Other vaccine developers—including Operation Warp Speed participants Janssen (owned by Johnson & Johnson), AstraZeneca, Novavax, Sanofi and Merck & Co.—are closely watching to see which path is taken.

They are in a race against time—a race that may not end well for those running late in getting their vaccine out. And halting those efforts could hurt billions of people elsewhere in the world whose lives and livelihoods will depend on the arrival of plentiful, cheap vaccines.

One problem is finding willing test subjects. As increasing numbers of Americans are vaccinated, and the virus recedes from our shores, “the fewer the number of people eligible to participate in trials,” said Susan Ellenberg, professor of biostatistics at the University of Pennsylvania.