METHODS: Through CDC’s Epidemiology Laboratory Capacity funding, health departments conducted active surveillance for varicella outbreaks in elementary, middle, and high schools during the academic years of 2012-2013, 2013-2014, and 2014-2015. Health departments were required to include at least 300 schools under surveillance. Outbreaks of varicella were defined as ≥5 cases in a school within at least one incubation period (21 days). Cases and outbreaks of varicella were reported to health departments by school nurses, healthcare providers, or laboratories; demographic, vaccination, and clinical data were collected. We conducted descriptive analyses to examine outbreaks by academic year and case characteristics.
RESULTS: Georgia, Houston, Maine, Minnesota, New York City, and Philadelphia participated all 3 school years, Puerto Rico and West Virginia participated during 2012-2013, and Arkansas and Kansas participated during 2014-2015. A total of 29 outbreaks (with 262 cases) were reported: 14 in 2012-2013 among 3751 schools under surveillance (0.4%), 1 in 2013-2014 among 2755 schools (0.04%), and 14 in 2014-2015 among 6026 schools (0.2%); p=0.6. No outbreaks were reported from Houston and Puerto Rico. Median size and duration of outbreaks did not significantly differ over time (p>0.2) and were 8 cases (range, 5-31 cases) and 31 days duration (range, 4-100 days), respectively. Among case-patients associated with the 13 larger outbreaks, defined as being greater than or equal to the median outbreak size (i.e. ≥8 cases), 55% were unvaccinated, 16% were 1-dose and 18% were 2-dose vaccine recipients. In contrast, 33% of case-patients in 16 outbreaks with 5-7 cases were unvaccinated, 17% were 1-dose and 41% were 2–dose vaccine recipients. Of 257 outbreak cases with data on age, 143 (56%) were 5-9 and 67 (26%) were 10-14 years of age. Data on number of lesions were available for 204 cases, of which 100 (49%) were mild (<50 lesions) and 80 (39%) were mild/moderate (50-249 lesions).
CONCLUSIONS: The majority of cases associated with outbreaks occurred in under-vaccinated children (unvaccinated and 1-dose vaccine recipients). Although outbreaks included cases among 2-dose vaccine recipients, those with a greater proportion of 2-dose vaccine recipients were smaller in size. Full implementation of the 2-dose program could lead to further control of varicella outbreaks.