Incremental effectiveness of second dose varicella vaccination for outbreak control at an elementary school in Philadelphia, pennsylvania, 2006.

Background: In 2006, the Philadelphia Department of Public Health conducted an investigation of a varicella outbreak at an elementary school in which second-dose vaccination for outbreak control (VOC) was implemented. We evaluated the effectiveness of this intervention. Methods: Self-administered questionnaires collected varicella disease and vaccination information. Students eligible for second-dose VOC were 1-dose vaccine recipients without prior varicella disease. A breakthrough varicella case was defined as a maculopapulovesicular rash in a student with onset >42 days after 1-dose vaccination without other apparent cause. Vaccine effectiveness was evaluated using survival analysis techniques and analyzed by vaccine status (first dose versus second dose). Multivariable Cox proportional hazard models were used to identify statistical interactions and adjust for confounders. Results: The questionnaire response rate was 92% (342/370). Of the 286 eligible students, 187 (65%) received a second-dose VOC. The crude attack rate was 9/187 (5%) among second-dose VOC recipients; 43/99 (43%) among 1-dose recipients, and 5/6 (83%) among unvaccinated students. Second-dose VOC recipients had milder rashes, compared with 1-dose or unvaccinated students. The adjusted incremental second-dose vaccine effectiveness was 76% (95% confidence interval: 44%―90%) for students with classroom exposure. Incremental effectiveness was similar (79%) when we extended the immune response time from 4 days to 7 days after second-dose VOC. Conclusions: Second-dose VOC resulted in a substantial reduction in varicella incidence for students with classroom exposure. Until high rates of routine second-dose vaccine coverage are achieved, clinicians should consider second-dose VOC an appropriate intervention to reduce disease transmission in institution-based outbreaks.