BACKGROUND: During 2012-2015 10 sentinel jurisdictions conducted surveillance for varicella outbreaks in school settings to assess feasibility of outbreak surveillance and monitor impact of the 2-dose varicella vaccination recommendation for children implemented in 2007. Varicella outbreak surveillance was determined to be feasible and expanded nationwide to increase robustness of findings.
METHODS: Starting in August 2015, through CDC’s Epidemiology Laboratory Capacity funding, participating jurisdictions conducted surveillance for clusters and outbreaks of varicella; clusters were defined as 3-4 varicella cases and outbreaks as ≥5 varicella cases in a defined setting within at least one incubation period (21 days). Health department staff collected case-based data on: outbreak setting, age, vaccination status, number of varicella vaccine doses, number of lesions, laboratory testing, and whether cases resulted in hospitalizations and/or had complications. Characteristics of cases associated with clusters and outbreaks did not differ; therefore, we present descriptive analyses of combined data.
RESULTS: During 8/1/2015-6/30/2016, 48 jurisdictions reported 124 clusters (416 cases) and 77 outbreaks (892 cases). Median size and duration of outbreaks was 7 cases (range, 5-212 cases) and 24 days (0-119 days), respectively. Twenty-eight percent (57) of clusters/outbreaks occurred in schools and 16% (33) in daycares. Of cluster- or outbreak-associated cases, 64% (818/1,287) were among children aged 1-9 years; the majority were in unvaccinated patients (71%; 866/1,223). Among vaccinated cases with information on number of doses received, 57% (188/327) were in one dose varicella vaccine recipients. Among 1,018 cases with information on number of lesions, 56% (570) had ≥50 lesions, of whom 78% were unvaccinated. For vaccinated patients, of the 160 cases with 1 dose of varicella vaccine, 66% (105) had <50 lesions while of the 111 cases with 2 doses of varicella vaccine, 72% (80) had <50 lesions (p=0.26). Four patients were hospitalized, all unvaccinated; an additional 13 had complications (1 vaccinated with 2 doses, 3 vaccinated with 1 dose). Only 7% (77) of cases were laboratory confirmed as varicella.
CONCLUSIONS: The first year of nationwide varicella outbreak surveillance provided data informing that clusters and outbreaks continue to occur most often in school-aged children. A majority of cases were unvaccinated. Vaccinated patients were more likely to have mild disease (<50 lesions) with few complications and no hospitalizations; number of lesions was not significantly different among 1- vs 2-dose vaccinees. Continued varicella outbreak surveillance is important to monitor trends in characteristics of varicella outbreaks and understand varicella epidemiology during the 2-dose varicella vaccination era.