Perhaps it would have, but without the vaccine that mutation may not be more fit than the original strain. If there was no difference in fitness, I'd might just emerge as a spontaneous new strain. For respiratory viruses those your of mutations usually lead to more contagious but less deadly strains. But the creation of new strains is dramatically accelerated by changes in environment.
Perhaps it would have, but without the vaccine that mutation may not be more fit than the original strain.
The strain's virulence would be identical vaccine or no vaccine. The vaccine cannot have an impact on the virulence of the new strain. If the new strain was more virulent it would take over anyway without the vaccine. If it was equally or less virulent it doesn't matter.
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