There are a great many viruses circulating in any populations, most of which have a large degree of immunity to them, it’s when you have a new virus strain that serious illness occurs. Influenza is a good example, the virus changes from year to year and scientists developed vaccines to protect against the most likely strains.
Because global travel is so widespread any new airborne virus is going to become a pandemic very quickly, we don’t know what the threat is in advance and it takes time to develop a vaccine. Corona virus is now endemic in the population, most of us have a good resistance, it’s not total herd immunity and vulnerable individuals will be seriously ill, just like the seasonal influenza we regularly get.
Coronavirus is not endemic. Just saying!