PIN86 Modeling Performance Parametrization of Varicella Vaccines

Abstract

Various modeling studies and clinical trials use different methods to report characteristics of varicella vaccines, which doesn’t allow for direct and appropriate comparison of vaccine performance. This study used a unified framework to estimate the effectiveness of two previously reported varicella vaccines [highly effective (HE) and moderately effective (ME)]. We developed and fitted a deterministic compartmental model for one- or two-dose regimens (states: Permanently Protected; Temporarily Protected; Vaccinated Susceptible; Breakthrough Infected) to the number of individuals infected with varicella over a 10-year period, per clinical trials. We assumed the same force of infection (15%) for both vaccines. Using maximum likelihood, we estimated the percentage of people permanently protected from breakthrough varicella with 1st dose (T1) and the percentage of people unprotected or temporarily protected with 1st dose that get permanently protected with second dose (T2). We also estimated the average duration of temporary protection for each vaccine (σ) for both regimens. We estimated that T1 was 90% and 62% for HE and ME vaccines respectively. Among individuals who were susceptible after the 1st dose (i.e. 10% for HE and 38% for ME), 69% and 85% gained permanent protection after 2nd dose respectively. Our model estimated that 97% of individuals were permanently protected with two-doses of HE vaccine compared to 95% with ME vaccine. The duration of temporary protection(σ) for each dose was 1.16 years for HE and 1.10 years for ME. The use of a unified framework and consistent methodology allows direct and robust comparison of the vaccine performance of different vaccine brands. Better performance of HE varicella vaccine may lead to the lower incidence of breakthrough varicella compared to ME vaccine. Further research is needed to better understand how vaccine performance parametrization can impact reduction in varicella cases and cost-effectiveness of the national immunization programs.