title: Challenges in confirming a varicella outbreak in the two-dose vaccine era.
abstract: BACKGROUND
  A second dose of varicella vaccine was recommended for U.S. children in 2006. We investigated a suspected varicella outbreak in School District X, Texas to determine 2-dose varicella vaccine effectiveness (VE).
  
  
  METHODS
  A varicella case was defined as an illness with maculopapulovesicular rash without other explanation with onset during April 1-June 10, 2011, in a School District X student. We conducted a retrospective cohort in the two schools with the majority of cases. Lesion, saliva, and environmental specimens were collected for varicella-zoster virus (VZV) PCR testing. VE was calculated using historic attack rates among unvaccinated.
  
  
  RESULTS
  In School District X, 82 varicella cases were reported, including 60 from Schools A and B. All cases were mild, with a median of 14 lesions. All 10 clinical specimens and 58 environmental samples tested negative for VZV. Two-dose varicella vaccination coverage was 66.4% in Schools A and B. Varicella VE in affected classrooms was 80.9% (95% CI: 67.2-88.9) among 1-dose vaccinees and 94.7% (95% CI: 89.2-97.4) among 2-dose vaccinees in School A, with a second dose incremental VE of 72.1% (95% CI: 39.0-87.3). Varicella VE among School B students did not differ significantly by dose (80.1% vs. 84.2% among 1-dose and 2-dose vaccinees, respectively).
  
  
  CONCLUSIONS
  Laboratory testing could not confirm varicella as the etiology of this outbreak; clinical and epidemiologic data suggests varicella as the likely cause. Better diagnostics are needed for diagnosis of varicella in vaccinated individuals so that appropriate outbreak control measures can be implemented.
authors.affiliation: National Center for Immunization and Respiratory Diseases, Centers for Disease Control and Prevention, Atlanta, GA 30333, USA. Amahamud@cdc.gov
authors.forename: Abdirahman
authors.initials: A
authors.lastname: Mahamud
authors.forename: Rachel
authors.initials: R
authors.lastname: Wiseman
authors.forename: Scott
authors.initials: S
authors.lastname: Grytdal
authors.forename: Candyce
authors.initials: C
authors.lastname: Basham
authors.forename: Jawaid
authors.initials: J
authors.lastname: Asghar
authors.forename: Thi
authors.initials: T
authors.lastname: Dang
authors.forename: Jessica
authors.initials: J
authors.lastname: Leung
authors.forename: Adriana
authors.initials: A
authors.lastname: Lopez
authors.forename: D Scott
authors.initials: DS
authors.lastname: Schmid
authors.forename: Stephanie R
authors.initials: SR
authors.lastname: Bialek
mesh_terms: D000293:Adolescent; D002644:Chickenpox / Q000175:diagnosis* / Q000453:epidemiology* / Q000473:pathology; D019433:Chickenpox Vaccine / Q000008:administration & dosage* / Q000276:immunology*; D002648:Child; D002675:Child, Preschool; D015510:Clinical Medicine / Q000379:methods; D004196:Disease Outbreaks; D004783:Environmental Microbiology; D005260:Female; D014645:Herpesvirus 3, Human / Q000302:isolation & purification; D006801:Humans; D008297:Male; D025202:Molecular Diagnostic Techniques / Q000379:methods; D016133:Polymerase Chain Reaction / Q000379:methods; D012189:Retrospective Studies; D012463:Saliva / Q000821:virology; D012867:Skin / Q000821:virology; D013781:Texas / Q000453:epidemiology
chemical_list: D019433:Chickenpox Vaccine