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New vaccine for chickenpox shown to be highly effective in study by Yale researchers

Yale University : 28 March, 2001  (New Product)
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 percent effective in preventing varicella infection and 97 percent effective in preventing moderately severe and severe disease,' said Marietta Vazquez, a postdoctoral fellow in infectious diseases and pediatrics at Yale 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 said 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 said. '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, data collection was completed for 330 potential cases, of which 243 were in children who had positive tests for varicella-zoster virus, or chickenpox. Of the 56 vaccinated children with chickenpox, 86 percent had mild disease while only 48 percent of the 187 unvaccinated children with chickenpox had mild disease.

Among the 202 children with chickenpox and their 389 matched controls, 23 percent of the children with chickenpox and 61 percent of the matched controls had received the vaccine, for an effectiveness rate of 85 percent. In addition, the vaccine was 97 percent 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 said. '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 said 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.
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