Lack of association between age at varicella vaccination and risk of breakthrough varicella, within the Northern California Kaiser Permanente Medical Care Program

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 x 2 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 >= 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.