Effects of One-dose Varicella Vaccination on Disease Severity in Children during Outbreaks in Seoul, Korea

BACKGROUND: In Korea, the incidence of varicella has increased despite the introduction of a universal one-dose vaccination for children aged 12–15 months in 2005. A previous study demonstrated that the vaccine effectiveness was insufficient to prevent against varicella. We assessed the effect of the varicella vaccination on disease severity. METHODS: Epidemiologic investigation of varicella cases in Seoul metropolitan area from 2015 to 2017 were used. Varicella-related symptoms such as rash were determined by the clinical practitioners. Disease severity of patients was assessed by the number of skin lesions and divided into mild (≤ 50) and moderate (51–249) to severe (≥ 250). Unconditional logistic regression analysis was performed and age was controlled. RESULTS: Among a total of 1,008 varicella cases reported, 869 cases were breakthrough cases and 139 were unvaccinated cases. The risk for occurrence of moderate-to-severe disease in the breakthrough group was 0.57 times less than that of the unvaccinated group. CONCLUSION: These data suggest that national varicella vaccination may have a significant effect on attenuation of disease severity in children.