Varicella outbreak at a nursery school under routine immunization

Background and objective In Japan, routine administration of two-dose immunization for varicella to one-year-old children was introduced in October, 2014. The objective of this study was measurement of the effectiveness of routine two-dose immunization for varicella to onset and assessment of severity in a nursery school setting. Method The study period extended from the beginning of April, 2017 through March, 2018. The study area was Nursery school B in a city A. Subjects were 120 children in all. We analyzed vaccine efficacy (VE) as an observational study and assessed severity using Fisher’s exact test. We also assessed VE for severity using linear regression. Severity was defined as the length of school absence attributable to varicella infection. Results For one dose or more, VE was 48.1% for all ages and 49.2% among children three years old and older. No significant VE was found. Vaccination using one dose or more can reduce severity significantly. Discussion and conclusion Low VE was found in a nursery school setting, although results were not significant. VE for severity was confirmed, but a second dose might not reduce severity.