SARS-CoV-2 infection control implementation based on sources of infection showing directions for three age groups in Japan

Background Some aspects of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmission in children and adults remain unclear. This report describes different SARS-CoV-2 transmission patterns by age group in Japan. Methods and findings This retrospective observational case series study analyzed transmission patterns of real-time polymerase chain reaction (RT-PCR)-confirmed SARS-CoV-2 infections found by local health authorities and commercial laboratories during January 14 through July 31, 2020 in Japan. After ascertaining the infection source for every symptomatic case as clusters at households, daycare facilities, schools, hospitals and workplaces etc., their associated transmission patterns were analyzed. Identified cases were divided into three groups: underage, < 20; adults, 20- 59; and elderly people 60 years old and older. The reproductive number (R)s of respective transmission directions found for the respective age groups were compared. Of 26,986 total cases, 23,746 unknown cases were found, leaving 3,240 ascertained sources of infection (12.0%) comprising 125 (3.9%) underage, 2350 (72.5%) adult, and 765 (23.6%) elderly people. The respective Rs of underage infection sources directed to underage, adult, and elderly people were estimated respectively as 0.0415 (95% CI, 0.0138- 0.0691), 1.11 (95% CI, 0.9171-1.3226), and 0.2811 (95% CI, 0.2074-0.3687). The respective Rs of adult infection source directed to underage, adult, and elderly people were estimated respectively as 0.0140 (95% CI, 0.0120-0.0162), 0.5392 (95% CI, 0.5236-0.5550), and 0.1135 (95% CI, 0.1074-0.1197). The respective Rs of elderly infection source directed to underage, adult, and elderly people were estimated as 0.065 (95% CI, 0.0039-0.0091), 0.3264 (95% CI, 0.3059-0.3474), and 0.3991 (95% CI, 0.3757-0.4229). Conclusions The main sources of SARS-CoV-2 infection were adults and elderly people. The R of underage people directed to adults was greater than 1 because of close familial contact but they were unlikely to become carriers transmitting SARS-CoV-2 because they accounted for a minority for transmissions. Apparently, SARS-CoV-2 was transmitted among adults and elderly people, suggesting that infection control of SARS-CoV-2 should be managed specifically by generation.