A COVID-19 vaccine developed by Moderna Inc. and the National Institutes of Health, shown here, is one of three vaccines in Phase 3 trials in the U.S. With development and testing underway, health officials are asking states to prepare for limited distribution of a potential vaccine as soon as this fall — though some experts say that’s too early.

FILE – In this Monday, July 27, 2020 file photo, a nurse prepares a shot as a study of a possible COVID-19 vaccine, developed by the National Institutes of Health and Moderna Inc., gets underway in Binghamton, N.Y. Who gets to be first in line for a COVID-19 vaccine? U.S. health authorities hope by late next month to have some draft guidance on how to ration initial doses, but it’s a vexing decision. (AP Photo/Hans Pennink)