First, the "virus" in this sentence means "all the trillions of individual virions currently in our environment, including the trillions currently in living cells. This does not mean that individual virions are changing in response to any stimulus or lack thereof. What they are saying is that as those trillions beget many trillions more, there will be many random mutations. Most of those mutations will result in failure. Some will end up as successful as their antecedent. A tiny few will end up with mutations that make them more successful in the current environment. So the mutations happen regardless of anything we do or don't do.
But as we take more countermeasures, it becomes harder for the baseline virions to be successful (because the vaccines work, distancing works, etc.). Any mutations that are only as successful as baseline are going to have trouble spreading. So by default what we will see is more of the mutations that result in things like easier transmission. Because they are better suited to the current environment where people are taking countermeasures. And because baseline is not as successful, the proportion of the new variants will increase over time.
Putting this together, if there's no selection pressure, mutations don't confer any advantage and they will tend to disappear from the population. So through that lens, the new variants wouldn't necessarily have gained a foothold if people weren't using countermeasures. That's not specifically the vaccines, though. Delta is apparently more transmissible generally, and that could also be a response (via random mutation) to social distancing & masking measures.
So the vaccine didn't cause the mutation, but the presence of the vaccine and other human countermeasures shaped the evolution of the vaccine.
First, the "virus" in this sentence means "all the trillions of individual virions currently in our environment, including the trillions currently in living cells. This does not mean that individual virions are changing in response to any stimulus or lack thereof. What they are saying is that as those trillions beget many trillions more, there will be many random mutations. Most of those mutations will result in failure. Some will end up as successful as their antecedent. A tiny few will end up with mutations that make them more successful in the current environment. So the mutations happen regardless of anything we do or don't do.
But as we take more countermeasures, it becomes harder for the baseline virions to be successful (because the vaccines work, distancing works, etc.). Any mutations that are only as successful as baseline are going to have trouble spreading. So by default what we will see is more of the mutations that result in things like easier transmission. Because they are better suited to the current environment where people are taking countermeasures. And because baseline is not as successful, the proportion of the new variants will increase over time.
Putting this together, if there's no selection pressure, mutations don't confer any advantage and they will tend to disappear from the population. So through that lens, the new variants wouldn't necessarily have gained a foothold if people weren't using countermeasures. That's not specifically the vaccines, though. Delta is apparently more transmissible generally, and that could also be a response (via random mutation) to social distancing & masking measures.
So the vaccine didn't cause the mutation, but the presence of the vaccine and other human countermeasures shaped the evolution of the vaccine.