The Food and Drug Administration said Thursday night that it will “rapidly work toward” emergency authorization of Moderna’s coronavirus vaccine, just hours after agency advisers endorsed the shot. The announcement appeared to pave the way for another weapon against a pandemic that has killed about 310,000 people in the United States.

The FDA statement came after the agency’s vaccine advisory panel voted almost unanimously — 20 in favor, with one abstention — that the benefits of the highly effective vaccine outweighed its risks for people 18 years of age and older. The FDA intends to authorize the vaccine Friday, according to knowledgeable individuals who spoke on the condition of anonymity because they were not authorized to speak publicly about the schedule.

“I just want to make the point of what a remarkable scientific achievement this is, and pay thanks to all the scientists, present and past, who contributed to this,” James E.K. Hildreth, president of Meharry Medical College and a member of the advisory panel, said at the close of the committee meeting on Thursday. “To go from having a [genetic] sequence of a virus in January, to having two vaccines available in December, is a remarkable achievement.”

Anticipating the authorization decision, Gen. Gustave Perna, who is overseeing the federal effort to distribute vaccines, said Monday that the government was preparing to ship almost 6 million doses of the Moderna vaccine to 3,285 locations in the first week after approval.

“It will be a very similar cadence that was executed this week with Pfizer, where we’re hitting initial sites on Monday, [followed] on Tuesday and Wednesday,” Perna said.

The advisory committee met on the same day the nation set three grim single-day records, for cases (more than 250,000), hospitalized covid-19 patients (more than 114,000) and deaths (more than 3,400). California in the past 48 hours posted more than 100,000 new infections, and the governor has readied a mass fatality plan.

At the same time, questions flared about the availability of the first coronavirus vaccine to receive federal authorization, with officials in multiple states saying they had been told their second shipments of Pfizer-BioNTech’s inoculation had been slashed for next week.

That development sowed confusion, led to dueling statements from Pfizer and federal officials about who was to blame, and raised questions about whether a promise to deliver shots to 20 million people by the end of the year would be kept.

Looming supply issues made clear that even with unprecedented scientific success, the limited availability of vaccines would not prevent a dark winter.

The FDA authorized the Pfizer-BioNTech vaccine last Friday, hours after White House Chief of Staff Mark Meadows warned FDA Commissioner Stephen Hahn he would be asked to submit his resignation if the agency didn’t authorize the shot by the end of that day. The agency had intended to authorize the vaccine on Saturday. The first doses were administered Monday to health-care workers. Questions about the supply have persisted, as Pfizer and the U.S. government have been in negotiations about securing more doses after the first 100 million already purchased for nearly $2 billion.

Moderna developed its vaccine in partnership with the National Institute of Allergy and Infectious Diseases, with the government underwriting the research and development of the vaccine and the advanced purchase of 200 million doses, bringing the government investment to $4.1 billion.

Early in Thursday’s meeting of the advisory committee, the FDA addressed an issue that did not directly involve the Moderna vaccine: reports of allergic reactions to the Pfizer-BioNTech vaccine, including in two health-care workers in Alaska, one of whom was hospitalized. Both are recovering.

“While the totality of data at this time continue to support vaccinations under the Pfizer [emergency use authorization] without new restrictions, these cases underscore the need to remain vigilant during the early phase of the vaccination campaign,” FDA vaccine expert Doran Fink said.

The agency is working with the companies to revise fact sheets provided to patients and health-care professionals, Fink said, and he underscored an existing requirement that facilities administering the vaccine ensure that treatment for a severe allergic reaction is immediately available.

The heightened concern about possible rare allergic reactions will also pertain to the Moderna vaccine, because it uses the same genetic technology as Pfizer-BioNTech. An expert panel convened by the National Institutes of Health on Wednesday tried to discern which of the components of the vaccines might trigger an allergic reaction, Moderna chief medical officer Tal Zaks said, but he added that differences between the two vaccines make it uncertain whether they both could trigger the rare reactions.

During the meeting, he and other Moderna executives explained that they saw no cases of anaphylaxis in their 30,000-person coronavirus vaccine trials. In trials of eight different vaccines that use the same technology against different diseases, there had been a single report of anaphylaxis among a total of about 1,700 participants. That case occurred more than two months after the shot — suggesting there was no link.

The FDA will also closely monitor for cases of Bell’s palsy, a temporary facial paralysis, as the vaccine is rolled out to more people. There were four cases of Bell’s palsy among people that received the Pfizer-BioNTech vaccine in its trial and three in the group that received the Moderna vaccine in its trial. The FDA said there wasn’t a clear causal link but acknowledged that the combined data from the two trials raised questions.

