[paul cotter]

It is down to whether one is dealing with a DNA or RNA virus. DNA viruses, for example smallpox, are much more stable than RNA viruses such as influenza and covid. RNA viruses are prone to antigenic drift where small changes occur over time and antigenic shifts which involve where large changes rapidly occur. I was vaccinated against smallpox ~1960 and the consensus is that I would still have a degree of protection. Given that the vaccination was so effective one might wonder why it took so long for eradication- the answer is down to the fact that smallpox is wildly contagious.