TLDR: We knew the mRNA vaccines were effective at preventing serious disease in late 2020. We didn't know for sure (publicly) if the mRNA vaccines reduced transmission rates until mid 2021, as more research concluded. Mid 2021 is when it was demonstrated that mRNA vaccines reduce transmission of the variants of SARS-CoV-2 at the time. As SARS-CoV-2 continues to mutate, more research needs to be done.
Delta was named at the end of May 2021 and started to become the dominant strain worldwide in the following months. As shown in the chart under ”Prevention” (and described in the linked pages), Delta significantly reduced vaccine effectiveness against infection and symptomatic disease, though not against severe disease.
However, it’s also true that vaccine effectiveness, regardless of variant, significantly declines over the months following the dose - again, much less so for severe disease than for any symptomatic disease.
