Why two doses of the new COVID-19 vaccines are better than one

Pfizer and Moderna have enough doses to vaccinate up to 6 percent of the U.S. population against COVID-19 by the end of the year.

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An Uzbek doctor is holding a dose of the Pfizer COVID-19 vaccine.

But a simple change would double the number of people getting the vaccine. Instead of getting two doses, those who queue first can get one. A single shot isn’t that effective, but it does allow the immune system to defend itself against some degree of infection from the coronavirus. Spreading the supply would give millions of people more protection and probably save more lives – at least in the short term.

Charging error

“It is unprecedented for this death rate to occur on a daily basis,” said Christopher Gill, an infectious disease specialist at Boston University, referring to the current daily death rate from COVID-19. “Now is the time to think about spreading the vaccine as far as possible and treating as many nursing home residents and health workers as possible as early as possible to reduce mortality.”

Moderna and Pfizer are not testing how one dose of their vaccines compares to two, but experts can read between the lines of the available data. The first shot of the Moderna vaccine takes two weeks to elicit an immune response, and recipients are boosted 28 days later. In the meantime, the vaccine is 92 percent effective in preventing symptomatic COVID-19, epidemiologist Michael Mina and columnist Zeynep Tufecki wrote in the New York Times. After the second dose starts, the vaccine is 94 percent effective. “Two doses were better, but not noticeably better,” says Gill. It’s less clear how a single dose of the Pfizers vaccine stacks up against a double dose, which is 95 percent effective, but Gill says the data suggests one injection is about 90 percent effective.

© Golib Tolibov / Deposit photos
An Uzbek doctor is holding a dose of the Pfizer COVID-19 vaccine.

Some experts are wary of this suggestion, but not because of the difference in efficacy. They cannot propose different dosage guidelines without hard evidence. “We don’t know about how long or how strong a single’s immune response would be [Pfizer or Moderna] vaccine, ”Barry Bloom, a public health expert at Harvard’s TH Chan School of Public Health, said at a news conference.

Without studies, experts cannot predict when protection will run out with a single dose. Six months later, vaccinated people might go to crowded areas believing they are safe once their protections are really gone, and health officials wouldn’t know until it’s too late. “If scientists start to guess what the evidence should be, instead of building on evidence, it could save more lives in the short term,” Bloom said. “But if it goes away, we’ll get into a really sticky problem.”

William Hanage, an epidemiologist at Harvard’s TH Chan School of Public Health, recommends waiting for a clinical trial to change the dose. “Once we gather the evidence, we would be in a better position to make those kinds of recommendations,” he said at the same press conference as Bloom. But Boston University’s Gill fears that waiting for a trial will cost lives. “We don’t have the luxury of waiting another six months,” he says. “We have to make do with the information we have.”

If health care providers continued on a single dose without the data to back it up, Hanage fears the public will be reluctant to get a COVID-19 vaccine. Some people are already concerned that companies are rushing their vaccine trials, and an unsupported vaccination regimen could further undermine confidence. And while early vaccine recipients could get a booster if supplies are less limited, it’s hard to convince people to come back for that second injection, especially if the follow-up is months or years later. To that end, the best way to distribute COVID-19 vaccines may be to develop more of them and use them as their tested labels suggest.