Varicella Immunity Wanes with Time Since Vaccination

Second dose may improve immunity

WEDNESDAY, March 14 (HealthDay News) -- The risk of breakthrough varicella increases significantly with increased time since vaccination against the virus, and a second dose may improve immunity, according to a report in the March 15 issue of the New England Journal of Medicine.

Sandra S. Chaves, M.D., from the U.S. Centers for Disease Control and Prevention in Atlanta, and colleagues examined the incidence and severity of breakthrough varicella (onset of rash more than 42 days after vaccination) in 350,000 subjects vaccinated with Varivax (a live attenuated varicella zoster vaccine made by Merck) from 1995 to 2004.

The researchers found that 11,356 subjects developed varicella, of whom 9.5 percent had breakthrough disease. Children aged 8 to 12 years who were vaccinated five years earlier or more had a significantly higher risk of moderate or severe disease (risk ratio 2.6). The annual rate of breakthrough varicella increased with time since vaccination, from 1.6 per 1,000 person-years at one year after vaccination to 58.2 per 1,000 person-years at nine years after vaccination.

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

After completion of this study, one author became an employee of Merck. Another author has also received compensation from Merck.

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