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As hospitalizations and deaths from the pandemic soar, and frustrations mount over the slow rollout of COVID-19 vaccines, public health experts remain divided on how best to inoculate as many Americans as quickly as possible, including whether to delay the second dose of the vaccines or, in the case of Moderna’s shot, to cut the doses in half to expand availability.

Some public health experts believe the United States should administer its entire supply of COVID-19 vaccines, rather than reserving half for second doses, to provide more people with at least partial protection. At the same time, scientists at Cambridge-based Moderna and the National Institutes of Health are studying the possibility of cutting each of the vaccine’s two doses in half, effectively doubling the supply, according to The New York Times. A spokesman for Moderna declined to comment. But other specialists warn that changing any dosing regimen now could undermine public confidence in the vaccines or leave people undervaccinated and vulnerable.

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The vaccines developed by Pfizer-BioNTech and Moderna have efficacy rates of roughly 95 percent when administered in two doses, spaced 21 and 28 days apart, respectively. Limited clinical trial data indicates both vaccines confer some immunity after the first dose, but the depth and duration of that protection is unknown.

The Food and Drug Administration on Monday pushed back against possible changes to the vaccines’ dosing regimens or schedules, saying such suggestions are “premature and not rooted solidly in the available evidence.”

“Without appropriate data supporting such changes in vaccine administration, we run a significant risk of placing public health at risk, undermining the historic vaccination efforts to protect the population from COVID-19,” FDA Commissioner Stephen Hahn and Peter Marks, director of the agency’s Center for Biologics Evaluation and Research, said in a statement.

In a gamble to stem the tide of cases in the United Kingdom, British health officials announced they will prolong the interval between shots of the two doses required for the Pfizer and Moderna vaccines, potentially to as long as three months, and they will permit the first dose and second dose for any individual to be from different vaccine manufacturers, if the matching vaccine is not available.

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Dr. Ashish Jha, dean of the Brown University School of Public Health, said he initially supported following a “very narrow read” of the clinical trial data for the Pfizer and Moderna vaccines. But he changed his mind, he said, with the emergence of the new and highly contagious UK variant of the virus in the United States, which has now been confirmed in Colorado, California, Florida, and New York.

“I am looking out to the next two months of this pandemic and seeing what feels to me like some very, very bad days ahead,” Jha said. “If [the variant] is 50 percent more contagious, that’s a huge problem. It will take most of the states that are right now getting the virus under reasonable control and turn them into places with exponential growth.”

In an op-ed published this week in The Washington Post, Jha, along with Dr. Robert Wachter, chair of the department of medicine at the University of California at San Francisco, argued in favor of delaying the second dose of vaccines to distribute as many shots as possible to potentially save more lives as cases climb.

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“My take is, if you want to be a narrow reader of the evidence, you say, ‘Well, that’s uncharted territory. We don’t have strong evidence. We shouldn’t do it,’ and then we’re resigning ourselves to 100,000 to 150,000 more deaths over the next couple of weeks,” Jha said.

Dr. Christopher Gill, an associate professor of global health at Boston University, said he too supports delivering as many first doses as possible, based on the limited data on the efficacy of a single dose of the Pfizer and Moderna vaccines. The Pfizer vaccine, for example, has an efficacy rate of 52 percent between the first and second dose. Similarly, the Moderna vaccine showed 51 percent efficacy in the two weeks following the first shot.

“We don’t know what the persistence of the single dose will be, but I have to say, we don’t know what the persistence for two doses is, either, and that didn’t stop us,” Gill said. “This is another example where we as a public health community have to make public health policy decisions based on imperfect data.”

But Dr. Nahid Bhadelia, an infectious diseases physician and director of the Special Pathogens Unit at Boston Medical Center, fears changing the vaccination schedules without strong evidence would erode public trust. And while she’s worried about the new variant, she notes nonpharmaceutical interventions, like lockdowns, stay-at-home orders, and other restrictions, are also effective at curbing transmission.

“To get people to take a vaccine that has gone through this revolutionary fast evaluation — but a scientific evaluation — when the bedrock of this is this very good clinical trial data, any alteration from that needs to have convincing data, not just extrapolations,” she said.

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The country’s biggest stumbling block now, Bhadelia added, is administering the vaccines that have already been delivered. About 17 million doses have been distributed nationwide, but less than a third of those shots — roughly 4.8 million — have been injected, according to the latest figures from the Centers for Disease Control and Prevention.

“There are too few places where people can get vaccinated and too few people giving the vaccinations,” Bhadelia said. “Currently the way the vaccines are being distributed are only in health care settings and pharmacies, and those are places that are already strapped for people, and the organization of all of this stuff is falling on cities, states, and county public health officials.”

Dr. Lindsey Baden, an infectious diseases specialist at Brigham and Women’s Hospital and one of three investigators who led the nationwide late-stage trial of Moderna’s vaccine, agreed that the problem with the vaccine rollout is not inadequate supply, but logistics.

“We have a lot of doses on the sidelines, and they should be in people,” he said. “Until we have allocated the doses that we know are available as the [FDA’s emergency use authorization] was designed and approved based on the data, I wouldn’t want to start changing the regimen.”

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“I’m uncomfortable going places where the data don’t guide us,” he added, “unless we have a very strong rationale about why we need to do it.”

Baden also cautioned against the suggestion — first raised by Dr. Moncef Slaoui, the head of Operation Warp Speed — of cutting doses of the Moderna vaccine in half.

Baden said there was evidence that two 50-microgram doses of Moderna’s vaccine produced neutralizing antibodies at levels comparable to those produced by two 100-microgram doses. But the late-stage trial didn’t determine that halving the Moderna doses was as effective at preventing COVID-19, Baden said. The regimen authorized by the FDA — two 100-microgram doses — demonstrated about 95 percent efficacy at preventing COVID-19 and appeared to show 100 percent efficacy at preventing severe cases of the disease.

“Before we start monkeying around with the doses, our leaders should explain what’s happening” with millions of doses that have been distributed but not injected in people, Baden said.

Dr. Peter Hotez, a vaccine scientist and dean of the National School of Tropical Medicine at Baylor College of Medicine in Houston, also dismissed proposals to alter dosing schedules. He blamed the fumbled vaccination rollout on the Trump administration’s failure to control the pandemic at the outset with robust testing and tracing, and implement a federal vaccination strategy.

“People are now grasping at straws, and the straws they’re grasping at will only make things worse,” Hotez said. “All they’ll do is reduce the effectiveness of the vaccines and instead of putting good vaccines through a crummy infrastructure, they’ll put crummy vaccines through a crummy infrastructure. That’s not the answer. The answer is to fix the infrastructure, and we have to do it now.”

Jonathan Saltzman of the Globe staff contributed to this report.



Deanna Pan can be reached at deanna.pan@globe.com. Follow her on Twitter @DDpan.