IN THIS issue of the ARCHIVES, Clements and colleagues1 report on their 5-year cohort study of varicella among children from 11 day care centers in North Carolina. The observation period began shortly prior to licensure of the varicella vaccine in 1995; thus, it was possible to compare the rate of disease in the study population prior to and after initiation of the immunization program, as well as to monitor the increase in the uptake of the vaccine over time. During the study, the incidence of varicella was carefully monitored by both a passive reporting system for parents and day care center personnel and active surveillance conducted by the research team. Based on the data collected, it was estimated that varicella vaccine coverage in the day care centers rose from 4% in early 1995 to 63% by the end of 1999. As might be expected, the rate of varicella disease in immunized children was low—ranging from 5.35 cases per 1000 person-months before July 1996, to 1.01 cases per 1000 person-months in late 1999. However, concurrent with the increased uptake of the vaccine in the day care centers, the rate of disease in unvaccinated children also fell dramatically. Among those not vaccinated, the rate of varicella was 16.74 per 1000 person-months prior to July 1996; by the end of 1999 this rate had decreased to 1.53 per 1000 person-months. The authors conclude that this decrease in varicella in the unvaccinated children is a demonstration of herd immunity and is a welcome effect of the vaccine.
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