PARIS – France is weighing whether to give people who have recovered from COVID-19 only one vaccine dose instead of two, a practice that if enacted here and followed by other countries could free up tens of millions of doses.

“It’s likely that we’ll see similar moves elsewhere, given that we’re facing a shortage of vaccine doses,” said Tobias Kurth, the director of the Institute of Public Health at Berlin’s Charité hospital.

France’s health advisory body has recommended that one shot provides sufficient protection, acting like a booster shot, for previously infected people.

“People who have already been infected retain an immunological memory,” the advisory body said in a release justifying its changed guidance.

The nonbinding guidance still needs to be approved by the French government, which usually follows the body’s recommendations, though it has been hesitant to deviate from standard practices in its coronavirus vaccination campaign.

In a statement last week, vaccine maker Pfizer cautioned that “we do not have any data regarding this approach and our study was designed to dose all participants regardless of previous infection.”

But Florian Krammer, a professor of vaccinology at the Icahn School of Medicine at Mount Sinai in New York, said the one-dose recommendation in France was in line with results from recent research.

A study by Krammer and his colleagues, not yet peer-reviewed, found that previously infected people have an equal or even better antibody response after one dose than individuals who received two shots and had not had covid.

“We found that people who had COVID before mounted a very, very strong and a very quick response after the first shot,” he said.

The Mount Sinai paper’s conclusions were based on limited data from 109 people, of whom over a third used to be infected with the coronavirus.

Separate work published this month by researchers from the University of Maryland, New York University and a group of Israeli researchers has come to similar conclusions.

The NYU team found that the response of previously exposed individuals to a second dose was “muted,” raising the possibility that could be a waste of scarce resources.

“Of course, the official policy right now is to give two doses, but I think that should be up for discussion,” said Krammer. “The data is very strong.”

Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, told The Washington Post last week he is aware of “very recent data” indicating that a single dose could induce “a very good immune response against the virus.”

But he added that “since these data are very recent, I am not aware of any serious discussion” about a change in policy.

All the coronavirus vaccines currently approved by U.S. and E.U. regulators require two doses, though Johnson & Johnson has a single-shot candidate awaiting regulatory assessment.

The United States and the European Union have so far reported a total of about 50 million coronavirus cases, meaning that as many doses could be freed up if the French guidance were applied across the board.

“That’s enough to vaccinate 25 million people, which is more than the population of some smaller countries,” Krammer said.

In its guidance, France’s health advisory body said anyone with the coronavirus should wait at least three months and ideally “closer to 6 months” after the infection to get vaccinated.

The recommendation to wait several months appeared intended to maximize the share of the population with at least some degree of protection, and it was made under the assumption that a prior infection provides a degree of immunity for up to six months, though the exact length remains uncertain.

The same assumption has triggered calls in some countries to move previous COVID patients to the end of the vaccine line.

“To maximally preserve human life, we should prioritize our limited vaccine supply to those who have never had COVID-19,” Marty Makary, a professor at the Johns Hopkins University School of Medicine, wrote in a Washington Post op-ed last month.

The French recommendation represents a more cautious approach. The body said that in terms of vaccinating previously infected people, it does not matter “how long ago the infection was” – an assessment shared by Krammer and his colleagues.

A one-dose vaccination “might still work if you’re a few years down the line from an infection,” he said.

At least one other country appears to be looking into single doses. Howard Njoo, Canada’s deputy chief public health officer, told reporters last week that the country’s national advisory committee on immunizations is “currently discussing” the question.

Some researchers cautioned that a one-dose strategy would pose logistical challenges, including the question how authorities would identify previously infected individuals.

France’s advisory body recommended against antibody tests – traces of infection fade over time – but said either a PCR test or antigen test should be considered valid.

There’s potential for a false positive test to result in some people receiving only one vaccine dose even though they never really had the virus.

Krammer suggested a solution could be to only apply changed guidance to people who tested positive for the virus and showed symptoms at the time.

“I don’t think that we need to worry about false positives in that case,” he said.

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