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

Pfizer and Moderna have enough doses to vaccinate up to 6% of the United States population against COVID-19 by the end of the year.

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A Uzbek doctor shows a dose of the Pfizer COVID-19 vaccine.

But a simple change would double the number of people receiving the vaccine. Instead of receiving two doses, the first in line could receive one. A single injection is not as effective, but it allows the immune system to defend itself against a certain amount of coronavirus infection. Distributing the supply would give millions of people more protection and probably save more lives – at least in the short term.

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“It is unprecedented that this level of mortality happens daily,” said Christopher Gill, an infectious disease specialist at Boston University, referring to the current daily mortality rates for COVID-19. “This is the time to think about spreading the vaccine as far as possible and covering as many nursing home residents and health professionals as possible in the beginning 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 initial injection of the Moderna vaccine takes two weeks to elicit an immune response, and recipients receive a booster dose 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 New York Times. After the start of the second dose, the vaccine is 94 percent effective. “Two doses were better, but not much better,” says Gill. It is less clear how a single dose of the Pfizer vaccine compares to a double dose, which is 95% effective – but Gill says the data suggest that an injection is about 90% effective.

© Golib Tolibov / Photos of the deposit
A Uzbek doctor shows a dose of the Pfizer COVID-19 vaccine.

Some experts are suspicious of this proposal, but not because of the difference in effectiveness. They cannot suggest different dosing guidelines without hard evidence. “We don’t know anything about how long or how strong the immune response would be for a single [Pfizer or Moderna] vaccine, ”Barry Bloom, a public health expert at the Harvard School of Public Health TH Chan, told a news conference.

Without studies, experts cannot predict when the protection of a single dose will end. Six months later, vaccinated people can go to crowded areas on the assumption that they are safe when their protection ends, and health officials will not know until it is too late. “If scientists start to guess what the evidence should be, instead of being based on evidence, it could save more lives in the short term,” said Bloom. “But when it does, we have a very complicated problem.”

William Hanage, an epidemiologist at the Harvard School of Public Health TH Chan, recommends waiting for a clinical trial to make any dose changes. “After collecting the evidence, we would be in a better position to make that kind of recommendation,” he said at the same news conference as Bloom. But Gill, at Boston University, fears that waiting for a trial could cost lives. “We cannot afford to wait another six months,” he says. “We have to be content with the information we have.”

If healthcare professionals went ahead with a single dose without the data to prove it, Hanage fears that the public may be more reluctant to receive a COVID-19 vaccine. Some people are already concerned that companies are rushing their vaccine tests and a vaccine regimen that is not based on evidence could further undermine confidence. And while the first recipients of the vaccine may be boosted when supplies are less limited, it is difficult to convince people to return for their second injection, especially when follow-up takes months or years. To that end, the best way to spread COVID-19 vaccines may be to develop more of them and use them as suggested by the test labels.