Coronavirus disease 2019 transmission and symptoms in young children during the severe acute respiratory syndrome coronavirus 2 Delta variant and Omicron variant outbreaks

Abstract

Objective: Little is known about the transmission of coronavirus disease 2019 (COVID-19) in young children. This study aimed to clarify the risk of COVID-19 transmission among this population.

Methods: Between July 2020 and April 2022, 1660 0 to 3-year-old children underwent a nasopharyngeal swab for later reverse transcription-polymerase chain reaction testing at a mass screening test center in Japan. Their disease transmission rate and clinical symptoms were evaluated according to the predominant variant strains of that season.

Results: The secondary transmission rate after close contact of the Delta B.1.617.2 (17.4%) and Omicron B.1.1.529 (39.2%) variants was significantly higher than that of the conventional strains (B.1.1.284 and B.1.1.214; 4.5%) during the pandemic. The increased transmissibility with the Delta and Omicron variants was independent of close contact or location. The prevalence rates of cough, fatigability, and fever were similar in young children infected by the Delta and Omicron variants.

Conclusions: COVID-19 transmission in children aged 0 to 3 years increased by 3 to 4 fold during the Delta outbreak and by 8 to 10 fold during the Omicron outbreak compared with the conventional strain outbreak. The symptoms in young children were not different between the Delta and Omicron variants.

Keywords: Coronavirus disease 2019; Delta; Omicron; symptom; transmission; variant strain; young child.

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.