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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., (Copyright © 2014 by the American Academy of Pediatrics.)

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.

Background: Varicella vaccine currently is recommended for children between 12 and 18 months of age. However, rates of breakthrough varicella have been reported to be higher among children vaccinated before 14 or 15 months of age and to increase with time since vaccination., Methods: An ongoing study at the Northern California Kaiser Permanente Medical Care Program is evaluating vaccine efficacy in 7585 children vaccinated with Varivax in 1995, when they were between 12 and 23 months of age. Cases of chickenpox are identified by telephone interviews with each child's parent(s) every 6 months. Mean age at varicella onset and mean time from vaccination to onset were calculated on the basis of age, in months, at vaccination. Logistic regression was used to test for trend, and the chi2 test was used to test for differences in rates of breakthrough varicella by age., Results: Over the first 8 years of the study, a total of 1161 cases of breakthrough varicella were reported, for an average rate of 21.7 cases/1000 person-years. Vaccine effectiveness was 83.6% at year 8. The rate of breakthrough varicella did not change for each additional month of age at vaccination (P = .864), and no difference in the rate of breakthrough varicella was found between children vaccinated at <15 months of age and those vaccinated at > or =15 months of age., Conclusions: Our data do not show a difference in vaccine effectiveness with age at vaccination and thus support the current recommendations for initial vaccination between 12 and 18 months of age.

Background: A wide range in antibody titers has been found after immunization with the varicella vaccine, although the basis for these differences has not been described., Methods: To evaluate the contribution of a genetic component in the immune response to the varicella vaccine, concordance for six-week postimmunization antibody titers was evaluated among 248 biologic siblings who participated in varicella vaccine clinical trials by comparing all pairs of siblings (151 pairs) to all possible unrelated, nonsibling pairs created from within this same cohort (30,477 pairs)., Results: Postimmunization antibody titers after 1 varicella vaccine dose were within the range observed historically among healthy vaccinees, with 85.4% of subjects having antibody responses greater than the approximate correlate of protection of 5 gpELISA units. Postimmunization antibody titers within sibling pairs clustered together more than or less than 10 gpELISA units when compared with within nonsibling pairs (P < 0.0001). Postimmunization titers within sibling pairs were also quantitatively closer together than were those within unrelated, nonsibling pairs (P = 0.022). The age-adjusted intraclass correlation coefficient indicated that the heritability of the varicella vaccine immune response is 45% (95% confidence interval of 15-75%)., Conclusions: Similarities in siblings' response to varicella vaccine are supportive of the hypothesis that genetic factors play a role in the antibody response to the varicella vaccine.