A growing number of researchers agree that people who have already had COVID-19 need only one shot of the two-dose Moderna or Pfizer vaccines for sufficient protection. Giving these people a second dose would be a waste of vaccine, they say.

“For those who’ve been infected and recovered, which is tens of millions of people, they’ll only need one shot, which will make the vaccine go even further,” said Dr. James Hildreth, a prominent immunologist and CEO of Meharry Medical College, according to Business Insider.

Hildreth was on the Food and Drug Administration committee that gave emergency use authorization to the current vaccines including the one-dose Johnson & Johnson shot. If healthcare officials tweak vaccine recommendations to require only one dose for the 29 million Americans who have already had COVID-19, that would free up 15 million doses for others.

When a person contracts COVID-19, their immune system mounts a response by producing antibodies that recognize and defeat the invading virus should it strike again. Therefore, in a person who has already generated an immune response to SARS-CoV-2, the virus that causes COVID-19, one dose of a vaccine should be enough to manufacture a quicker and stronger defense, says Insider. Many studies have supported this theory.

In a letter published in The New England Journal of Medicine, researchers said that study subjects who already had SARS-CoV-2 antibodies “before the first vaccination injection rapidly developed uniform, high antibody titers within days of vaccination.”

Furthermore, the researchers from the Icahn School of Medicine at Mount Sinai found that the antibodies of people who had COVID-19 and received both mRNA vaccine doses had antibody levels that were several times higher than people who had two doses of the vaccine and did not previously have the illness.

The scientists concluded that the results of their studies require further investigation to see if a single-dose strategy should be considered for those previously infected with the coronavirus.

Another study, available in preprint, also found that healthcare workers who had COVID-19 “showed clear secondary antibody response to vaccination,” and authors suggested that a single-dose strategy would be sufficient. They also noted that people who have had the disease can be safely placed lower on the vaccination list since they already had protective levels of antibodies.

A British study also found that after a single dose of vaccine the antibody levels of people with prior COVID-19 was a whopping 140 times higher than before the shot. Yet, according to Insider, most countries still advocate two doses for everyone.

“This is where policy lags science,” said Akiko Iwasaki, a professor at Yale University. She said that people could have antibody tests before getting vaccinated if they had or suspected they had COVID-19 to show that their immune systems are still up for the fight. If their antibody levels are high, the second shot would be redundant, according to Insider.

“What is the point,” said Iwasaski, a leading immunologist. “It’s kind of a wasted shot.”

Dr. Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, told NBC News' Meet the Press in February that the idea of a single dose is worth investigating further.

“The data looked really quite impressive — that if you’ve been infected and then you get a single dose, the boost you get with that single dose is really enormous,” said Fauci. “That is one thing that you might want to consider, but we really want to look carefully at the data first.”