There is a good article in Nature explaining the research behind the South African COVID-19 variant which has changed its spike protein shape so much by mutations that antibodies from people infected earlier in the pandemic no longer bind to the virus to neutralize it.
Investigators have now used the serum from those vaccinated with the Moderna RNA virus and found a reduction in the ability of the antibodies to work but there are other factors in the immune process that could help provide protection and stimulate new antibodies faster in those vaccinated.
Antibodies are only part of the puzzle. Those vaccinated could still have a better outcome than those who aren’t vaccinated but this has to be determined by following the clinical course of those vaccinated and get COVID-19 with the new South African strain. When will the virus change so much that we have to call it COVID-21?