The mRNA-1273 vaccine against the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) elicited high neutralizing-antibody titers in phase 1 trial participants and has been shown to be highly efficacious in preventing symptomatic Covid-19 disease and severe disease. The emergence of SARS-CoV-2 variants in the United Kingdom (the B.1.1.7 lineage), South Africa (the B.1.351 lineage), Brazil (the P.1 lineage), and California (the B.1.427/B.1.429 lineage) has led to concerns about increased transmission and the potential of these variants to circumvent immunity elicited by natural infection or vaccination. The recent identification in the United Kingdom of a B.1.1.7 variant that includes the E484K mutation (B.1.1.7+E484K) furthers these concerns.