Human Astrovirus Infection and Outbreaks in Japanese Children Before and After the Emergence of COVID-19 Pandemic.

Human astroviruses (HAstVs) were examined in 1625 stool samples collected from outpatient children with diarrhea who visited clinics in 4-6 prefectures in Japan. The study was conducted over a period of 4 years from July 2018 to June 2022, spanning the period before and after the emergence of COVID-19. The HAstV and other diarrheal viruses including group A rotavirus, norovirus and sapovirus were screened by RT-PCR. Of these, HAstV was detected in 140 out of 1625 (8.6%). When the stool samples were categorized by the year of collection, HAstV was detected in 2018-2019, 2019-2020, 2020-2021, and 2021-2022 with the prevalences of 3.1%, 6.6%, 3.0%, and 20.1%, respectively. Among 140 HAstV positive cases, HAstV1, MLB1, MLB2, HAstV3, MLB3, and VA2 were detected in 77, 46, 10, 3, 2, and 2 samples, respectively. High infection rate was found in children 1-3 years of age (95/140; 67.9%). Severity of the disease increased with the co-infection with norovirus. During the surveillance of 2021-2022, two outbreaks of HAstV were detected, one was an outbreak of HAstV1 and MLB2 in September, 2021 in Kyoto, the second was an outbreak of HAstV1 and MLB1 in December, 2021 in Kyoto and Shizuoka. It was interesting to observe that mixed-infection of astroviruses between HAstV1 and MLB1 was reported for the first time in this study. Characterization of the subgenotypes in HAstV1 and HAstV3 indicated that most HAstV1 circulating in Japan belonged to HAstV1a and only a single strain was HAstV1b, whereas all HAstV3 was identified as the HAstV3c subgenotype. In conclusion, several HAstV genotypes, including classic HAstV, novel MLB, and VA genotypes, were detected in this study. The incidence of astrovirus outbreaks was also reported after the pandemic of COVID-19 in 2021-2022.