Universal varicella vaccination: efficacy trends and effect on herpes zoster.

In 1995, the Varicella Active Surveillance Project (VASP) was established in Antelope Valley (California), a geographically distinct high-desert community of 300,000 residents, as one of three sites in the nation in a cooperative agreement with the Centers for Disease Control and Prevention (CDC) to collect baseline demographic and clinical data and to monitor trends in varicella (chickenpox) following introduction of varicella vaccine. Herpes zoster (shingles) was added to the active surveillance January 1, 2000. The universal varicella program has proven effective in terms of reducing the number of reported verified varicella cases by 85%, from 2,934 in 1995 to 412 in 2002. Prior to this dramatic reduction, immunologic boosting due to exogenous exposures to wild-type varicella-zoster virus (VZV) in the community (1) caused mean serum anti-VZV levels among vaccines to increase with time after vaccination and (2) served as a mechanism that helped suppress the reactivation of herpes zoster (HZ), especially among individuals with a previous history of wild-type varicella. That immunologic boosting might play a significant role in both varicella and the closely related HZ epidemiology is evidenced by (1) a decline in vaccine efficacy by over 20%, from 95.7% (95% C.I., 82.7% to 98.9%) in 1999 to 73.9% (95% C.I., 57.9% to 83.8%) in 2001 and (2) an unexpectedly high cumulative (2000 to 2003) true incidence rate of 223 (95% C.I. 180-273) per 100,000 person-years (p-y) among children <10 years old with a previous history of varicella. Because capture-recapture methods demonstrate a likely lower bound of 50% underreporting, the actual rate is likely double or 446 per 100,000 p-y, approaching the HZ rate reported among older adults. Other recent studies based on VASP data have mitigated against discovery of the above trends that challenge several initial assumptions inherent to the universal varicella program, namely, (1) a single dose confers long-term immunity and (2) there is no immunologically mediated link between varicella and HZ incidence. As vaccinated children replace those with a prior history of wild-type varicella in the <10 age group, increasing HZ incidence among this cohort will be of less concern in the near future. However, previous scientific studies, including the present preliminary results from active surveillance indicate that HZ may be increasing among adults. It may be difficult to design booster interventions that are cost-effective and meet or exceed the level of protection provided by immunologic boosting that existed naturally in the community in the prelicensure era.