“It’s something we are looking into and thinking much about,” said Rachel Zhang, FDA medical officer.

The Pfizer-BioNTech and Moderna vaccines both use a new genetic vaccine technology, and in addition to the impact the vaccines could have on the pandemic, there is hope the new technology could be used to rapidly create vaccines for other diseases. But the novelty of the technology, which has been tested for years in people but never deployed in an approved medical product, has been used to foster doubt about the vaccines in some social media posts. Moderna executives used Thursday’s advisory committee hearing to address those questions head on.

Melissa J. Moore, Moderna’s chief scientific officer, explained that a snippet of genetic material, called messenger RNA, is encapsulated in a tiny fat bubble and delivered to the body’s lymph node cells. There, cells’ protein-making machinery follow the genetic instructions to build the coronavirus spike protein. Immune cells interact with the spike to muster a protective response.

Some social media posts have stoked fear that messenger RNA will change people’s DNA. Moore explained that the messenger RNA cannot be integrated into the genome.

Questions have also circulated about the vaccine’s ingredients, especially following the reports of rare allergic reactions. Moore explained that the vaccine contains the synthetic messenger RNA and a fat bubble. It does not contain many ingredients commonly found in other vaccines, such as preservatives or adjuvants, which are used to make vaccines work better — and is not manufactured in human or animal cells.

Moderna’s vaccine was shown to be 94 percent effective in its large clinical trial; Pfizer-BioNTech’s vaccine was 95 percent effective. The efficacy was similar across age, gender and racial groups.

Paul A. Offit, director of the Vaccine Education Center at the Children’s Hospital of Philadelphia and a member of the FDA advisory committee, said Wednesday on CNN that the two vaccines are similar in safety and efficacy. Asked whether people will have a choice of which to get, he replied, “There is not a lot of vaccine out there, so you are going to be asked to get the vaccine that the area has.”

In Moderna’s trial, 30,000 people were randomly assigned to receive either two shots of a vaccine given four weeks apart, or two saline shots. Neither the participants nor the people running the trial knew who was in which group.

Investigators then waited as people were exposed to coronavirus in their daily lives, to see if there were more cases of covid-19 in the group that did not receive the real vaccine. They counted cases starting two weeks after the second dose, to measure how well the vaccine protected participants after the immune system had mustered a full response.

The numbers were decisive: There were 196 cases in the trial, all but 11 of them were in the group that received the placebo shots. There were 30 cases of severe covid-19 and one death from covid-19 in the trial, all in the group that received the placebo.

There was a suggestion that a first dose of vaccine afforded some protection from the virus, but some key information is lacking.

One of the main questions debated by the panel was what to do about the trial once a vaccine is authorized.

Moderna is proposing notifying everyone in the placebo group immediately after the vaccine is cleared and allowing them access to the vaccine, using leftover supply from the clinical trial that will expire soon and can’t be given to people as part of the emergency authorization. That’s different from the Pfizer-BioNTech plan, which will allow individual participants who become eligible for a shot under health agency guidelines to request access to the vaccine. At that point, those participants could opt to find out if they were in the placebo group — and if they were, request the vaccine.

Steven Goodman, professor of medicine at Stanford University, argued that the Moderna plan could erode the ability of the trial to answer key questions about the duration of immunity — and might endanger future trials.

“We have a very strong interest in developing good information for the other vaccines,” Goodman said. “There will be a precedent, as soon as something has been shown to be effective and available, that it’s unethical to ask people to wait any more time to be immunized in any way — and this is a precedent you may not want to set.”

Moderna argued against a plan that was more logistically difficult to implement or that would defer offering the vaccine, in part because of the ethics and because so many trial participants are in high-risk groups who will begin to be offered the vaccine soon and might drop out of the trial otherwise.

Lindsey Baden, the co-principal investigator of the trial at Brigham and Women’s Hospital, said there were two to three severe cases of disease among placebo recipients in the trial each week.

“It’s important we carefully consider the volunteer viewpoint as we navigate fairness, equity, trust, transparency as well as a larger societal interest,” Baden said. “Without them, clinical research cannot function and we have unique obligations to handle the study properly.”

Yasmeen Abutaleb, Josh Dawsey, Jacqueline Dupree, Isaac Stanley-Becker, Lena H. Sun and Reis Thebault contributed to this report.