New data from Moderna offer hope that booster shots against Covid-19 could become at least somewhat more effective than they already are. But the data also point to how difficult it could be to determine exactly which Covid shots to give as annual boosters.
On Tuesday Moderna released data testing a booster shot that is bivalent, meaning it contains equal amounts of vaccine from two different strains of the virus. This booster, currently known by the code number mRNA-1273.211, contains equal mRNA amounts of ancestral SARS-CoV-2 and spike proteins from the Beta variant of SARS-CoV-2, which originally emerged in South Africa. It does not contain vaccine targeted specifically against the Delta or Omicron variants that caused the most recent waves of Covid-19.