A Macro-Level Association of Vaccination Rate with the Number of Confirmed COVID-19 Cases in the United States and Japan

Aiming to evaluate a macro-level association of vaccination rate as well as booster vaccination rate with the number of confirmed COVID-19 cases in the United States and Japan, a cross-sectional study was conducted using data in each jurisdiction. Data on the total number of people who were fully vaccinated as of the end of December 2021, data on the total number of people who have received a booster dose as of the end of March 2022 and data on the cumulative number of confirmed COVID-19 cases were obtained from the website of the national governments. A generalized regression model was used to examine the association. This study showed that a higher vaccination rate was associated with a lower number of confirmed COVID-19 cases per year in 2021 for both the United States and Japan. The number of confirmed COVID-19 cases per 1000 population per year (95% confidence intervals) as a 1% increment of the vaccination rate was −0.74 (−1.29, −0.20), p = 0.007 for the United States and −1.48 (−1.95, −1.00), p < 0.0001 for Japan. A similar association was observed for the booster vaccination rate in 2022, although the association was attenuated in a multivariable model, particularly for the United States. This study provided macro-level evidence that vaccination may reduce the number of confirmed COVID-19 cases.