One of the things that made the second wave of Spanish flu so lethal was that the normal rules of viral transmission were upended by World War I. Ordinarily, the normal course of viral mutation is for the less lethal strain to be preferentially spread. Instead, soldiers in the trenches who became very ill were sent on trains to crowded hospitals, while those who developed mild cases stayed where they were. I don't want to say it's likely that mass vaccination might have a similar adverse effect, but it seems possible.
Isn't there also a fine line where a virus becomes so deadly that it can no longer spread effectively before killing the host or becoming "harmless" yet spread much better and more effective than a deadlier version?
I remember reading not long ago about a study that showed the common cold is so dominant that I can prevent covid from replicating in an infected person.
Immunologists today are promoting that everybody should get vaccinated against Covid. Maybe not everybody but vast majority of people that's what they say.
Masks, social distancing and washing hands was and is enough.
Maybe vaccine should've been rolled out more slowly not everywhere at once.