Publication date: Jul 15, 2023

Although the effectiveness of BNT162b2 messenger RNA vaccines against the Omicron variant has been reported in several countries, data are limited in children living in Asian countries. Therefore, this study aimed to estimate the effectiveness of the pediatric primary two-dose monovalent mRNA vaccine series in preventing symptomatic coronavirus disease 2019 (COVID-19) in Japan. We conducted a test-negative case-control study (262 test-positive cases and 259 test-negative controls) in patients aged 5-11 years who presented with COVID-19-like symptoms during the Omicron BA. 2- and BA. 5-predominant periods. Vaccination status, demographic data, underlying medical conditions, lifestyle, personal protective health behaviors, living environment, and PCR test results were obtained using parent-administered questionnaires and clinical records. Vaccine effectiveness (VE) against symptomatic COVID-19 was calculated using a multivariate logistic regression analysis. Of the test-positive cases and test-negative controls, 9. 2% (n=24) and 12. 7% (n=33) received two vaccine doses, respectively. Having siblings and a BA. 5-dominant period were significantly associated with symptomatic COVID-19. After adjusting for age, siblings, study period, and duration after the last vaccination, the overall VE of two-dose vaccination was 50. 0% (95% confidence interval [CI], 5%-74%). VE was 72% (95%CI: 24%-89%) within 3 months after the two-dose vaccination, while it decreased to 24% (95%CI: -80% to 68%) after 3 months. Two BNT162b messenger RNA vaccine doses provided moderate protection against symptomatic COVID-19 during the Omicron variant period. A time-dependent decrease in VE was noted after the second dose; thus, a booster dose 3 months after the second dose is warranted.

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