Modeling the impact of one- and two-dose varicella vaccination on the epidemiology of varicella and zoster.

In many countries, policymakers are being asked to make recommendations regarding the introduction of a 2-dose varicella vaccination program. The objective of this study was to examine the potential impact of 1-dose versus 2-dose varicella vaccination programs on varicella and zoster incidence, using Canada as an example. We developed a deterministic realistic age-structured model that fits 1- and 2-dose vaccine efficacy, varicella force of infection and zoster incidence. Assuming 90% coverage, the base case model (range: min; max) predicts that 1-dose vaccination will reduce varicella and zoster cases by 64% (14%; 96%) and 5% (-2%; 22%), respectively, over 80-years. Adding a second dose is predicted to reduce varicella and zoster by an additional 22% (0%; 82%) and 6% (0%; 14%), respectively. Most varicella cases prevented by the second dose are breakthrough infections. Although the incremental effectiveness of adding the second dose is highly sensitive to vaccine efficacy and mixing, predictions of the overall benefit of a 2-dose program is relatively robust to model assumptions. Adding a 2-dose program may help guarantee high population-level effectiveness against varicella. However, the incremental benefit of a second dose is highly dependant on the effectiveness of the first dose and its impact on zoster.