Estimated coverage of vaccines for children in Japan between 2011 and 2022: a descriptive study utilizing nationwide monthly market data.

Background: Japan lacks comprehensive reports on the nationwide voluntary vaccine coverage. The effectiveness of public subsidies in promoting vaccination has not been fully investigated. Therefore, we aimed to estimate the nationwide coverage of voluntary vaccines, compare it with that of national immunization program (NIP)-included vaccines, and investigate the effectiveness of public subsidies.

Methods: We obtained nationwide monthly vaccine market data for rotavirus, Haemophilus influenzae type b (Hib), diphtheria, tetanus toxoid, acellular pertussis, inactivated poliovirus (DTaP-IPV), and mumps vaccines; estimated recipient numbers; and calculated coverage as the proportion of children from October 2011 to March 2022. Regarding the NIP-included vaccine, we compared vaccine coverage calculated from nationwide annual market data with that estimated by World Health Organization (WHO)/United Nations Children's Fund (UNICEF), using Bland-Altman analysis.

Results: The estimates of Hib and DTaP-IPV vaccine coverage derived from market data were slightly higher than the WHO/UNICEF estimates, with mean differences of 0.05 (95% CI: 0.02-0.07) for Hib and 0.03 (95% CI: 0.01-0.05) for DTaP-IPV. The coverage of the rotavirus vaccine gradually increased long before the implementation of national subsidies, reaching 0.9 in 2020. Hib vaccine coverage had already achieved 1.0 by January 2012. The coverage of the DTaP-IPV vaccine was approximately 0.6-0.8 in 2013, reaching 1.0 in 2014. The coverage of mumps vaccine increased gradually from 2011 to 2021.

Conclusions: Despite the possibility of overestimation, our estimates may serve as a valuable surrogate for actual vaccine coverage in Japan. An increasing trend in rotavirus and mumps vaccine coverage was observed when these vaccines were categorized as voluntary. Although vaccination policies differ from country to country, it would be beneficial to share findings on the impact of subsidies in Japan with other countries.