Analysis of varicella vaccine breakthrough rates: implications for the effectiveness of immunisation programmes

The objective of this study was to quantify key parameters describing varic ella tester virus (VZV) vaccine efficacy. To do so a mathematical model was developed to represent breakthrough cases as a function of time after Vacc ination in vaccine efficacy trials. Efficacy parameter sets were identified by fitting the predicted annual number of breakthrough infections with tha t observed in three clinical trials chosen to represent the plausible range of vaccine efficacy. Results suggest that varicella vaccination seems to r esult in a high proportion of individuals who are initially totally protect ed (97% for the base-case). However, individuals lose full protection relat ively rapidly (3% per year for the base-case). Once total protection has wa ned individuals have a high probability of developing a breakthrough infect ion if exposed to varicella (73% of the probability in unvaccinated suscept ibles for the base-case). Results also highlight that vaccine efficacy para meters should be estimated concurrently to take into account dependencies b etween parameters. (C) 2000 Elsevier Science Ltd. All rights reserved.