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The participants in the study included persons who were 60 years of age or older and who had been fully vaccinated before March 1, 2021, had available data regarding sex, had no documented positive result on polymerase-chain-reaction assay for SARS-CoV-2 before July 30, 2021, and had not returned from travel abroad in August 2021. The number of confirmed infections in each population is shown in parentheses.
