This is an absolute myth. There is no evidence of "immunity debt", and serious public officials parrotting this nonsense are doing real damage. It's a tempting explanation because it's simple and seems to make intuitive sense, but it's just wrong. The explanation that has much more evidence is that everyone and their dog caught COVID, and COVID has been directly linked to lasting immune system damage, making everyone more susceptible to catching these other viruses.
https://globalnews.ca/news/9272293/immunity-debt-covid-19-mi...