title: An elementary school outbreak of varicella attributed to vaccine failure: policy implications.
abstract: BACKGROUND
  Since licensure in the United States, studies have shown that varicella vaccine's overall effectiveness ranges from 44% to 100%, with substantial protection against moderate and severe varicella; however, breakthrough illness has been documented in up to 56% of vaccinated individuals.
  
  
  METHODS
  A varicella outbreak occurred in a Minnesota school with 319 students. Phone surveys were conducted with students' parents. Information was collected on students who had recent varicella infections, including onset date, rash characteristics, duration, and underlying medical conditions.
  
  
  RESULTS
  Fifty-four cases occurred after a primary breakthrough case. Twenty-nine (53%) students had been vaccinated. Unvaccinated students had an increased risk of moderate varicella, compared with vaccinated students (relative risk [RR], 4.4 [95% confidence interval [CI], 2.2-9.1]; P<.001). The vaccine was 56% effective at preventing any varicella and 90% effective against moderate illness. Students vaccinated >or=5 years before the outbreak had a greater risk of breakthrough varicella than did those vaccinated within <or=4 years (RR, 2.6 [95% CI, 1.3-5.4]; P<.01).
  
  
  CONCLUSIONS
  Vaccinated students presented with milder varicella symptoms than did unvaccinated students. Individuals with breakthrough illness can be highly infectious. Time since varicella vaccination was associated with illness. Despite 29 breakthrough cases, the varicella vaccine conferred a high degree of protection against moderate illness.
authors.affiliation: Immunization, Tuberculosis, and International Health Section, Infectious Disease Epidemiology, Prevention, and Control Division, Minnesota Department of Health, Minneapolis 55414, USA. brian.lee@health.state.mn.us
authors.forename: Brian R
authors.initials: BR
authors.lastname: Lee
authors.forename: Shelly L
authors.initials: SL
authors.lastname: Feaver
authors.forename: Claudia A
authors.initials: CA
authors.lastname: Miller
authors.forename: Craig W
authors.initials: CW
authors.lastname: Hedberg
authors.forename: Kristen R
authors.initials: KR
authors.lastname: Ehresmann
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; D006291:Health Policy; D014645:Herpesvirus 3, Human / Q000276:immunology*; D006801:Humans; D008297:Male; D008910:Minnesota / Q000453:epidemiology; D012307:Risk Factors; D012574:Schools; D012720:Severity of Illness Index; D017211:Treatment Failure; D014611:Vaccination
chemical_list: D019433:Chickenpox Vaccine