A nationwide sense of relief grew Tuesday as a second coronavirus vaccine appeared poised for approval just days after thousands of health-care workers received doses of the first one in an unprecedented mass inoculation campaign that has raised hopes for a return to normalcy.

The second vaccine, developed by the biotechnology company Moderna, was “highly effective” in a clinical trial and carried no serious safety concerns, according to a detailed review by the Food and Drug Administration published Tuesday. The agency is likely to authorize the two-shot regimen as soon as Friday, according to a person with knowledge of the situation who was not authorized to discuss the issue.

Such an approval would clear the way for the shipment of almost 6 million doses of the Moderna vaccine in the first week — about double the amount of doses being sent out this week, Gen. Gustave Perna, who is overseeing the distribution effort, said Monday.

Pfizer, partnering with BioNTech, and Moderna have created effective coronavirus vaccines that scientists hope will lead to medical breakthroughs using mRNA. (The Washington Post)

Those numbers represent a tiny fraction of the doses that will be required to end or even slow the pandemic in the United States. Still, the rollout of the just-approved Pfizer-BioNTech vaccine Tuesday, and the glimmer of hope it offered at the end of a bleak year, sparked elation and tears of joy among health-care workers and ordinary people alike. A video of staffers at Boston Medical Center jubilantly dancing on a sidewalk, shared on Twitter Monday evening by the hospital’s chief executive, circulated widely Tuesday.

In bittersweet briefings, governors hailed the arrival of vaccines in their states but warned that American life would not — could not — return to usual for many months to come. The initially limited supplies of vaccine will not meet demand until well into 2021, and scientists do not yet know whether the Pfizer-BioNTech and Moderna vaccines, or others in development, prevent recipients from spreading the virus — or only from being sickened by it.

The elation was also shadowed by the grim cadence of rising coronavirus case counts and new deaths one day after the nationwide death toll reached 300,000, as infections seeded over the Thanksgiving holiday began to show up in state reports. On Tuesday, over 190,000 new cases and more than 2,800 deaths were reported.

Hospitalizations, meanwhile, threaten to overwhelm facilities across the nation as the December holidays approach. More than 110,000 people were hospitalized with covid-19 on Tuesday.

“Seeing vaccinations underway gives us hope at the end of a hard year. But this virus continues to be extremely contagious and deadly,” said Gov. Roy Cooper (D) of North Carolina, where 2,735 covid-19 patients were in hospitals on Tuesday — nearly twice as many as on Nov. 15. He urged residents to “keep using our tried and true safety precautions.”

Speaking by video during a briefing with Ohio Gov. Mike DeWine (R), the chief clinical officer of Mercy Health Toledo, Kevin Casey, warned: “This is just the beginning of the end. It is not the end.” Even so, Casey acknowledged, the first inoculations of members of the hospital’s staff “is an exciting day for us.”

Public health officials hope that at least 70 percent of the population will agree to get vaccinated to achieve herd immunity and stop the virus’s spread. As truckloads of vaccine fanned out across the country, that goal seemed like it might be attainable as a new survey suggested willingness to be vaccinated may be growing in the United States, despite substantial hesitancy in some communities.

The Kaiser Family Foundation poll, conducted before the approval of the Pfizer-BioNTech vaccine and released Tuesday, found that 71 percent of respondents said they would definitely or probably opt for vaccination if it is determined by scientists to be safe and is free. That is a higher figure than has been reported in several previous surveys, including one by the foundation in September, which found 63 percent said they would get vaccinated.

Blacks and Hispanics reported significant increases in willingness to be vaccinated since September — with the rates rising from 50 to 62 percent among Black adults, and from 60 to 71 percent among Hispanics. White adults’ willingness also rose, from 65 percent to 73 percent. Still, more than one-quarter of those polled said they probably or definitely would not get a vaccine.

“Many who are hesitant are in wait-and-see mode, and their concerns include worries about side effects and whether the vaccine can cause covid-19, which may dissipate as people get more information and see the vaccine introduced successfully among people they know,” said Drew Altman, chief executive of the Kaiser Family Foundation.

The 54-page FDA document published Tuesday morning positions the Moderna vaccine to follow the same historic track as the Pfizer-BioNTech vaccine. After the agency gave a positive review of the Pfizer-BioNTech vaccine exactly a week ago, the shots were endorsed by a panel of outside experts on Thursday and granted a green light by regulators a day later.

Assessing Moderna’s vaccine, regulators said the regimen was 94 percent effective at preventing disease in the trial and particularly effective against severe disease. Of 30 cases of severe covid-19 in the trial, none was in the group that got the vaccine.

They also confirmed the vaccine worked well in people of all ages, races and genders. Side effects were widespread but typically short-lived and bearable. They included fatigue, headaches, muscle aches and pain at the injection site.

One point of contention about the Pfizer-BioNTech vaccine — whether it should be authorized for 16- and 17-year-olds — will probably be avoided with Moderna’s because the company is seeking clearance to use the vaccine only in adults. Last week, the firm began testing its vaccine in adolescents.

Moderna’s vaccine will be reviewed Thursday by the same panel of independent experts that last week recommended the ­Pfizer-BioNTech vaccine. The data it will consider echoes the evidence that led to a 17-to-4 vote to authorize the Pfizer-BioNTech vaccine, which was 95 percent effective.

Paul Duprex, director of the Center for Vaccine Research at the University of Pittsburgh, predicted the Moderna vaccine would be authorized soon because it looked effective and safe. That vaccine’s simpler logistics are also an advantage, he said. It does not require specialized, ultracold storage conditions like the ­Pfizer-BioNTech doses.

“I like the fact that now we have something that can be stored in a minus-20 [degrees Celsius] fridge — the same kind of fridge you keep your ice cream in,” Duprex said.

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 its investment to $4.1 billion.

The trial randomly assigned 30,000 people 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 the novel coronavirus in their daily lives to see whether more covid-19 infections arose in the group that did not receive the real vaccine. They counted cases starting two weeks after the second dose, measuring how well the vaccine protected participants after the immune system had mustered a full response.

The numbers were decisive: Of 196 coronavirus cases in the trial, all but 11 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, also all in the group that received placebo shots.

One major outstanding question about this and other coronavirus vaccines is whether they prevent infection as well as disease. Data filed by Moderna suggested the vaccine might prevent asymptomatic infections, not just symptomatic cases of disease. But the FDA review said the data was limited and called for further studies.

Because most cases of covid-19 are asymptomatic, that question has major implications for whether people will need to continue wearing masks and social distancing despite being vaccinated.

In an addendum Moderna filed to the FDA, the company presented a tantalizing hint: Upon receiving their second dose of vaccine or placebo, 52 people tested positive for the coronavirus without symptoms. Most of those people were in the placebo group, indicating the vaccine may have protected participants who received the vaccine against asymptomatic infection.

Enrollment in Moderna’s trial was slowed this summer because scientists were concerned it lacked sufficient racial diversity to determine whether it truly protected those at highest risk of the disease. In the end, one-fifth of participants were Hispanic, 10 percent were Black, and 5 percent were of Asian descent. More than 40 percent of the participants were either over the age of 65 or had medical conditions that increased their risk of severe disease.

“The trial enrolled the right people at risk,” said Larry Corey, a virologist at the Fred Hutchinson Cancer Research Center who co-leads the government’s clinical trial network responsible for testing coronavirus vaccines. “For us who were designing the trial and enrolling the people, that’s so fantastic.”

Laurie McGinley and Scott Clement contributed to this report.