Effectiveness over time of varicella vaccine.

CONTEXT Reports of outbreaks of varicella in highly immunized groups have increased concern about the effectiveness of varicella vaccine. OBJECTIVE To assess whether the effectiveness of varicella vaccine is affected either by time since vaccination or by age at the time of vaccination. DESIGN Case-control study conducted from March 1997 through June 2003. SETTING Twenty different group practices in southern Connecticut. PARTICIPANTS Case subjects, identified by active surveillance of all practices, consisted of 339 eligible children 13 months or older who were clinically diagnosed as having chickenpox and who also had a polymerase chain reaction (PCR) test result that was positive for varicella-zoster virus DNA. For each case subject, 2 controls were selected, matched by both age and pediatric practice. MAIN OUTCOME MEASURES The effectiveness of the vaccine, especially the effects of time since vaccination and age at the time of vaccination, adjusted for possible confounders. RESULTS Although the adjusted overall effectiveness of the vaccine was 87% (95% confidence interval, 81%-91%; P<.001), there was a substantial difference in the vaccine's effectiveness in the first year after vaccination (97%) and in years 2 to 8 after vaccination (84%, P =.003). The vaccine's effectiveness in year 1 was substantially lower if the vaccine was administered at younger than 15 months (73%) than if it was administered at 15 months or older (99%, P =.01), 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% vs 88%, P =.17). Most cases of chickenpox in vaccinees were mild. CONCLUSIONS Although varicella vaccine is effective, its effectiveness decreases significantly after 1 year, although most cases of breakthrough disease are mild. If administered at younger than 15 months, the vaccine's effectiveness was lower in the first year after vaccination, but the difference in effectiveness was not statistically significant for subsequent years.