It’s a bit of everything. RSV is a big daycare/ preschool disease (it’s very contagious). So generally every year a certain number of kids build up immunity. The last 2 and half years prevented a lot of spread many ways- kids with any sign of cough were immediately sent home from school or daycare and were told to stay there until it was completely gone. Parks were closed for a while, kiddie gyms were closed, birthday parties cancelled, less travel, less visiting relatives, not to mention parents who were particularly concerned not bringing their kids to stores and restaurants. Older siblings were podded, masked and distanced at school (my daughter had no snack, outdoor lunches, windows open year round, water breaks in the coat room, etc). All that suppresses spread, not simply masks.

Now that it’s all gone, the disease has lots more room to spread. My younger daughter is in preschool. 15 kids in the class, about a third have older siblings. Typically they probably would have already been exposed/ have caught RSV. This year, the whole class is probably vulnerable (obviously I haven’t actually tested all these kids, just an example).