The change is partly that the effectiveness wanes after a few months and partly that its less effectively again new variants like Delta and Omicron. You can create delta+omicron mRNA vaccines to deal the second problem, but the first problem is much harder--and maybe impossible--to deal with.
This is why we had low cases during the summer of 2021. The vaccine mostly worked at stopping the spread. It no longer does.
The change is partly that the effectiveness wanes after a few months and partly that its less effectively again new variants like Delta and Omicron. You can create delta+omicron mRNA vaccines to deal the second problem, but the first problem is much harder--and maybe impossible--to deal with.
This is why we had low cases during the summer of 2021. The vaccine mostly worked at stopping the spread. It no longer does.