Incidence of herpes zoster, before and after varicella-vaccination-associated decreases in the incidence of varicella, 1992-2002

Background. Varicella zoster virus (VZV) causes varicella and, later in the life of the host, may reactivate to cause herpes zoster (HZ). Because it is hypothesized that exposure to varicella may boost immunity to latent VZV, the vaccination-associated decrease in varicella disease has led some to suggest that the incidence of HZ might increase. We assessed the impact that varicella vaccination has on the incidence of varicella and of HZ. Methods. Codes for cases of varicella and of HZ in an HMO were determined in automated databases of inpatients and outpatients, on the basis of the Ninth Revision of the International Classification of Diseases. We calculated the incidence, during 1992 - 2002, of varicella and of HZ. Results. The incidence of HZ remained stable as the incidence of varicella decreased. Age-adjusted and - specific annual incidence rates of varicella decreased steadily, starting with 1999. The age-adjusted rates decreased from 2.63 cases/1000 person-years during 1995 to 0.92 cases/1000 person- years during 2002; among children 1 - 4 years old, there was a 75% decrease between 1992 - 1996 and 2002. Age-adjusted and - specific annual incidence rates of HZ fluctuated slightly over time; the age-adjusted rate was highest, at 4.05 cases/1000 person-years, in 1992, and was 3.71 cases/ 1000 person- years in 2002. Conclusions. Our findings revealed that the vaccination-associated decrease in varicella disease did not result in an increase in the incidence of HZ. These early findings will have to be confirmed as the incidence of varicella disease continues to decrease.