Children's Vaccines Health Center

Chicken Pox Vaccine Smarter Than You Think

March 28, 2000 (Cleveland) -- Although up to 30% of children vaccinated with the chicken pox vaccine could have a recurrence of chicken pox, parents should not be concerned. According to a study in the April issue of the journal Nature Medicine, the virus's reactivation only produces mild symptoms and may actually be beneficial by producing a boost in immunity.

The vaccine has been available in the U.S. since 1995, and is a live, weakened strain of varicella-zoster virus (VZV), which causes chicken pox. The vaccine has no serious side effects, and has been recommended universally for everyone over 1 year by the American Academy of Pediatrics Committee on Infectious Diseases and the CDC.

Philip R. Krause and Dennis M. Klinman, of the FDA's Center for Biologics Evaluation and Research, analyzed the antibody levels and infection rates in more than 4,600 children immunized with the vaccine over a four-year period. They wanted to determine whether the vaccine would continue to produce long-term protection against chicken pox.

In the study, chicken pox infection rates in vaccinated children were low, at 2.7% per year, compared with 13.0% per year in children who were not vaccinated. The researchers also found that the reactivation of the chicken pox virus in vaccinated children did occur. But this reactivation of the disease was mild and seemed to offer a boost in immunity. When levels of antibodies that fight off the virus were low, the weakened vaccine virus already in the body caused infection. The vaccination seemed to have a 'sense' of when a boost in immunity was necessary.

"At the point where you might become susceptible to infection if you bumped into it in the community, your own virus reactivates. But since it's a [weakened] virus, it doesn't seem to make the children so ill," co-author Klinman tells WebMD.

So vaccination is still recommended, and may benefit children even more than initially believed. "[Varicella vaccination] is safe and highly effective, and should be used universally in children," says John Mills, MD. "The fact that it may reactivate to boost immunity, but only when immunity is low, is mostly a virtue," he tells WebMD. In his accompanying editorial piece, Mills says that this capacity to induce latent infection and become reactivated are most likely positive, rather than negative, "if they boost and maintain lifelong immunity and the reactivation events are clinically inconsequential."

The hazards of disease reactivation are twofold, says Mills, who is a professor of medicine at the University of California, San Francisco and professor of microbiology at the University of Melbourne and Monash University, both in Melbourne, Australia. The hazards include illness in the child who was vaccinated and the potential to spread the disease to others.

However, these are of minimal concern, Mills adds. "[Further] spread to others is only of possible concern if the person exposed has severe immune deficiency," he says.