They even prevented transmission to a certain extent - but we have NEVER tried to vaccinate our way out of a raging pandemic, chasing a virus adapting to the human population.
Like all respiratory viruses it undergoes antigenic drift selected for by antibodies, most likely in persistent infections IMO.
These variants then occupy the niche such selection afford them, becoming the next variant
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What is crucial is that DESPITE this evasion of protection from transmission with later variants the first wave of vaccine DID STILL prevent serious illness on the majority.