQuad. Both contain the same chickenpox component. The difference is that ProQuad rolls four vaccines into one shot, which means fewer injections for children. However, ProQuad carries a slightly higher risk of fever and febrile seizures when given as the first dose at 12 to 15 months, so your child’s provider may recommend separate injections at that age. For the second dose at age 4 to 6, either option works equally well.
Who Still Needs It as an Adult
Many adults assume childhood chickenpox made them immune, and for most born before 1980 that’s likely true. But if you were born in 1980 or later and never had chickenpox or received the vaccine, the CDC recommends getting two doses. This is especially important for healthcare workers, teachers, college students, international travelers, and anyone living with someone who has a weakened immune system. Adults who catch chickenpox tend to get sicker than children, with higher rates of pneumonia and hospitalization, so vaccination is well worth it if you’re unsure of your immunity.