A Single Vaccine Dose Appears To Protect Against COVID-19. So Why Are We Giving Two?

Two vials of Pfizer’s COVID-19 vaccine at Tufts Medical Center. (Jesse Costa/WBUR)
Two vials of Pfizer’s COVID-19 vaccine at Tufts Medical Center. (Jesse Costa/WBUR)

Tucked inside the Food and Drug Administration’s analyses of Moderna and Pfizer’s vaccines was a pleasant discovery: A single injection of either of the two-dose vaccines appears to provide strong protection against the coronavirus.

With supplies of the vaccines limited — and hundreds of millions of people waiting for inoculation — this leaves epidemiologists grappling with a complicated question. Should the nation vaccinate fewer people with the best protection possible, or provide twice the number of people with a single shot, covering more of the population but with slightly weaker protection?

“[The] question is a really difficult moral and scientific one,” says Barry Bloom, an epidemiologist at Harvard University. “If the second vaccine dose were superfluous, and we knew [it] didn’t extend the duration of protection, the principle would be to protect as many people and save as many lives as possible.” 

The right answer, in other words, depends on science wehaven’t yet completed. For one, the protection from a single dose of Pfizer’s vaccine hasn’t been definitively tested. Scientists can only infer from the trial data that Pfizer’s vaccine would provide protective antibodies to roughly half of people who get one dose.

But Dr. Chris Gill, an infectious disease specialist at Boston University, points out that a single injection of Pfizer’s vaccine may be even more effective than this estimate suggests. Looking at data from a smaller window between the time the first injection should have started working and before the second injection kicked in, Gill says the Pfizer-BioNTech vaccine may have an efficacy rate as high as 80 or 90% with just a single dose.

Moderna actually collected data from people who only received one dose of its vaccine, Gill says. Some 2,000 participants in Moderna’s phase three clinical trial received just a single injection of either a placebo or the vaccine. In that population, the efficacy of the single vaccine dose was roughly 80 to 90%.

“[Moderna] was not shy about showing that a single dose was so effective, and they do the math right,” Gill says. “After 14 days, the [single dose] vaccine is remarkably effective.”

In light of that, Gill argues it might be better for as many people as possible to get one vaccine shot now, while supplies are limited and the coronavirus is infecting and killing record numbers of Americans. A few months from now, when vaccine makers expect to scale up their manufacturing to produce hundreds of millions of doses a month, Gill says those who only got one dose can come back for their second booster shot.

“We could save a lot of lives. We can give two doses to people now, but in the interim a bunch of people who could have gotten the vaccine are going to die,” Gill says. “Is this not an example of where, yet again, the perfect is the enemy of the good?”

Dr. Benjamin Linas, an epidemiologist also at Boston University, is still mulling over the question. There’s still a lot of information that is yet to be revealed, he says. For example, is it less effective to receive the second dose of the vaccine a few months later than recommended schedule?

“Probably not, but no one knows,” Linas says.

And nobody knows how long the protection from a single dose will last. Of course, nobody knows how long the protection from two doses of the vaccines will last, either. Beyond the roughly two-month period of the clinical trials, those studies haven’t been done yet.

“The only way to know is to follow over time, and we haven’t had time,” Linas counters. “It’s a great question. I’m ready to start doing some simulation modeling, but I don’t have the data now to talk about it.”

There is another concern throwing its shadow over the proposal to vaccinate more people now with a single dose: How to convince millions of people to show up for a second dose at an unspecified point in the future. It’s hard enough to get people to arrive at pre-scheduled appointments for a second shot a month later, Linas says, let alone an unknown date based on an unknown supply of vaccines.

“If we gave all the vaccine now and back fill the second doses later, do we really have the logistical support to do that without entering chaos?” Linas says. “It makes me a little nervous.”

Harvard’s Bloom agrees this is a significant question.

“The probability of finding [vaccines] and getting them to return for a booster shot will be almost impossible,” he says. “That [may] leave a significant number of vaccinated people susceptible. If the booster prevents that, then two shots will lead ultimately to more lives saved over the long run.”

Gill is undeterred, though. The logistical issues in this plan would be real, he admits but, he arguesnot insurmountable. At the very least, he says, the idea of vaccinating as many people as possible with one dose should be seriously floated.

“But you know, we can solve those problems,” he says. “This is a one-way trip. It creates a moral urgency to at least have this discussion properly.”