To stave off a potential fourth surge of Covid-19, some health experts are calling for the United States to postpone people's second doses of Pfizer/BioNTech and Moderna's vaccines to maximize the number of people receiving at least one dose of either vaccine—but others say that strategy could be backfire.

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Why some experts want the US to delay second shots

Current practice in the United States is to administer two doses of Pfizer/BioNTech and Moderna vaccines three to four weeks—which is the interval the drugmakers used in their clinical trials, Ezekiel Emanuel, director of the Healthcare Transformation Institute at the University of Pennsylvania; Govind Persad, assistant professor at the University of Denver's Sturn College of Law; and William Parker, assistant professor of medicine and assistant director of the MacLean Center for Clinical Medical Ethics at the University of Chicago wrote in an op-ed for USA Today.

However, the authors contended that drugmakers chose this interval "to rapidly prove efficacy in clinical trials," not because they were trying to assess "the optimal way of using the vaccines to quell a pandemic." And while this "three- or four-week follow-up is safe and effective, there is no evidence it optimizes either individual benefit or population protection," the authors write, particularly as the country faces the potential for another wave of infections.

Moreover, the authors cite research showing that a "single dose of an mRNA vaccine is 80% effective and durable for 12 weeks," whereas the full two-dose regime is "about 90% effective." As they put it, "In the battle against the surge, a first dose provides eight times more benefit than the incremental increase in protection achieved by using that dose for a second shot."

That's why some other countries, like Britain, have delayed second doses in favor of getting more people their first shot, they write. In fact, according to the New York Times, Britain is currently delaying shots by up to 12 weeks—allowing it to more rapidly give at least partial protection to 46% of its population—and a government advisory committee in Canada has advised second shots be delayed by up to four months.

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Emanuel and colleagues argue that the United States should do the same. According to their estimates, if the United States were to continue administering doses at its current rate of about 3 million per day but focused on first doses alone, the nation could catch up to Britain's vaccination levels in two or three weeks—and likely "quell the fourth surge."

Emanuel and his colleagues aren't alone in their opinion, the New York Times reports. Atul Gawande, a surgeon at Brigham and Women's Hospital, tweeted that it's time for the Biden administration "to delay 2nd vax doses to 12 weeks," arguing that it's "urgent" to get "as many people as possible a vax dose."

And Michael Osterholm of the University of Minnesota said as early as January that "we need to get as many one doses in as many people over 65 as we possibly can to reduce…serious illness and deaths that are going to occur over the weeks ahead."

Why other experts express concern over delaying second shots

However, other experts have voiced several concerns about increasing the delay between doses.

For instance, Luciana Borio, former acting chief scientist of FDA, and other critics have cited research showing Pfizer/BioNTech and Moderna's vaccines work less effectively against certain variants of the new coronavirus.

Instead of changing the vaccine schedule, Borio said it would be better to focus more on preventive measures like mask-wearing. "It's crucial that we don't just reopen into a big national party," she said.

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Several experts have also cautioned against attributing Britain's drop in cases entirely to its vaccine approach. "They've done a few other things, like shut down," Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said.

"I think the real test will be whether we see a rebound in cases now that the [United Kingdom] is reopening," Céline Gounder, an infectious disease specialist at Bellevue Hospital Center and a member of President Joe Biden's coronavirus advisory board, added.

But perhaps the biggest concern, according to experts, is a worry that delaying doses to allow new variants to breed in partially vaccinated people who can't quickly stave off infection. "Let's go with what we know is the optimal degree of protection," Fauci said.  

Gounder echoed those sentiments, arguing it would be risky to take an untested approach in the middle of an epidemic—especially since there's limited research on how long the partial protection from a single dose could last.

"When you're talking about doing something that may have real harm, you need empirical data to back that," she said. "I don't think you can logic your way out of this."

However, several critics have pushed back against that concern, the Times reports. Catherine Schuster-Bruce, a British health care writer, noted that the possibility that variants could evolve to evade vaccine protection in partially vaccinated people is only theoretical, and that there's no data backing the claim up—just as there's no data demonstrating that a three- or four-week gap is ideal. 

Similarly, Sarah Cobey, an epidemiologist and evolutionary biologist at the University of Chicago, said she's not concerned about the possibility of new variants breeding in partially vaccinated people. "I would put my money on it having the opposite effect," she said, suggesting that wider vaccination could instead help arrest the emergence and spread of new variants (Zimmer, New York Times, 4/9; Leonhardt, New York Times, 4/13; Emanuel et. al., USA Today, 4/8, Tabarrok, Marginal Revolution, 4/9).