The UK is making a risky bet to stretch its supply of coronavirus vaccines, and scientists are split on the untested strategy

• UK officials announced Tuesday that they would prioritize getting people their first doses of coronavirus vaccines instead of holding enough to ensure they can get a second dose on time.
• Pfizer's coronavirus vaccine is supposed to be given as two injections 21 days apart.
• The new guidelines allow people to wait up to 12 weeks between receiving their first and second doses.
• The UK is the first country to publicly commit to the untested strategy, although some scientists in the US had also advocated for a single-dose strategy.

Scientists are split on the UK's plan to give initial doses of coronavirus vaccines to as many people as possible, even at the risk of delaying their booster shots.

UK officials announced Tuesday that the country would prioritize getting its population first doses of the coronavirus vaccine instead of holding enough for a second dose. The idea is to boost the number of people getting vaccinated with the country's initial supply of the Pfizer and BioNTech vaccine.

"At this stage of the pandemic, prioritising the first doses of vaccine for as many people as possible on the priority list will protect the greatest number of at risk people overall in the shortest possible time," UK officials said in a statement Tuesday.

The vaccine is about 95% effective at preventing COVID-19 when given as two doses 21 days apart, and that's how it's being administered in the US. The UK's strategy runs the risks of leaving people vulnerable to the coronavirus, because no one has tested what happens when doses are given farther apart.

Read more: One striking chart shows just how well Pfizer's vaccine works to prevent COVID-19 infections

"We don't have the data on the efficacy of one dose. It might be too long to wait for a second dose," Dr. Paul Offit, a University of Pennsylvania professor of vaccinology and director of the Vaccine Education Center, told Business Insider. "That's why you do the studies. It hasn't answered those questions."

UK officials released guidelines on Tuesday that allow people to wait up to 12 weeks between doses.

On Wednesday, the British Medical Association released a statement calling the strategy "unreasonable and totally unfair" to at-risk patients who were previously scheduled to receive a second dose.

"The decision to ask GPs, at such short notice, to rebook patients for three months hence, will also cause huge logistical problems for almost all vaccination sites and practices. For example, to make contact with even just two thousand elderly or vulnerable patients will take a team of five staff at a practice about a week, and that's simply untenable," chair of the BMA GP committee, Dr. Richard Vautrey wrote.

Scientists around the globe have advocated for similar strategies, arguing that some limited data shows that the initial shot provides at least some protection, and that the aim should be to vaccinate people as quickly as possible.

Dr. Scott Gottlieb, a former US Food and Drug Administration commissioner and current Pfizer board member, and University of Pennsylvania emeritus professor of pediatrics Dr. Stanley Plotkin are among those who say the US should vaccinate more people more quickly.

"I feel very strongly that we should get as many shots in arms as possible, right away. The reality is that one dose is partially protective," Gottlieb wrote in a USA Today op-ed on December 7.

Plotkin agreed, saying that providing some protection for a larger percentage of the population is better than none at all. However, he said he still supports providing two full doses to high-risk individuals to ensure they are as protected as possible.

"At this point, it's clearly an insufficient supply for the larger population," Plotkin told Business Insider. "If we insist on two doses, that will mean we'll be vaccinating half as many people as if we use one dose."

Scientists want more data to understand the implications of leaving more time between doses

Because the strategy remains untested, both Plotkin and Offit emphasized how difficult it is to measure the effectiveness rates of a single dose over a period of several weeks. It could be as high as 60% or 70%, they estimated based on previous trials, but that remains far below the 95% efficacy for people who get both doses.

Pfizer said in a statement that decisions on dosing are up to health authorities, but that there's no data to show how the shot works if people don't get their second injection on time.

"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," Pfizer and BioNTech said.

Clear messaging could help boost public confidence

The nuance of measuring vaccine effectiveness is particularly crucial to boost the public's confidence in the vaccine. Too many changes in strategy with mixed messages have already undermined the public's trust in the vaccine, an issue health officials have been working to combat.

"There's fragile vaccine confidence, and if it looks like it's been done haphazardly, which it does, you could lose the faith of the public," Offit said.

Even so, the benefit of providing people some protection far outweighs the risk, according to Plotkin. That protection could help the UK and US control the pandemic, Plotkin said in a letter to Centers for Disease Control and Prevention officials and other public health experts overseeing the US coronavirus response. He shared a copy with Business Insider.

Offit said he sees the appeal of such a strategy given the current strain on supplies, but added that he's hesitant to back the strategy in the US until it can collect and analyze additional data.

"It's a troubled time. It's a pandemic that has brought us to our knees. We want to act quickly, but we need the trials," Offit said.

This story was updated on December 31 to include a statement from the British Medical Association.