The impact of varicella vaccination in the United States

The addition of varicella vaccine to the universal childhood immunization schedule in the United States in 1995 can be seen as a bold step. Shown to be safe and efficacious against varicella in extensive prelicensure studies, it is nonetheless the first vaccine against a herpesvirus and, furthermore, it is a live, attenuated vaccine. Both wild-type and vaccine strain varicella zoster virus (VZV) are noteworthy for their ability to establish latent infection within the host, with the subsequent possibility of reactivation. Therefore, at the population level, a successful vaccination program could result in the eventual displacement of wild-type VZV by the attenuated vaccine virus. The immediate objective of universal vaccination, however, was to reduce the significant morbidity and mortality associated with primary VZV infection. Data now accumulating suggest that the varicella vaccine as used in the United States has so far been highly effective. The challenge for the future is to predict how the resulting substantial reduction in circulation of VZV will affect immunity among both vaccinees and the unvaccinated. Vaccination strategies likely will need to be adjusted as the epidemiology of VZV in the United States continues to evolve.