Is a Single Dose of the Pfizer COVID-19 Vaccine Enough?

As the COVID-19 vaccine rollout continues to lag behind ideal numbers around the world, some health experts are looking at the potential for giving a single dose of the mRNA COVID-19 vaccines, instead of the two-dose regimen.

Two Canadian doctors recently made the argument that the Pfizer-BioNTech mRNA vaccine is effective enough to warrant a single-dose regimen when communities have a limited supply of the shots. Their letter, published in mid-February when Canada was experiencing a delay in vaccine shipments, states that the vaccine had an efficacy of 92.6% beginning two weeks after the first dose, and before the second was administered. The authors based their analysis on documents Pfizer submitted to the FDA.

They pointed out that this high efficacy was similar to the first-dose efficacy of Moderna’s mRNA vaccine, which came in at 92.1%.

“With such a highly protective first dose, the benefits derived from a scarce supply of vaccine could be maximized by deferring second doses until all priority group members are offered at least one dose,” the authors wrote of the Pfizer vaccine. 

They’re not the only ones who have suggested distributing single doses first instead of ensuring everyone have access to the two-dose regimen as intended. And while Canada's National Advisory Committee on Immunization (NACI) hasn't gone as far as suggesting a single dose is all you need, it does recommend extending the window between doses to four months for all three of its currently approved vaccines: Pfizer, Moderna, and AstraZeneca. NACI says one dose of a vaccine is still effective during this time, and such an approach would get more people vaccinated faster.

What We Know About Using a Single Dose 

The U.K.'s public health authority published findings that suggest a single shot of the Pfizer vaccine can cut hospitalizations in adults by around 80% three to four weeks after the shot. The U.K. has delayed giving second doses to citizens in order to give more people a single jab first.

Research out of Israel published in The Lancet found that the first dose of the Pfizer COVID-19 vaccine was 85% effective two to three weeks after a single dose. Another study found that after one dose of the vaccine, between days 14 to 20, it offered 57% effectiveness for symptomatic COVID-19, 74% for hospitalization, and 62% for severe disease.

So does the high efficacy of Pfizer’s vaccine mean that it is time to allow a single dose? In certain circumstances, possibly, Vanessa Raabe, MD, an infectious diseases specialist at the NYU Langone Vaccine Center in New York, tells Verywell, but it’s too soon to roll out this strategy.

“Our data on how well this works is so limited,” Raabe says. “Studies that have been done for a single dose were not done in a clinical trial; they were done in a real-world setting and only looked really at the first few weeks after the first vaccine dose.”

This means what happens in the months following a single dose, and how well protection lasts, is not known. What we do know is how much protection is offered after the two-dose regimen that was studied in clinical trials, she says. 

“I think the data out there certainly makes it worth investigating further—I think it's a promising signal and it needs some follow up—but I have a little bit of caution relying on the limited data we have for that [single dose] compared to the nine months or so of data that we have on the two-dose regimen,” Raabe says.

A Single Dose May Benefit Some

That being said, there are possibly some circumstances where a single dose may prove to be beneficial. France recently said that a single dose of an mRNA vaccine like the Pfizer or Moderna options may work for people previously infected with COVID-19.

People previously infected likely have some immunity and the single dose can act as additional protection or a “booster.”

Raabe says for patients who recovered from COVID-19, a single dose may be warranted if access to both doses is in short supply. She says there are a number of preprint papers from different groups showing that people who had COVID-19 show immune responses after a single dose that are at least as high—if not higher—than people who have not had COVID-19.

“This is all based on antibody [data] and data from the immune response, rather than actual efficacy data, which takes a much longer time,” Raabe says. “You have to study a much larger group for a much longer time to know, but just looking at purely the immune responses…based off of the data that's available, it would be something to think about as a reasonable strategy.”

Further Muddling Vaccine Rollout

Timothy C. Y. Chan, PhD, a professor of industrial engineering at the University of Toronto and the Canada research chair in Novel Optimization and Analytics in Health, tells Verywell that a single dose of mRNA vaccines is an idea “worth exploring” when supplies are limited or supply chains are interrupted. 

Still, it’s not a one-size-fits-all approach and there are logistical issues to consider. A regimen that differs from the one laid out by the vaccine manufacturer could add a level of confusion or disorganization to vaccine efforts, he says. 

“It does add a bit of an additional challenge when we're thinking about keeping track of who already has had one dose and making sure they come back…to get the second one,” he says. “What if someone misses their [second] appointment, and they come back a week later and say, ‘Oh, I'd like my shot now.’? You'd have to decide, do you give it to this person? Or do you give it to the person who's getting their first shot?”

Logistical issues aside, Raabe says a short delay between the recommended timing between the first and second dose of the Pfizer vaccine may not be cause for major concern, but pushing weeks or months out? It’s too early to tell what effects that may have. We don’t have good data for what happens when you significantly delay the second dose, she says. 

“But one of the positive things about what we’re seeing from single-dose studies is that for the people who maybe had side effects from their first dose, who aren't getting their second dose of the vaccine, they probably have some protection,” she says. “How long does that last compared to the two doses? We don't know. But I think it’s reassuring…that you probably still have some partial protection.”

Above All, the Vaccines are Safe

The important message that Raabe says shouldn't get lost in larger conversations around dosing is how safe and important the vaccines are. Without them, we cannot combat COVID-19 and end the pandemic.

“The vaccine trial process for these [COVID-19 vaccines]—even though the timeframe has gone by quickly—is exactly the same as what we would do in clinical trials for all the other vaccines,” she says. 

“There are so many layers of safety protection built-in, including all the protocols that are done, and in the U.S., vaccines also have to be approved by the FDA," she says. "They work very well, and we are in a public health emergency.”

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