Vaccine-era varicella epidemiology and vaccine effectiveness in a public elementary school population, 2002-2007.

OBJECTIVE: We conducted ongoing varicella surveillance of public elementary school students to track changes in incidence from 2002 to 2007. In school year 2002-2003, we also conducted a retrospective cohort study to measure varicella vaccine effectiveness, assess relationships between risk factors for varicella in vaccine recipients (breakthrough varicella) identified in earlier studies, and assess the ability of school nurse surveillance to detect varicella cases.

PATIENTS AND METHODS: Varicella was defined as acute illness with maculo-papulo-vesicular rash without another apparent cause persisting >24 hours, regardless of previous episodes of the same or a similar illness. Using case data reported by school nurses, we described breakthrough varicella rates (percentage of vaccinated students with varicella), annual varicella-incidence rates (varicella cases per 100 public elementary school students), vaccine effectiveness, risk factors for breakthrough varicella, clinical characteristics of vaccinated and susceptible varicella patients, and sensitivity and positive predictive value of school nurse surveillance.

RESULTS: During school years 2002-2007, 502 elementary school students met the varicella case definition. Breakthrough varicella rates among exposed students ranged from 6% to 8% per school year; annual incidence rates ranged from 0.2% to 0.3% of public elementary school students; and varicella was more severe and lasted longer in susceptible than in vaccinated students. The positive predictive value of school nurse surveillance was 94%, and sensitivity was 90%. Vaccine effectiveness was 81%.

CONCLUSIONS: School nurse surveillance has both high positive predictive value and sensitivity and is a useful means of tracking varicella occurrence. Annual incidence rates of varicella are low. Vaccine effectiveness and breakthrough varicella rates are comparable to findings of other studies.