Respiratory virus circulation in children after relaxation of COVID-19 restrictions in fall 2021-A nationwide register study in Finland

Social restrictions interrupted the normal respiratory virus circulation in Spring 2020. This report describes virus circulation in the pediatric population before and after the restrictions ended in Finland in September 2021. We used data from the Finnish Infectious Disease Register. Nationwide influenza A and B, rhinovirus, severe acute respiratory syndrome coronavirus 2 (SARS-Cov-2), and respiratory syncytial virus (RSV) findings from January 2018 to December 2021. Age stratified (0-4, 5-9, and 10-14 years) weekly incidences per 100 000 children were calculated. School and day-care closures interrupted completely the circulation of all other respiratory viruses than rhinovirus in spring 2020. After restrictions were relaxed in September 2021, SARS-Cov-2 detections increased majorly. We observed high RSV season atypically early. SARS-Cov-2 was detected in older children whereas RSV season peaked especially among children aged under 5. Influenza seemed to return to normal circulation. In conclusion, we report that the ending of social restrictions in September 2021 led to an increase in SARS-Cov-2 detections and high epidemic peaks of RSV and parainfluenza in atypical timing in children. Our results highlight the importance of continuous pathogen surveillance during the pandemic, as atypical surges of non-COVID-19 respiratory viruses were observed.