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Study period of each eligible study.
Three calendar periods were defined based on phase of pandemic and emergence of the delta variant of SARS-CoV-2: ‘early pandemic’ through 8 May 2021, when the rate of delta variant infection was ≤1%; ‘mid pandemic’ between 9 May and 30 June 2021, when incidence of delta variant infection increased; and ‘late pandemic’ from 1 July 2021, when delta was the predominant strain. Each horizontal line represents study period of each paper.
