By David Douglas
NEW YORK (Reuters Health) Apr 03 - Since its introduction in the mid-1990s, varicella vaccine has reduced the average incidence of chickenpox by as much as tenfold, according to researchers.
The vaccine was licensed in the US in 1995 for children at least 12 months old. In 2006, a second dose was recommended.
For a study reported Monday in Pediatrics, Dr. Roger Baxter of the Kaiser Permanente Vaccine Study Center, Oakland, California and colleagues tracked 7,585 children vaccinated in 1995 when they were in their second year of life. This group included 2826 youngsters who received a second dose between 2006 and 2009. All were members of the Kaiser Permanente Northern California health delivery system.
The study showed "the long-lasting effectiveness of varicella vaccine, and the benefit of the second dose," Dr. Baxter told Reuters Health by email.
More than 97% of the children enrolled completed the study. Overall, over 14 years, the average incidence of varicella was 15.9 per 1000 person-years, nine- to tenfold lower than in the prevaccine era.
Annual vaccine effectiveness ranged from 73% to 80% in the first two years of the study to 80% to 90% in the last 10 years. Effectiveness did not seem to wane.
Most cases of varicella were mild and occurred early after vaccination, at which time varicella virus zoster was still widely circulating. No child developed varicella after a second dose. In addition, herpes zoster cases were mild and rates were lower in the vaccine era than earlier (relative risk, 0.61).
The investigators concluded, "Varicella vaccine was effective at preventing chicken pox, and no evidence of waning protection was noted over a 14-year period. One dose provided excellent protection against moderate to severe disease, and most cases occurred shortly after the cohort was vaccinated."
SOURCE: http://bit.ly/17dteyU Pediatrics 2013.