OBJECTIVES:

Varicella is a common, vaccine-preventable disease associated with significant socio-economic burden. Following the experience from eight pilot regions, Italy has made varicella vaccination mandatory since 2017. This study compares the impact of two monovalent/tetravalent vaccines in a 2-dose schedule as recommended in Italy.

METHODS:

A previously published age-structured dynamic model of varicella vaccination was used to compare the two vaccines. Given the differences in the immunogenicity profiles, an efficacy of 67.2% and 78.0% was considered for GSK and MSD first dose monovalent vaccines, respectively. GSK and MSD tetravalent vaccines were assumed to result in 95.4% and 98.3% of full protection with the second dose, respectively. The average vaccination coverage of 84.2% at 24 months in the eight Italian pilot regions was conservatively assumed for both first and second doses. Model outputs include the incidence of wild-type (WT) and breakthrough (BKT) varicella; and varicella inpatient days.

RESULTS:

Varicella incidence before vaccination introduction was 910 per /100,000 person-year. At the post-immunization equilibrium, annual WT infections were similar between the two vaccines with 18.1% and 17.5% of the pre-vaccination period for GSK and MSD vaccines, respectively. BKT infections represented 72.6% and 72.4% of all infections for GSK and MSD vaccines, respectively. Both vaccines resulted in a similar age distribution at equilibrium; with 72.9% of WT and 54.9% of BKT infections occurring in age groups less than 25 years old with GSK vaccine compared to 70.5% of WT and 52.3% of BKT infections with MSD vaccine. The average age of WT and BKT infections shifted to the 15-24 age group with both vaccines. Varicella inpatient days were reduced by 67.0% and 67.1% with GSK and MSD vaccines, from the pre-vaccination era, respectively.

CONCLUSIONS:

The difference in efficacy between the two varicella vaccines does not translate into differences in varicella epidemic dynamics in Italy.