From the March 15, 2007 issue of the New England Journal of Medicine.

Loss of Vaccine-Induced Immunity to Varicella over Time
Sandra S. Chaves, M.D., M.Sc., Paul Gargiullo, Ph.D., John X. Zhang, Ph.D.,
Rachel Civen, M.D., Dalya Guris, M.D., M.P.H., Laurene Mascola, M.D.,
M.P.H., and Jane F. Seward, M.B., B.S., M.P.H.

Background The introduction of universal varicella vaccination in 1995 has
substantially reduced varicella-related morbidity and mortality in the
United States. However, it remains unclear whether vaccine-induced immunity
wanes over time, a condition that may result in increased susceptibility
later in life, when the risk of serious complications may be greater than in
childhood.

Methods We examined 10 years (1995 to 2004) of active surveillance data from
a sentinel population of 350,000 subjects to determine whether the severity
and incidence of breakthrough varicella (with an onset of rash 42 days
after vaccination) increased with the time since vaccination. We used
multivariate logistic regression to adjust for the year of disease onset
(calendar year) and the subject's age at both disease onset and vaccination.

Results A total of 11,356 subjects were reported to have varicella during
the surveillance period, of whom 1080 (9.5%) had breakthrough disease.
Children between the ages of 8 and 12 years who had been vaccinated at least
5 years previously were significantly more likely to have moderate or severe
disease than were those who had been vaccinated less than 5 years previously
(risk ratio, 2.6; 95% confidence interval [CI], 1.2 to 5.8). The annual rate
of breakthrough varicella significantly increased with the time since
vaccination, from 1.6 cases per 1000 person-years (95% CI, 1.2 to 2.0)
within 1 year after vaccination to 9.0 per 1000 person-years (95% CI, 6.9 to
11.7) at 5 years and 58.2 per 1000 person-years (95% CI, 36.0 to 94.0) at 9
years.

Conclusions A second dose of varicella vaccine, now recommended for all
children, could improve protection from both primary vaccine failure and
waning vaccine-induced immunity.