Record high parainfluenza season in children after relaxation of COVID-19 restrictions in fall 2021-A nationwide register study in Finland.

Social restrictions interrupted the circulation of common respiratory viruses among children in spring 2020. In the winter season 2020-2021, only rhinovirus spread in Finland. As the restrictions were ended in September 2021, we saw record high epidemic peak of parainfluenza. Typically, the epidemic peak is in springtime, but now, it started in the early fall 2021. The monthly incidence among children aged 0-4 years (120 per 100,000 children) was six times higher than the second highest reported monthly incidence (21 per 100,000 children) during the last 10 years. Our finding highlights the importance of active surveillance of viral respiratory pathogens during the pandemic.