title: Long-term effectiveness of varicella vaccine: a 14-Year, prospective cohort study.
abstract: BACKGROUND
  Varicella vaccine was licensed in the United States in 1995 for individuals ≥12 months of age. A second dose was recommended in the United States in June 2006. Varicella incidence and vaccine effectiveness were assessed in a 14-year prospective study conducted at Kaiser Permanente Northern California.
  
  
  METHODS
  A total of 7585 children vaccinated with varicella vaccine in their second year of life in 1995 were followed up prospectively for breakthrough varicella and herpes zoster (HZ) through 2009. A total of 2826 of these children received a second dose in 2006-2009. Incidences of varicella and HZ were estimated and compared with prevaccine era rates.
  
  
  RESULTS
  In this cohort of vaccinated children, the average incidence of varicella was 15.9 per 1000 person-years, nine- to tenfold lower than in the prevaccine era. Vaccine effectiveness at the end of the study period was 90%, with no indication of waning over time. Most cases of varicella were mild and occurred early after vaccination. No child developed varicella after a second dose. HZ cases were mild, and rates were lower in the cohort of vaccinated children than in unvaccinated children during the prevaccine era (relative risk: 0.61 [95% confidence interval: 0.43-0.89]).
  
  
  CONCLUSIONS
  This study confirmed that varicella vaccine is effective at preventing chicken pox, with no waning noted over a 14-year period. One dose provided excellent protection against moderate to severe disease, and most cases occurred shortly after the cohort was vaccinated. The study data also suggest that varicella vaccination may reduce the risks of HZ in vaccinated children.
authors.affiliation: Kaiser Permanente Vaccine Study Center, Oakland, CA 94612, USA. roger.baxter@kp.org
authors.forename: Roger
authors.initials: R
authors.lastname: Baxter
authors.forename: Paula
authors.initials: P
authors.lastname: Ray
authors.forename: Trung N
authors.initials: TN
authors.lastname: Tran
authors.forename: Steve
authors.initials: S
authors.lastname: Black
authors.forename: Henry R
authors.initials: HR
authors.lastname: Shinefield
authors.forename: Paul M
authors.initials: PM
authors.lastname: Coplan
authors.forename: Edwin
authors.initials: E
authors.lastname: Lewis
authors.forename: Bruce
authors.initials: B
authors.lastname: Fireman
authors.forename: Patricia
authors.initials: P
authors.lastname: Saddier
mesh_terms: D000293:Adolescent; D017677:Age Distribution; D002140:California / Q000453:epidemiology; D002644:Chickenpox / Q000453:epidemiology* / Q000276:immunology / Q000517:prevention & control*; D019433:Chickenpox Vaccine / Q000008:administration & dosage / Q000276:immunology*; D002648:Child; D002675:Child, Preschool; D015331:Cohort Studies; D005260:Female; D005500:Follow-Up Studies; D006562:Herpes Zoster / Q000175:diagnosis / Q000453:epidemiology*; D006801:Humans; D015994:Incidence; D007223:Infant; D008297:Male; D011446:Prospective Studies; D018570:Risk Assessment; D017678:Sex Distribution; D013997:Time Factors; D014611:Vaccination / Q000379:methods
chemical_list: D019433:Chickenpox Vaccine
keywords: chicken pox; children; effectiveness; vaccine; varicella; zoster