According to background information in the article, chickenpox (varicella) vaccine is recommended for routine administration to healthy children at 12 to 18 months of age and to older children who have not yet had chickenpox. Recent reports of outbreaks of chickenpox in groups with substantial rates of immunization have increased concern about the effectiveness of the current recommendations for administration of the vaccine.

Marietta Vazquez, M.D., of the Yale University School of Medicine, New Haven, Conn., and colleagues examined the influence of age at the time of chickenpox vaccination and the time since vaccination on the vaccine's effectiveness. The study, conducted from March 1997 through June 2003, included 339 children 13 months or older who were clinically diagnosed as having chickenpox. For each case subject, two controls were selected, matched by both age and pediatric practice.

The researchers found that although the overall effectiveness of the vaccine was 87 percent, there was a substantial difference in the vaccine's effectiveness in the first year after vaccination (97 percent) and in years 2 to 8 after vaccination (84 percent). "The vaccine's effectiveness in year 1 was substantially lower if the vaccine was administered at younger than 15 months (73 percent) than if it was administered at 15 months or older (99 percent), although the difference in effectiveness overall for children immunized at younger than 15 months was not statistically significantly different than for those immunized at 15 months or older (81 percent vs. 88 percent). Most cases of chickenpox in vaccinees were mild," the authors write.
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Contact: Karen Peart
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JAMA and Archives Journals
17-Feb-2004