The changing epidemiology of varicella incidence after implementation of the one-dose varicella vaccination policy.

The varicella vaccine has been available in the Taiwan market since July 1997. Beginning 1998-1999, Taipei City and Taichung City/County as the early launch areas included the varicella vaccine in their free pediatric vaccination programs. By contrast, the national free vaccination program was not implemented until 2004. We aim to investigate the changing epidemiology of varicella incidence through an analysis of age-period-cohort effects. With the greatest decrease in varicella incidence occurring in children aged below 6, the incidence of varicella shifted to older age groups as reflected in different birth cohorts. The current study provides important implications for the current vaccination policy.