Evidence for frequent reactivation of the Oka varicella vaccine strain in healthy vaccinees.

Serum antibody levels and infection rates were followed for 4 years in 4,631 children immunized with the recently licensed Oka strain varicella vaccine. Anti-VZV titers declined over time in high-responder subjects, but rose in vaccinees with low titers. Among subjects with low anti-VZV titers, the frequency of clinical sequelae and immunological boosting significantly exceeded the 13%/yr rate of exposure to wild type varicella. These findings indicate that the Oka strain of VZV persisted in vivo, and reactivated as serum antibody titers declined after vaccination. This mechanism may improve vaccine-associated long-term immunity.