title: Effectiveness of 2 doses of varicella vaccine in children.
abstract: BACKGROUND
  Because of ongoing outbreaks of varicella, a second dose of varicella vaccine was added to the routine immunization schedule for children in June 2006 by the Centers for Disease Control and Prevention.
  
  
  METHODS
  We assessed the effectiveness of 2 doses of varicella vaccine in a case-control study by identifying children ≥4 years of age with varicella confirmed by polymerase chain reaction assay and up to 2 controls matched by age and pediatric practice. Effectiveness was calculated using exact conditional logistic regression.
  
  
  RESULTS
  From July 2006 to January 2010, of the 71 case subjects and 140 matched controls enrolled, no cases (0%) vs 22 controls (15.7%) had received 2 doses of varicella vaccine, 66 cases (93.0%) vs 117 controls (83.6%) had received 1 dose, and 5 cases (7.0%) vs 1 control (0.7%) did not receive varicella vaccine (P < .001). The effectiveness of 2 doses of the vaccine was 98.3% (95% confidence level [CI]: 83.5%-100%; P < .001). The matched odds ratio for 2 doses vs 1 dose of the vaccine was 0.053 (95% CI: 0.002-0.320; P < .001).
  
  
  CONCLUSIONS
  The effectiveness of 2 doses of varicella vaccine in the first 2.5 years after recommendation of a routine second dose of the vaccine for children is excellent. Odds of developing varicella were 95% lower for children who received 2 doses compared with 1 dose of varicella vaccine.
authors.affiliation: Department of Pediatrics, Yale University School of Medicine and Graduate School of Arts and Sciences, New Haven, Connecticut 06520-8064, USA. eugene.shapiro@yale.edu
authors.forename: Eugene D
authors.initials: ED
authors.lastname: Shapiro
authors.forename: Marietta
authors.initials: M
authors.lastname: Vazquez
authors.forename: Daina
authors.initials: D
authors.lastname: Esposito
authors.forename: Nancy
authors.initials: N
authors.lastname: Holabird
authors.forename: Sharon P
authors.initials: SP
authors.lastname: Steinberg
authors.forename: James
authors.initials: J
authors.lastname: Dziura
authors.forename: Philip S
authors.initials: PS
authors.lastname: LaRussa
authors.forename: Anne A
authors.initials: AA
authors.lastname: Gershon
mesh_terms: D016022:Case-Control Studies; D002644:Chickenpox / Q000517:prevention & control*; D019433:Chickenpox Vaccine / Q000008:administration & dosage* / Q000276:immunology*; D002648:Child; D005260:Female; D006801:Humans; D007115:Immunization Schedule; D008297:Male; D016133:Polymerase Chain Reaction
chemical_list: D019433:Chickenpox Vaccine