RSV (respiratory syncytial virus) season in most regions of the United States typically starts in the fall and lasts through the winter, with seasonal administration of RSV antibodies generally recommended from October through the end of March. In some places, especially for pregnant people, the RSV vaccine is recommended during the season from September through January. The timing of RSV activity can vary geographically, and local health authorities may adjust this period somewhat based on local epidemiology.

In Canada, RSV season generally begins in late fall and lasts until spring, with most cases occurring from December through March.

In summary, RSV season is largely from fall to early spring , roughly October to March or January to March , depending on the specific population and region.