Partial uptake of varicella vaccine and the epidemiological effect on varicella disease in 11 day-care centers in North Carolina

Background: The increasing use of varicella vaccine in children attending day care has rapidly decreased the incidence of wild-type varicella disease. The herd immunity noted is significant and will have an effect on the epidemiology of natural varicella. Objective: To monitor the change in varicella incidence in day-care attendees after the licensure of varicella vaccine. Design: A prospective observational cohort study design. Setting: Eleven private day-care centers and preschools in North Carolina participated in the study from January 1, 1995, through December 31, 1999. Participants: All children in the 11 centers were eligible for participation. Some participated more actively, supplying information on a regular basis. Others participated passively. Day-care personnel provided information about all cases of varicella. Interventions: None. Main Outcome Variables: The change in the incidence of varicella disease was documented as the use of varicella vaccine increased. Results: Varicella vaccine coverage increased substantially from 4.4% in 1995 to 63.1% in December 1999. The vaccination rate accelerated dramatically in 1996 and 1997, leveled off in 1998, and rose again in 1999. Cumulative varicella incidence decreased from 16.74 cases per 1000 person-months in July 1996 to 1.53 cases per 1000 person-months in December 1999 in unvaccinated children. Conclusions: The varicella vaccination rate continued to increase slowly in the day-care population after an initial rapid uptake. The decrease in varicella disease is greater than the increase in varicella vaccination. This herd effect is welcome and even apparent in the unvaccinated children younger than 1 year.