Childhood vaccination against varicella: Persistence of antibody, durationof protection, and vaccine efficacy

Objective: To document the duration of protection afforded by Oka/Merck var icella vaccine over a 7-year period. Study design: The subjects were healthy children 1 to 12 years of age origi nally enrolled in clinical studies to evaluate the primary immune response to varicella vaccine 6 weeks after vaccination. Each was monitored for anti body persistence, breakthrough infection, and household exposure to varicel la to produce estimates of vaccine efficacy. Results: The 6-year cumulative varicella antibody persistence rate was 99.5 % (95% CI: 98.9%, 100.0%). The annual breakthrough rate through 7 years ran ged from 0.2% to 2.3% per year; the estimated cumulative event rate,vas 6.5 %. Comparison of the observed average annual breakthrough rate with the age -adjusted expected annual incidence rate of varicella in unvaccinated child ren corresponded to an estimated vaccine efficacy of 93.8% to 94.6%. Eighty vaccinated children were exposed to varicella in the household, resulting in 8 (10%) cases of infection. When compared with the historical attack rat e of 86.8% in unvaccinated susceptible persons exposed to varicella in the household, this yields an estimated vaccine efficacy of 88.5% (95% CI: 80.9 %, 96.1%). Varicella cases in vaccinated children generally were mild. Conclusion: The live attenuated varicella vaccine is highly effective in in ducing persistent immunity and long-term protection against breakthrough va ricella infection.