Figure 1.

Flow diagram of the study design. Individuals who had a nasopharyngeal swab RT-PCR test at a large screening test center in Japan between July 2020 and April 2022 were initially recruited. In this population, young children aged 0 to 3 years were enrolled. The study period was then divided into the following four seasons: July 2020 to January 2021 (B.1.1.284 and B.1.1.214 outbreaks), May 2021 to June 2021 (B.1.1.7 Alpha outbreak), August 2021 to September 2021 (B.1.617.2 Delta outbreak), and January 2022 to April 2022 (B.1.1.529 Omicron outbreak). Secondary transmission rates stratified by contact closeness or locations and symptoms in RT-PCR test-positive young children were then compared between the periods.

SARS-CoV-2, severe acute respiratory syndrome coronavirus 2; RT-PCR, reverse transcription-polymerase chain reaction.