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

The objective of this study was to quantify key parameters describing varicella zoster virus (VZV) vaccine efficacy. To do so a mathematical model was developed to represent breakthrough cases as a function of time after vaccination in vaccine efficacy trials. Efficacy parameter sets were identified by fitting the predicted annual number of breakthrough infections with that observed in three clinical trials chosen to represent the plausible range of vaccine efficacy. Results suggest that varicella vaccination seems to result in a high proportion of individuals who are initially totally protected (97% for the base-case). However, individuals lose full protection relatively rapidly (3% per year for the base-case). Once total protection has waned individuals have a high probability of developing a breakthrough infection if exposed to varicella (73% of the probability in unvaccinated susceptibles for the base-case). Results also highlight that vaccine efficacy parameters should be estimated concurrently to take into account dependencies between parameters.