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Background In 2007, a routine second dose of varicella vaccine was recommended in the United States for children aged 4 to 6 years to better control varicella-zoster virus circulation and outbreaks. Sentinel varicella outbreak surveillance was established to assess feasibility of surveillance and describe outbreaks that are occurring. Methods Through the Centers for Disease Control and Prevention Epidemiology Laboratory Capacity funding, health departments conducted active surveillance for varicella outbreaks in schools from 2012 to 2015. Outbreaks of varicella were defined as ≥5 cases in a school within at least 1 incubation period (21 days). School nurses, healthcare providers, or laboratories reported cases and outbreaks of varicella to health departments; demographic, vaccination, and clinical data were collected. Results Georgia, Houston, Maine, Minnesota, New York City, and Philadelphia participated in all 3 years; Puerto Rico and West Virginia participated in 2012 to 2013; and Kansas and Arkansas participated in 2014 to 2015. Twenty-nine outbreaks including 262 cases were reported. The median size of the outbreaks was 7 cases (range, 5–31 cases), and the median duration was 31 days (range, 4–100 days). Of the case-patients associated with larger outbreaks (≥8 cases), 55.4% were unvaccinated, and 15.7% and 18.1% had received 1 or 2 doses of vaccine, respectively. In small outbreaks (5–7 cases), 33.3% of case-patients were unvaccinated, and 16.7% and 38.5% had received 1 or 2 doses of vaccine, respectively. Conclusions The majority of cases associated with outbreaks occurred in undervaccinated children (unvaccinated and 1-dose vaccine recipients). Outbreaks with a greater proportion of 2-dose vaccine recipients were smaller. Varicella outbreak surveillance is feasible, and continued monitoring of outbreaks remains important for describing the epidemiology of varicella during the 2-dose varicella vaccination program. [ABSTRACT FROM AUTHOR]