INTRODUCTION

COVID-19 vaccination for children has been debatable. Compiling scientific evidence on vaccine safety and effectiveness was indispensable for the effective vaccine strategy. This study aimed to estimate the effectiveness and protection duration of the COVID-19 vaccine among children and adolescents in Japan.

MATERIAL AND METHODS

A retrospective cohort study was conducted targeting residents aged 5-17 years in 11 cities in Japan. Combining the national SARS-CoV-2 surveillance record and vaccination registry, Cox proportional hazard regression analyses were performed to estimate the effectiveness of the original monovalent vaccine against Omicron infection compared with unvaccinated. The duration of protection was also estimated.

FINDINGS

A total of 1,908,012 records were included. The effectiveness of primary series vaccination, the monovalent and bivalent first booster dose vaccination against any infection with Omicron BA.5 was 2 % (95 % CI: 0-3 %) (median 9.6 months after vaccination), 39 % (38-40 %), and 41 % (39-44 %) among those aged 12-17 years, respectively. The effectiveness against severe infection with the Omicron BA.5 was 49 % (35-60 %), 54 % (39-65 %), and 73 % (46-87 %), respectively. The result was similar for children aged 5-11 years. The protection of hybrid immunity was estimated at 83-97 %. The vaccine-derived protection waned to nearly 0 % at ten months after primary series vaccination; however, the protection against severe infection persists more than that.

DISCUSSION

The monovalent vaccine's protection against Omicron infections was limited, partly due to the rapid waning of vaccine-derived immunity. The protection against severe diseases was moderate, especially when the bivalent vaccine was used. The results reassure the importance of updated vaccination.