The prospect of a fourth wave of coronavirus, with new cases skyrocketing in the upper Midwest, has sparked renewed debate among vaccine experts about how long to wait between first and second doses. Extending this period would quickly increase the number of people with partial protection from a single shot, but some experts fear that this could also lead to dangerous new variants.

In the United States, two-dose vaccines are three to four weeks apart, which is what has been tested in clinical trials. In the UK, however, health officials have postponed the dosage by up to 12 weeks in order to reach more people faster. And in Canada, where vaccines are few and far between, a government advisory council recommended on Wednesday that the second dose be delayed even longer, up to four months.

Some health professionals believe the United States should follow suit. Dr. Ezekiel J. Emanuel, co-director of the Healthcare Transformation Institute at the University of Pennsylvania, has suggested that all U.S. vaccines should go to people on their first dose in the next few weeks.

“That should be enough to suppress the fourth surge, especially in places like Michigan like Minnesota,” he said in an interview. Dr. Emanuel and his colleagues posted the proposal in USA Today on Thursday.

However, opponents, including health advisors to the Biden government, argue that delaying dosing is a bad idea. They warn that the country will be prone to variants – those that are already in circulation, as well as new ones that could develop in the bodies of partially vaccinated people who are unable to fight off infection quickly.

“Postponing the second dose to a later date is a very dangerous suggestion,” said Dr. Luciana Borio, the former acting chief scientist of the Food and Drug Administration. Dr. Anthony S. Fauci, the nation’s leading expert on infectious diseases, agreed. “Let’s move on to what we know is the optimal level of protection,” he said.

The cornerstone for the debate was laid in December when clinical studies first gave scientists a good look at how vaccines work. For example, in the clinical trial for the Pfizer BioNTech vaccines, volunteers enjoyed robust protection from Covid-19 two weeks after the second dose. But just 10 days after the first dose, the researchers found that the volunteers got sick less often than those who received the placebo.

In the same month, the UK saw a surge in cases caused by a new, highly communicable variant called B.1.1.7. After the UK government approved two vaccines – from Pfizer-BioNTech and AstraZeneca – it decided to combat the variant by delaying the second dose of both formulations by 12 weeks.

In January, some researchers campaigned for the United States to follow Britain’s lead.

“I think right now, before this surge, we need to take as many single doses as possible in as many people over 65 as possible to reduce the serious illness and deaths that will occur in the coming weeks,” said Michael T. Osterholm of the University of Minnesota said “Meet the Press” on NBC’s Jan. 31st.

But the government stayed on track, arguing that it would be unwise to venture into the unknown in the middle of a pandemic. Although the clinical trials showed early protection from the first dose, no one knew how well this partial protection would last.

“When you’re talking about doing something that can do real harm, you need empirical data to back it up,” said Dr. CĂ©line R. Gounder, Infectious Disease Specialist at the Bellevue Hospital Center and member of the Coronavirus Advisory Service for Mr Biden Tafel. “I don’t think you can make your way out of it logically.”

Over the past few weeks, however, those in favor of dosing delay have been able to point to mounting evidence suggesting that an initial dose can provide effective protection that lasts for several weeks.

Updated

April 9, 2021, 9:38 a.m. ET

The Centers for Disease Control and Prevention reported that two weeks after a single dose of the Moderna or Pfizer BioNTech vaccine, a person’s risk of developing coronavirus infection was reduced by 80 percent. And researchers in the UK have found that the first dose provides protection for at least 12 weeks.

Dr. Emanuel argued that the UK’s campaign to get more people first doses played a role in the 95 percent drop in cases since their peak in January. “It was pretty breathtaking,” said Dr. Emanuel.

He cites such data as further evidence that the United States should extend vaccination. He and his colleagues estimate that if the country had used a 12-week schedule from the start of its introduction, by April 5 an additional 47 million people would have received at least one dose.

Sarah E. Cobey, an epidemiologist and evolutionary biologist at the University of Chicago, said the United States had lost a valuable opportunity to save many lives with such a strategy. “We missed a window and people died,” she said.

But even now, said Dr. Emanuel, it is worth postponing the dosage. The United States issues about three million vaccines every day, but nearly half goes to people who have already received a shot. All of the nation’s offering, he argued, should go to first-timers instead.

If so, according to his team’s calculations, it would take the United States two or three weeks to catch up with Britain. The extra protection would not only save the lives of those vaccinated, it would also help reduce the transmission of the virus to people who are not yet protected.

Still, some scientists say it is premature to acknowledge the belated vaccination schedule for the decline in cases in the UK.

“They did a couple of other things like shutdown,” said Dr. Fauci.

“I think the real test will be whether we see a rebound in cases where the UK reopens.” Said Dr. Gounder.

Rather than experimenting with vaccination schedules, critics think it wiser to take basic preventive measures like wearing masks seriously. “It is crucial that we don’t just rejoin a big national party,” said Dr. Borio.

You and others are also concerned about recent studies showing that a single dose of Moderna or Pfizer-BioNTech doesn’t work as well against certain variants like B.1.351, which were first found in South Africa.

“Relying on a dose of Moderna or Pfizer to stop variants like B.1.351 is like using a BB gun to stop a charging rhino,” said John P. Moore, virologist at Weill Cornell Medicine.

Dr. Moore said he also feared delaying dosing could encourage the spread of new variants that vaccines can better resist. When coronaviruses multiply in the bodies of some vaccinated people, they can acquire mutations that allow them to evade the antibodies produced by the vaccine.

But Dr. Cobey, who studies virus evolution, said she wasn’t worried about delayed doses that produce more variants. “I would bet my money on it, with the opposite effect,” she said.

Last week, she and her colleagues posted a comment in Nature Reviews Immunology to defend the delay of doses. Vaccinating more people – even with moderately less protection – could curb the spread of the virus in a community more than if fewer people had more protection, they said. And that decline wouldn’t just mean more lives were saved. Variants would also have a lower chance of showing up and spreading.

“There are fewer infected people who can have variants,” she said.

Dr. Adam S. Lauring, a University of Michigan virologist who was not involved in the comment, said he felt that Dr. Cobey and her colleagues had come up with a compelling case. “The arguments in this piece really agree with me,” he said.

While the United States is unlikely to change course, its northern neighbor has adopted a delayed strategy to deal with a booming pandemic and vaccine shortage.

Dr. Catherine Hankins, a public health specialist at McGill University in Montreal and a member of Canada’s Covid-19 Immunity Task Force, approved this decision based on the emerging evidence for single doses. And she said that she thought other countries facing even worse deficits should consider this too.

“I will advocate, on a global level, that countries look closely at Canada’s strategy and think seriously about it,” said Dr. Haskins.