You have just explained, in some more detail than I did, how selection pressures effect the evolution of organisms. Evolution is an emergent response to environmental changes, in this case the presence of specific vaccine induced immune response.
I did not say that the vaccine CAUSES the virus to change directly. It changes the environment within which the virus will evolve to survive (or not). The mutation may be random, but the selection is not.
I don't think we are in disagreement at all really.
What I mean is that the mutation would have happened without the vaccine, and it would have spread just as much without the vaccine. The vaccine just highlights the new strain by removing the old strain.
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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