Some experts believe people who have had COVID-19 should get just one shot of two-dose coronavirus vaccines.
This would give these people protection while freeing up vaccine doses for others, they say.
However, other experts warn the approach could be logistically difficult and scientifically "risky."
Experts are divided on whether people who have already had COVID-19 need a full coronavirus vaccine course to protect them from reinfection.
Early studies show these people produce robust immune responses after just one dose of two-dose COVID-19 vaccines, such as those made by Pfizer and BioNTech, AstraZeneca and Oxford University, and Moderna.
Some experts are arguing that waiving the second shot for these people would be safe, and would free up vaccines for others. Effectively, they say a previous COVID-19 infection could act like the first shot. France made this position its official policy on February 12.
Other experts, including Dr. Anthony Fauci, the leading infectious-disease expert in the US, have said it is too early to make this call - but even he says the results of these early trials are "really quite impressive."
More than 115 million people have been infected with COVID-19 worldwide, according to Johns Hopkins University. That figure includes more than 28.8 million people in the US, more than 4.2 million in the UK, and upwards of 3.8 million in France.
"If you could save a dose for every person that has had COVID-19, that's a lot of doses," Lawrence Young, a virologist at the University of Warwick in the UK, told Insider.
Fauci, President Joe Biden's chief medical advisor, told NBC on February 21 that the immunity "boost" for giving a single COVID-19 vaccine shot to people who have had the virus was "enormous," and the data was "really quite impressive."
The US government was therefore "looking very carefully" at whether one dose was enough - but it was too early to make a change yet, Fauci said.
Johnson & Johnson's COVID-19 vaccine, authorized in the US in February, only requires one shot.
Evidence is mounting of single-shot protection for those who have had COVID-19
A US study of more than 100 people showed that those previously infected with COVID-19 had an antibody response 10 times higher after one dose of vaccine than people who hadn't been infected and received two doses.
The study's authors, from Mount Sinai, New York, also said that those who previously had COVID-19 experienced more side-effects after immunization - such as a more painful or redder injection site - than those who hadn't been infected. Giving just one shot could lessen those side effects, they said.
The study, published February 1, was a pre-print, and not yet scrutinized by other experts in a peer review. It was published as correspondence to the New England Journal of Medicine March 11.
Another US group from Maryland medical school found that healthcare workers who had been previously infected had a 500-fold increase in antibody response from baseline at 14 days, after a single shot. "The response was bigger and faster, than those who had not had COVID-19," Mohammad Sajadi, associate professor of Medicine at the Institute of Human Virology at Maryland University, told Insider. The study was published in the Journal of the American Medical Association on March 1.
Initial clinical trials didn't see this phenomenon because volunteers at the time probably hadn't caught COVID-19 before, Sajadi explained.
A UK-based study of 51 health workers published in correspondence to the Lancet medical journal on February 25 reported a 140-fold increase in antibody levels from baseline in people previously infected with COVID-19 after one dose - more than those who hadn't caught the virus before.
Another UK-based study of 72 health workers in London, published in correspondence to the same journal on the same day, showed that those who had a previous COVID-19 infection produced 25 times more antibodies against COVID-19 than people who had not been infected previously, and their T-cell responses - another crucial part of the immune system - were approximately 10 times higher, 21 days after the first dose.
The Mount Sinai researchers said that changing policy to give individuals who have had COVID-19 one dose of vaccine could free up many "urgently needed" doses.
Sajadi also said that this was a solution to vaccine supply shortages that wouldn't take up many resources.
"In the US, 9% of people have been diagnosed with PCR or antigen tests so that would free up a 4-5% increase in vaccine supply without doing any extra screening for antibodies," Sajadi said.
Assuming the total US vaccine supply was 500 million doses by the end of June, this could therefore free up tens of millions of doses.
Sajadi said we don't yet know how long people would be protected after their first shot if they've previously had COVID-19, but he said there was evidence that antibody responses can last up to nine months. Immunity to other coronaviruses wanes over time.
He said that waiving the second dose wouldn't work for everyone. People with weakened immune systems, or low antibody levels after COVID-19 infection - such as older people - would be in a different group. This is the case in France.
The authors of the UK study of 75 health workers, who do not advocate a change in vaccine policy, said that their study suggested those aged over 50 and not previously infected with COVID-19 had a weaker immune response to a single dose of the vaccine.
Sajadi said that it was also not yet clear what advice would be given to people with "long COVID," i.e., people with COVID-19 symptoms lasting more than a few weeks, because the single-dose approach hasn't been tested in this group.
Can't guarantee protection
Other experts are sceptical. "We don't know what type of immunity, duration of immunity or potency of the immunity that two-dose vaccine gives, compared with a single dose on top of past infection," Jonathan Ball, professor of Molecular Virology at the University of Nottingham, told Insider.
"Therefore whilst you might argue that it is a way of saving a dose of vaccine, you can't guarantee it is as much protection as someone that receives both."
Ball said that in terms of "serious side effects," none have appeared to his knowledge for those previously infected with COVID-19 who have received two vaccine shots.
"Scientifically it's not crazy, we don't have the same amount of data as we do with clinical trials [that tested two doses]," Ball said. "It would be a brave decision to suddenly introduce that as policy."
Ball said that where there was a serious supply issue, we may need to try new ways of meeting demand, such as prioritizing those who haven't had COVID-19 for first doses, but we should still ensure that everyone gets two doses eventually.
Professor Paul Morgan, director of the Systems Immunity Research Institute at Cardiff University, told Insider that only giving one shot to those who already had COVID-19 was "a good idea but risky."
Morgan said that immunity varies between individuals. "Some are better prepared to defend themselves than others," he said.
Morgan said that it would be "cumbersome, time-consuming, and a lot of money" to figure out who would be eligible for such a scheme.
Young also said that the mass vaccination effort was already a logistical challenge, and changing the approach could delay the vaccine rollout.
"People won't know whether they have had COVID-19 or not, and we would also have to figure out how much of an immune response they have had," he said.
It is also not clear yet whether the science holds for other, more contagious coronavirus variants.
"That changes the equation," Sajadi said. "The vaccine may have a role in terms of boosting or changing the immune responses in those who have had COVID-19 before, but more data is needed."
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