The study, published in the February 18, 2004 issue of the Journal of the American Medical Association, also suggests that the vaccine might be less effective in the first year after vaccination if it is administered to children less than 15 months of age. Vazquez said this difference in effectiveness disappears after the first year and overall is not significant.
The ongoing study conducted over the past seven years addresses concerns about varicella outbreaks in highly- immunized groups that have raised controversy about the effectiveness of the varicella vaccine. The authors assessed whether the effectiveness of varicella vaccine is affected either by time since vaccination or age at the time of vaccination. They studied 339 children ages 13 months or older who were clinically diagnosed with chicken pox after they had been vaccinated with varicella. Two controls were selected for each study participant, matched by age and pediatric practice.
The researchers found the significant decrease in effectiveness one year after vaccination, but most cases of breakthrough disease are mild.
"The vaccine's effectiveness against moderate or severe disease is excellent throughout the period of the study," says Vazquez.
Vazquez and her team stress that it will be important to continue monitoring effectiveness of the vaccine since boosts to immunity from exposure to varicella will become increasingly rare as the incidence of varicella diminishes.