UK's new COVID-19 vaccination plan prioritizes 1st doses. What does that mean?

In a major departure from vaccine rollouts around the world, the United Kingdom announced this week it would change its vaccine distribution plan to prioritize giving first doses of the COVID-19 vaccine, even if it means delaying second doses.

"Everyone will still receive their second dose and this will be within 12 weeks of their first. The second dose completes the course and is important for longer term protection," the U.K. government said in a statement. "With two vaccines now approved, we will be able to vaccinate a greater number of people who are at highest risk, protecting them from the disease and reducing mortality and hospitalization."

Professor Sarah Gilbert of Oxford University, a lead researcher for the Oxford/Astra Zeneca vaccine, said that part of the decision to change the vaccine distribution plan was the severity of the outbreak in the U.K., where more than 22,000 people are currently hospitalized, according to the National Health Service.

Because hospitals are struggling, "it's been recommended that the first dose should be given to as many at-risk people as possible," Gilbert said. "Then come back three months later and give them their second dose, which will maintain the response for a good long period of time." The new plan "enables getting a lot of people vaccinated quickly so that we can start to make some impact," she added.

But since clinical trials were developed around a two-dose regiment, there's not sufficient data about how much immunity a single dose provides.

"Pfizer and BioNTech’s Phase 3 study for the COVID-19 vaccine was designed to evaluate the vaccine’s safety and efficacy following a 2-dose schedule, separated by 21 days," Pfizer told MarketWatch.

"The safety and efficacy of the vaccine has not been evaluated on different dosing schedules as the majority of trial participants received the second dose within the window specified in the study design."

Scott Gottlieb, the former commissioner of the Food and Drug Administration, who now sits on Pfizer's board, expressed support for prioritizing first doses weeks before the U.K. announcement.

"I feel very strongly that we should get as many shots in arms as possible, right away," Gottlieb told USA Today in early December. "The reality is that one dose is partially protective," he said. "I don't think we should be holding on to supply now, anticipating that something goes wrong."

In contrast, Dr. Anthony Fauci expressed caution in an interview with NBC's "Today Show" on Thursday. From clinical trials, he said, "we know that the optimal time to give the second doses is 28 days after the first dose for Moderna and 21 days for Pfizer."

"If you want to stick with the data, that's the way you should do it," Fauci said. "You can make an argument, and some people are, about stretching out the doses and giving a single dose and hoping you're going to get a second dose in time."

The U.K.'s chief medical officers acknowledged potential downsides of the new plan in a statement on Thursday. "As with all decisions during this pandemic it is about balance of risks and benefits," they wrote. "We recognize that the request to reschedule second appointments is operationally very difficult, especially at short notice, and will distress patients who were looking forward to being fully immunized," they added. But with COVID-19 running rampant in U.K. communities, they had to act with speed, they said. "We believe the public will understand and thank us for this decisive action."

Given that the United States has fallen far short of its goal of vaccinating 20 million Americans by the end of 2020, it's not clear that the U.S. is ready for a plan that involves hitting tight vaccination deadlines.

Still, Fauci added, the first dose priority plan is "under consideration."

Any change to the U.S. distribution plan would be at the discretion of the Department of Health and Human Services and Operation Warp Speed.

ABC News has yet to receive a request for comment from Pfizer.

ABC News' James Longman and Zoe Magee contributed to this report.

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