Data from phase 3 clinical trials of messenger RNA (mRNA) vaccines through November 2020 showed 94.1% efficacy for the prevention of symptomatic severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection at 14 days after the second dose of the mRNA-1273 vaccine (Moderna)
1 and 95% efficacy at 7 days after the second dose of the BNT162b2 vaccine (Pfizer).
2 Since the results of these trials were published, a nationwide surge in coronavirus disease 2019 (Covid-19) has been noted, SARS-CoV-2 variants with increased infectivity have emerged, the Food and Drug Administration has granted emergency use authorization for these two mRNA vaccines, and vaccination has been initiated across the United States.