Effectiveness of COVID-19 Vaccines in Japanese Pediatric Patients (Autumn 2022 to Autumn 2023): A Case-Control Study

Kajiume T

Published on: 2025-03-17

Abstract

Objective: The coronavirus (COVID-19) pandemic, which began in 2019, has led to numerous infections and deaths worldwide. A vaccine was developed within 1 year, largely owing to the efforts of the pharmaceutical industry. However, the vaccination rate among children in Japan remains low, possibly owing to the insufficient disclosure of the effectiveness of vaccination. Since September 2022, the Ministry of Health, Labour and Welfare of Japan has not publicly disclosed vaccine effectiveness data, contributing to distrust among parents. This study investigated vaccine effectiveness in children during recent COVID-19 waves.

Methodology: This study included children who were ≥6 months of age and visited the Mukainada Child Clinic with a fever (≥37.5°C) between November 2022 and October 2023. Clinical data were collected from electronic medical records, and SARS-CoV-2 testing was conducted using rapid antigen testing of nasopharyngeal swabs. Vaccine effectiveness was estimated using a test-negative case-control design.

Results: Among 1,616 children tested, the COVID-19 morbidity rates in unvaccinated and vaccinated patients were 33.6% and 31.3%, respectively, during the eighth wave, indicating a vaccine effectiveness of 7.1% (p = 0.874), and 16.4% and 29.7%, respectively, during the ninth wave, indicating a vaccine effectiveness of –80.9% (p = 0.010). These results suggested that vaccine effectiveness varied by SARS-CoV-2 variant. The mean body temperature was 38.83°C and 38.71°C in unvaccinated and vaccinated patients, respectively, suggesting no significant effect of vaccination on COVID-19 severity.

Conclusions: These results highlight the need for the rapid adaptation of vaccines to prevailing SARS-CoV-2 variants and ongoing surveillance of vaccine effectiveness.