On Dec 18, 2020, the US Food and Drug Administration issued an Emergency Use Authorization (EUA) for the Moderna COVID-19 vaccine, adding to the earlier EUA for the Pfizer-BioNTech COVID-19 vaccine Although trial evidence for both vaccines indicates partial protection against COVID-19 illness after 1 dose, the vaccines are authorized only as 2-dose series and have not yet been evaluated for single-dose use In the US, distribution plans for initial supply of vaccine doses withhold half of the available supply for second doses to be administered weeks later With COVID-19 surging, there are important tradeoffs to consider between the health costs of deferring benefits of earlier protection for half of people who could be vaccinated from initial supply, weighed against risks of possible vaccine supply disruptions that could delay receipt of second doses in the absence of sufficient reserves