title: Incremental effectiveness of second dose varicella vaccination for outbreak control at an elementary school in Philadelphia, pennsylvania, 2006.
abstract: 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.
authors.affiliation: Epidemic Intelligence Service, Centers for Disease Control and Prevention, Atlanta, GA, USA.
authors.forename: Michael D
authors.initials: MD
authors.lastname: Nguyen
authors.forename: Dana
authors.initials: D
authors.lastname: Perella
authors.forename: Barbara
authors.initials: B
authors.lastname: Watson
authors.forename: Mona
authors.initials: M
authors.lastname: Marin
authors.forename: Mia
authors.initials: M
authors.lastname: Renwick
authors.forename: C Victor
authors.initials: CV
authors.lastname: Spain
mesh_terms: D002644:Chickenpox / Q000453:epidemiology* / Q000517:prevention & control*; D019433:Chickenpox Vaccine / Q000008:administration & dosage* / Q000276:immunology; D002648:Child; D002675:Child, Preschool; D004196:Disease Outbreaks; D005260:Female; D006801:Humans; D007117:Immunization, Secondary / Q000379:methods*; D015994:Incidence; D008297:Male; D015143:Philadelphia / Q000453:epidemiology; D012574:Schools; D011795:Surveys and Questionnaires; D016896:Treatment Outcome
chemical_list: D019433:Chickenpox Vaccine