The effectiveness of the varicella vaccine in clinical practice.

Background A live attenuated varicella vaccine was approved for use in the United States in March 1995 and is recommended for all susceptible persons 12 months of age or older. Methods To assess the effectiveness of the varicella vaccine, we conducted a case–control study with two controls per child with chickenpox, matched according to both age and pediatric practice. Children with potential cases of chickenpox were identified by active surveillance of pediatric practices in the New Haven, Connecticut, area. Research assistants visited the children on day 3, 4, or 5 of the illness, assessed the severity of the illness, and collected samples from lesions to test for varicella–zoster virus by the polymerase chain reaction (PCR). Results From March 1997 through November 2000, data collection was completed for 330 potential cases, of which 243 (74 percent) were in children who had positive PCR tests for varicella–zoster virus. Of the 56 vaccinated children with chickenpox, 86 percent had mild disease, wherea...