If most of the population is vaccinated against COVID-19, then the coronavirus wouldn’t have enough new hosts and the pandemic would burn out. That transformational phenomenon is known as “herd immunity.”

But the United States, like so many other countries, has limited vaccine supplies, slower than expected distribution, and relentlessly rising case numbers.

That’s why some public health experts are proposing a gamble to achieve widespread immunity as quickly as possible: Instead of the approved regimen of two doses about a month apart, they suggest giving only one dose, or extending the interval between doses, or giving half-doses, or mixing vaccines from different manufacturers.

The U.S. Food and Drug Administration on Monday said such deviations are “premature,” unstudied, and could wind up “placing public health at risk.” Meanwhile, in Britain, where a coronavirus variant that seems more transmissible has taken hold, the government is allowing changes to approved vaccine protocols.

Here are the knowns — and unknowns — shaping the latest chapter in the pandemic.

Why is vaccine distribution so slow?

Last month, the FDA authorized emergency use of two vaccines, one developed by Pfizer-BioNTech, the other by Moderna. Both use novel mRNA technology, and both conferred 95% protection against coronavirus infection after two shots given three or four weeks apart.

Currently, frontline health workers and nursing home residents — the top priority groups totaling about 24 million people — are eligible to get the shots.

The federal government projected that 20 million Americans would be vaccinated by the end of 2020. Instead, about 17 million doses have been delivered by manufacturers, and only 4.8 million have been injected into arms, according to data published Wednesday by the Centers for Disease Control and Prevention.

Among the many bumps in the rollout: Holiday interruptions. The logistics of keeping the Pfizer vaccine at ultracold temperatures. Vaccine refusers. Bureaucracy. Staffing shortages.

In the state of New York, the initial phase has been so slow that Gov. Andrew Cuomo on Monday ordered hospitals to pick up the pace or face fines. “You have the allocation; we want it in people’s arms as soon as possible,” he said at a news briefing.

» READ MORE: Your vaccine questions, answered

Is it better to give more people one dose or fewer people two doses?

Given that the U.S. is struggling to administer the vaccine it already has, some observers say the problem is not vaccine production.

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

But others, pointing to surging cases and swamped hospitals, argue that the goal of vaccination should be to provide partial protection to as many people as possible as quickly as possible.

“As Mike Tyson famously said, ‘Everybody has a plan until they’ve been punched in the mouth.’ When it comes to COVID-19, we’re being punched in the mouth over and over again,” prominent public health researchers Robert M. Wachter and Ashish K. Jha wrote in a Washington Post opinion piece. “It’s time to change the plan; namely, we should give people a single vaccination now and defer their second shot until more doses of vaccine become available.”

» READ MORE: When can I get the COVID-19 vaccine?

They contend that giving 100 million people — especially those at risk of severe COVID-19 — one dose that provides substantial protection “will save more lives than giving 50 million people two shots that are 95% effective.”

In clinical trials, both the Pfizer-BioNTech and Moderna vaccines began warding off infection a few weeks after the first shot. But since almost all participants got their second dose on schedule, it is not known how much protection one dose confers, or how long that protection lasts.

Pfizer-BioNTech spokespersons have stressed those unknowns.

“Although data from the [pivotal] study demonstrated that there is a partial protection from the vaccine as early as 12 days after the first dose, there is no data to demonstrate that protection after the first dose is sustained after 21 days,” the companies said in a statement to CNBC News.

Steven Joffe, a pediatric oncologist and bioethicist at the University of Pennsylvania, said, “Those unknowns are why some people say, ‘We should stick with what we know. By all means, do the trials to test [varied regimens], but don’t just wing it.’ Others say, ‘We are in a race against the virus.’ "

He added, “I’m not going to come down on one side or the other.”

Could the vaccination plan make the pandemic worse?

Paul Offit, a Children’s Hospital of Philadelphia pediatrician who specializes in infectious diseases and vaccines, has come down hard against winging it.

Even with two vaccine doses, a small number of participants in the Pfizer and Moderna clinical trials got sick with COVID-19. Offit fears that if lots of vaccinated people were to get sick because they didn’t get the proven vaccine regimen, then the consequences could undermine the whole immunization effort. Virus transmission might not be slowed, which might increase the number of vaccine refusers.

“What you’ve done then is shaken what is already a fragile vaccine confidence in this country,” Offit, who serves on a panel that advises the FDA on vaccines, told the Cut, an online magazine. “I saw someone on CNN say, ‘You don’t want perfect to be the enemy of the good,’ and I’m all for that. The problem here is that you don’t want the unknown to be the enemy of the known.”

The FDA echoed that sentiment.

“If people do not truly know how protective a vaccine is, there is the potential for harm because they may assume that they are fully protected when they are not and alter their behavior to take unnecessary risks,” FDA Commissioner Stephen M. Hahn and Peter Marks, director of the center for biologics evaluation, wrote in the statement issued Monday.

More vaccines, including ones by Astra-Zeneca and Johnson & Johnson, are on track to seek emergency authorization from the FDA this year. The agency has vowed not to cut corners.

“We have committed time and time again to make decisions based on data and science,” Hahn and Marks wrote. “Until vaccine manufacturers have data and science supporting a change, we continue to strongly recommend that health care providers follow the FDA-authorized dosing schedule for each COVID-19 vaccine.”