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BACKGROUND: As varicella incidence has declined with implementation of the routine varicella vaccination program, national varicella surveillance has improved such that we can now use these data to describe the epidemiology of varicella-related hospitalizations during the two-dose vaccination era.
METHODS: We analyzed varicella hospitalization data collected by 3 states; California, Illinois and Michigan, during 2007–2010. The data were combined and only variables collected by all three states were included in the analysis. We defined disease severity by the number of lesions reported, with those having less than 50 lesions defined as mild disease and those having greater than 50 lesions as moderate/severe disease.
RESULTS: During 2007-2012, 499 varicella-related hospitalization were reported; 108 from California, 311 from Illinois and 80 from Michigan. Median age was 27 years; 67 (13.4%) of the cases were < 1 year of age, 63 (12.6%) 1–9 years, 74 (14.8%) 10–19 years and 295 (59.1%) ≥ 20 years. Of the 389 (78.0%) case with laboratory testing, 234 (60.1%) were laboratory confirmed. One-nineteen (34.5%) of the hospitalized cases were noted to have an immunocompromising condition. Of the 340 with data on vaccination history, 298 (87.60%) were either unvaccinated or had unknown vaccination status. Of the 42 (12.4%) who had received varicella vaccine, 35 (83.3%) had received one dose. Most (68.0%) of those who were unvaccinated or had unknown vaccination status were ≥20 years old. A higher proportion of persons who were vaccinated reported having mild disease as compared to those who were unvaccinated (85.7% vs. 24.7%; p<0.001). More vaccinated cases also had an immunocompromising condition compared to unvaccinated cases (70.6 vs. 26.6; p<0.001). There was no difference in complications and duration of hospitalization among those vaccinated compared to unvaccinated cases. Of the six deaths reported among varicella-related hospitalized cases, one case had received one dose of varicella vaccination; however there was no difference in outcome by vaccination history.
CONCLUSIONS: In the two-dose varicella vaccination era, varicella-related hospitalizations continue to be reported mainly among unvaccinated adults. Milder disease was reported among vaccinated hospitalized cases and immunocompromised conditions appear to be a risk factor for hospitalization among vaccinated persons. Thirty–two additional states have been identified to have varicella hospitalization data and we plan to include those data for future analyses when available. More complete implementation of the two dose varicella vaccination has the potential to further reduce the burden of severe varicella and varicella hospitalizations.