title: Impact of vaccination on the epidemiology of varicella: 1995-2009.
abstract: BACKGROUND
  When varicella vaccine was licensed in the United States in 1995, there were concerns that childhood vaccination might increase the number of adolescents susceptible to varicella and shift disease toward older age groups where it can be more severe.
  
  
  METHODS
  We conducted a series of 5 cross-sectional studies in 1994 to 1995 (prevaccine), 2000, 2003, 2006, and 2009 in Kaiser Permanente of Northern California to assess changes in varicella epidemiology in children and adolescents, as well as changes in varicella hospitalization in people of all ages. For each study, information on varicella history and varicella occurrence during the past year was obtained by telephone survey from a sample of ∼8000 members 5 to 19 years old; varicella hospitalization rates were calculated for the entire membership.
  
  
  RESULTS
  Between 1995 and 2009, the overall incidence of varicella in 5- to 19-year-olds decreased from 25.8 to 1.3 per 1000 person-years, a ∼90% to 95% decline in the various age categories (5-9, 10-14, and 15-19 years of age). The proportion of varicella-susceptible children and adolescents also decreased in all age groups, including in 15- to 19-year-olds (from 15.6% in 1995 to 7.6% in 2009). From 1994 to 2009, age-adjusted varicella hospitalization rates in the general member population decreased from 2.13 to 0.25 per 100,000, a ∼90% decline.
  
  
  CONCLUSIONS
  In the 15 years after the introduction of varicella vaccine, a major reduction in varicella incidence and hospitalization was observed with no evidence of a shift in the burden of varicella to older age groups.
authors.affiliation: Kaiser Permanente Vaccine Study Center, Oakland, California; roger.baxter@kp.org.
authors.forename: Roger
authors.initials: R
authors.lastname: Baxter
authors.affiliation: Department of Epidemiology, Merck Sharp & Dohme Corp, Whitehouse Station, New Jersey;
authors.forename: Trung N
authors.initials: TN
authors.lastname: Tran
authors.affiliation: Kaiser Permanente Vaccine Study Center, Oakland, California;
authors.forename: Paula
authors.initials: P
authors.lastname: Ray
authors.affiliation: Kaiser Permanente Vaccine Study Center, Oakland, California;
authors.forename: Edwin
authors.initials: E
authors.lastname: Lewis
authors.affiliation: Kaiser Permanente Vaccine Study Center, Oakland, California;
authors.forename: Bruce
authors.initials: B
authors.lastname: Fireman
authors.affiliation: Center for Global Health, Cincinnati Children's Hospital, Cincinnati, Ohio; and.
authors.forename: Steve
authors.initials: S
authors.lastname: Black
authors.affiliation: University of California San Francisco Medical Center, San Francisco, California.
authors.forename: Henry R
authors.initials: HR
authors.lastname: Shinefield
authors.affiliation: Department of Epidemiology, Merck Sharp & Dohme Corp, Whitehouse Station, New Jersey;
authors.forename: Paul M
authors.initials: PM
authors.lastname: Coplan
authors.affiliation: Department of Epidemiology, Merck Sharp & Dohme Corp, Whitehouse Station, New Jersey;
authors.forename: Patricia
authors.initials: P
authors.lastname: Saddier
mesh_terms: D000293:Adolescent; D000367:Age Factors; D002644:Chickenpox / Q000453:epidemiology* / Q000517:prevention & control*; D019433:Chickenpox Vaccine; D002648:Child; D002675:Child, Preschool; D003430:Cross-Sectional Studies; D005260:Female; D014645:Herpesvirus 3, Human; D006760:Hospitalization / Q000706:statistics & numerical data; D006801:Humans; D015994:Incidence; D008297:Male; D013997:Time Factors; D014611:Vaccination
chemical_list: D019433:Chickenpox Vaccine
keywords: chicken pox; epidemiology; vaccination; vaccine impact; varicella