Effectiveness of varicella vaccine as post-exposure prophylaxis during a varicella outbreak in Shanghai, China.

OBJECTIVES: Varicella vaccine (VarV) is recommended as effective post-exposure prophylaxis (PEP) within 3-5days to control outbreaks. However, the effectiveness of PEP at >5days after exposure and the administration of a second dose to those with a history of one dose prior to exposure have not been fully examined. This study evaluated the vaccine effectiveness (VE) of PEP in preventing disease during a varicella outbreak in Shanghai, China in 2013.

METHODS: Self-administered questionnaires were used to obtain the students' varicella history, vaccination status, and willingness to receive PEP. One dose of VarV was provided free of charge to eligible students. The VE of PEP was calculated as [1-relative risk (RR)]×100%. Analyses were restricted to grade 8 students, as no students from the other grades or teachers developed varicella during the outbreak.

RESULTS: Twenty-seven varicella cases were identified, 16 (59%) of which were infected after the PEP campaign. Sixty-five students received one dose of VarV on day 13 or 19 after the index case. Attack rates were 28% (9/32), 16% (15/94), 0% (0/10), and 6% (3/55) among unvaccinated, one-dose Pre-PEP, first dose as PEP, and second dose as PEP recipients, respectively. Cases among second dose as PEP recipients tended to have less fever compared with unvaccinated or one-dose Pre-PEP recipients. Compared with unvaccinated students, the VE of first dose as PEP recipients was 100% and of the second dose as PEP recipients was 60% (95% confidence interval -72% to 91%).

CONCLUSIONS: Post-exposure vaccination should be given as soon as possible after exposure. Nevertheless, vaccination is still recommended even at more than 5days post-exposure to control varicella outbreaks.