title: Childhood vaccination against varicella: persistence of antibody, duration of protection, and vaccine efficacy.
abstract: OBJECTIVE
  To document the duration of protection afforded by Oka/Merck varicella vaccine over a 7-year period.
  
  
  METHODS
  The subjects were healthy children 1 to 12 years of age originally enrolled in clinical studies to evaluate the primary immune response to varicella vaccine 6 weeks after vaccination. Each was monitored for antibody persistence, breakthrough infection, and household exposure to varicella to produce estimates of vaccine efficacy.
  
  
  RESULTS
  The 6-year cumulative varicella antibody persistence rate was 99.5% (95% CI: 98.9%, 100.0%). The annual breakthrough rate through 7 years ranged from 0.2% to 2.3% per year; the estimated cumulative event rate was 6.5%. Comparison of the observed average annual breakthrough rate with the age-adjusted expected annual incidence rate of varicella in unvaccinated children corresponded to an estimated vaccine efficacy of 93.8% to 94.6%. Eighty vaccinated children were exposed to varicella in the household, resulting in 8 (10%) cases of infection. When compared with the historical attack rate of 86.8% in unvaccinated susceptible persons exposed to varicella in the household, this yields an estimated vaccine efficacy of 88.5% (95% CI: 80.9%, 96.1%). Varicella cases in vaccinated children generally were mild.
  
  
  CONCLUSIONS
  The live attenuated varicella vaccine is highly effective in inducing persistent immunity and long-term protection against breakthrough varicella infection.
authors.affiliation: Merck Research Laboratories, Merck & Co, Inc, West Point, Pennsylvania 19482, USA.
authors.forename: S J
authors.initials: SJ
authors.lastname: Vessey
authors.forename: C Y
authors.initials: CY
authors.lastname: Chan
authors.forename: B J
authors.initials: BJ
authors.lastname: Kuter
authors.forename: K M
authors.initials: KM
authors.lastname: Kaplan
authors.forename: M
authors.initials: M
authors.lastname: Waters
authors.forename: D P
authors.initials: DP
authors.lastname: Kutzler
authors.forename: P A
authors.initials: PA
authors.lastname: Carfagno
authors.forename: J C
authors.initials: JC
authors.lastname: Sadoff
authors.forename: J F
authors.initials: JF
authors.lastname: Heyse
authors.forename: H
authors.initials: H
authors.lastname: Matthews
authors.forename: S
authors.initials: S
authors.lastname: Li
authors.forename: I S
authors.initials: IS
authors.lastname: Chan
mesh_terms: D017677:Age Distribution; D000914:Antibodies, Viral / Q000276:immunology*; D002644:Chickenpox / Q000453:epidemiology / Q000276:immunology* / Q000517:prevention & control; D019433:Chickenpox Vaccine / Q000276:immunology*; D002648:Child; D002675:Child, Preschool; D005260:Female; D006801:Humans; D015994:Incidence; D007223:Infant; D008297:Male; D012307:Risk Factors; D013997:Time Factors
chemical_list: D000914:Antibodies, Viral; D019433:Chickenpox Vaccine