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Change in exposure history by the number of influenza seasons with low activity. With no low influenza activity (“no COVID effect”), or 1 year, 2 years, or 3 years of low influenza activity since 2020–2021 (“1-year effect,” “2-year effect,” “3-year effect”), population immunity in the beginning of seasons starting from 2021 was mapped to 4 groups: naïve, natural infection, vaccination, and both. Abbreviation: COVID, coronavirus disease.
