The Durability of Vaccine Efficacy on the Incidence of Herpes Zoster Provided by ZOSTAVAX

Background: ZOSTAVAX was licensed in the U.S. for the prevention of herpes zoster (HZ) in persons ≥60 years of age based on the Shingles Prevention Study (SPS). A subset of SPS subjects was subsequently enrolled into an extension study (Short-Term Persistence Study, STPS) to collect longer-term vaccine efficacy (VE) data. To assess the long-term cost-effectiveness of ZOSTAVAX, the durability of VE was evaluated based on SPS/STPS.  While it is important to consider durability of VE for all three study endpoints: HZ burden-of-illness (BOI), incidence of HZ, and incidence of postherpetic neuralgia (PHN), only durability of VE on incidence of HZ (VE_HZ) is discussed here. VE measured by HZ BOI and PHN were both well-maintained in older subjects, while VE_HZ declined with increasing age. This needs to be considered in cost-effectiveness analyses.

Objectives: To evaluate the durability of VE_HZ provided by ZOSTAVAX, by investigating the effects of age and time since vaccination.

Methods: Combined HZ incidence data from the SPS/STPS were parsed into bins defined by concurrent age and year since vaccination. Poisson regression models were then used, with the number of HZ cases in each bin as the dependent variable, and concurrent age, year since vaccination, whether in the first year postvaccination, treatment, and interactions between treatment and each of the above variables as potential independent variables, and follow-up time as the off-set parameter.

Results: All models indicate a statistically significant age effect on VE_HZ, but the effect of time since vaccination was not significant since most of the time effect occurred during the first year post vaccination. 

Conclusions: It is critical to consider the age effect on durability of VE on incidence of HZ, along with HZ BOI and PHN, when assessing the long-term cost-effectiveness of ZOSTAVAX. The methodology for evaluating the durability of VE_HZ will be discussed.