Viruses are shape-shifters, naturally mutating as they go up against the immune defenders of their hosts. This forced evolution is why the influenza vaccine must be updated annually and one reason why no successful shot for HIV has been developed.
Coronaviruses like SARS-COV-2 mutate more slowly than either of those viruses. But changes to their genetic code inevitably accumulate, with the chance that some may affect their infectiousness, lethality or resistance to vaccines. That appears to now be playing out with SARS-CoV-2, as the emerging variants identified in the U.K., South Africa and Brazil seem to spread more easily.