Vaccine
http://www.sciencedirect.com/science/journal/0264410X
Volume 30, Issue 17 pp. 2707-2804 (5 April 2012)

Regular Papers
Estimating the age-specific duration of herpes zoster vaccine protection: A matter of model choice?
Original Research Article
Pages 2795-2800
Joke Bilcke, Benson Ogunjimi, Frank Hulstaert, Pierre Van Damme, Niel Hens, Philippe Beutels

Abstract
Introduction
The estimation of herpes zoster (HZ) vaccine efficacy by time since vaccination and age at vaccination is crucial to assess the effectiveness and cost-effectiveness of HZ vaccination. Published estimates for the duration of protection from the vaccine diverge substantially, although based on data from the same trial for a follow-up period of 5 years. Different models were used to obtain these estimates, but it is unclear which of these models is most appropriate (if any). Only one study estimated vaccine efficacy by age at vaccination and time since vaccination combined. Recently, data became available from the same trial for a follow-up period of 7 years.

Aim and methods
We aim to elaborate on estimating HZ vaccine efficacy (1) by estimating it as a function of time since vaccination and age at vaccination, (2) by comparing the fits of a range of models, and (3) by fitting these models on data for a follow-up period of 5 and 7 years.

Results
Although the models’ fit to data are very comparable, they differ substantially in how they estimate vaccine efficacy to change as a function of time since vaccination and age at vaccination.

Discussion
An accurate estimation of HZ vaccine efficacy by time since vaccination and age at vaccination is hampered by the lack of insight in the biological processes underlying HZ vaccine protection, and by the fact that such data are currently not available in sufficient detail. Uncertainty about the choice of model to estimate this important parameter should be acknowledged in cost-effectiveness analyses.