Yale Bulletin and Calendar

April 6, 2001

Volume 29, Number 25

Study verifies effectiveness
of vaccine for chickenpox

The vaccine for chickenpox, approved by the U.S. Food and Drug Administration in 1995, is highly effective in preventing chickenpox or minimizing the severity of the disease in children, according to a study by researchers at Yale.

"We know from these data that the vaccine, as used in clinical practice, is 85% effective in preventing varicella infection and 97% effective in preventing moderately severe and severe disease," says Marietta Vazquez, a postdoctoral fellow in infectious diseases and pediatrics at the School of Medicine and principal investigator of the study, published in the March 29 issue of The New England Journal of Medicine. "The study showed that in those cases in which vaccinated children go on and develop the disease, their disease is very mild."

The vaccine was developed in Japan in the 1970s and is recommended for persons 12 months of age or older who are susceptible to chickenpox. Vazquez says that after licensure, there were uncertainties regarding how the vaccine's efficacy in clinical practice would differ from the efficacy found in early clinical trials.

"There are certain questions that practitioners have for all new vaccines -- how well it works, how effective it is in different age groups and how effective will it be over time," she says. "There were long-term studies done in Japan looking at protective antibodies over 20 years, but the vaccine is not widely used in Japan so there is a natural boosting effect. Exposure to chicken pox tends to boost immunity against chickenpox."

To help answer these questions, Vazquez and her colleagues conducted a case control study with two controls per child with chickenpox. Children with potential cases of chickenpox were identified by active surveillance of pediatric practices in the New Haven area.

From March 1997 through November 2000, 243 children were identified who had positive tests for varicella-zoster virus, or chickenpox. Of the 56 vaccinated children with chickenpox, 86% had mild disease while only 48% of the 187 unvaccinated children with chickenpox had mild disease.

Complete data was available for 202 of the 243 children who had chicken pox and their matched controls. Of these, 23% of the children with chickenpox and 61% of the matched controls had received the vaccine, for an effectiveness rate of 85%. In addition, the vaccine was 97% effective against moderately severe and severe disease.

"These results indicate that the effectiveness of the vaccine as it is used in actual practice is excellent, at least in the short term," Vazquez says. "Virtually all the vaccinated children in whom chickenpox subsequently developed had very mild disease. We conclude that, thus far, the varicella vaccine, as it is used in clinical practice, is highly effective."

She says it may be too soon to assess the duration of vaccine induced immunity to chickenpox. The virus is still circulating in the population, so boosting of vaccine induced immunity may still occur commonly. However, as the incidence of chickenpox declines, such natural boosting of immunity will become increasingly rare.

"Because ours is an ongoing study, we expect that, over time, we will be able to detect any waning of immunity that occurs," Vazquez says.

-- By Jacqueline Weaver

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