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A 14-year study has confirmed that varicella vaccine is highly effective for preventing chicken pox and that it reduces the risk of herpes zoster in vaccinated children.
Researchers followed 7,585 children in the Kaiser Permanente Northern California health care system who were vaccinated with varicella vaccine when they were aged 12 through 23 months between June and November 1995 and evaluated them for varicella and herpes zoster through 2009. A total of 2,826 children received a second dose between 2006, when the Advisory Committee on Immunization Practices recommended a second dose of vaccine, and November 2009, when the study ended.
Incidence of breakthrough varicella in study participants was measured every 6 months and annual rates were estimated. In the 14 study years, 1,505 breakthrough cases of varicella were reported after vaccination. The average incidence of varicella in this cohort was 15.9 per 1,000 person-years, a rate 9-fold to 10-fold lower when compared with historical rates in children of the same age in the prevaccine era.
Vaccine effectiveness at the end of the study was 90% without any evidence of waning over time. Cases of varicella that did occur were mild and appeared early after vaccination. No children developed varicella after the second dose of vaccine.
Herpes zoster cases also were mild, and rates were lower among vaccinated children compared with unvaccinated children during the prevaccine period.
Researchers say their study demonstrates the lasting effectiveness of varicella vaccine and the benefit of the second dose when children are vaccinated according to the schedules recommended by the Centers for Disease Control and Prevention.