Effectiveness of second-dose varicella vaccination as post-exposure prophylaxis: a prospective cohort study

Although administration of a second dose of varicella vaccine (2^nd-dose VarV) to individuals who have previously received one-dose VarV has been recommended as a post-exposure prophylaxis (PEP) strategy for outbreak control, the effectiveness of this strategy remains unclear. We evaluated the vaccine effectiveness (VE) of 2^nd-dose VarV as PEP among students involved in 129 varicella outbreaks in Shanghai, China from 2013 to 2016.

Students who had received one-dose VarV more than 5 years prior to varicella exposure were eligible to receive 2^nd-dose VarV as PEP. We evaluated the VE using the following formula: VE = (1 - hazard ratio (HR)) × 100%.

A total of 6762 students were eligible for 2^nd-dose VarV, of whom 58.6% accepted PEP after varicella exposure. The adjusted VE of 2^nd-dose VarV as PEP was 77% (95% confidence interval 64-85%). In addition, the adjusted VE of 2^nd-dose VarV as PEP in affected classrooms with high vaccine uptake was higher than that in classrooms with lower vaccine uptake (87% vs. 69%). The adjusted VE was also higher in students who received 2^nd-dose VarV within 3 days of exposure than those who received it more than 3 days post exposure (77% vs. 64%).

These Results suggest that administration of 2^nd-dose VarV as PEP is an appropriate intervention for outbreak control in countries where two-dose VarV has not been adopted.