But in Britain, doctors have been told to postpone appointments for second doses that had been scheduled for January, so that those doses can be given instead as first shots to other patients. Officials are now pushing the second doses of both the Pfizer-BioNTech and Oxford-AstraZeneca vaccines as far back as 12 weeks after the first one.

In a regulatory document, British health officials said that AstraZeneca’s vaccine was 73 percent effective in clinical trial participants three weeks after the first dose was given and before the second dose was administered. (In cases in which participants never received a second dose, the interval ended 12 weeks after the first dose was given.)

But some researchers fear the delayed-dose approach could prove disastrous, particularly in the United States, where vaccine rollouts are already stymied by logistical hurdles and a patchwork approach to prioritizing who gets the first jabs.

“We have an issue with distribution, not the number of doses,” said Saad Omer, a vaccine expert at Yale University. “Doubling the number of doses doesn’t double your capacity to give doses.”