The Food and Drug Administration late Saturday granted emergency use authorization to Johnson & Johnson’s single-dose coronavirus vaccine, providing the United States with a long-anticipated tool that adds firepower and flexibility to the nation’s accelerating inoculation effort — but also presents new challenges.

Health authorities now have three effective vaccines, a singular scientific achievement that few would have predicted a year ago, when a pandemic emerged that has killed 2.5 million people worldwide, including more than 500,000 in the United States. It comes at a particularly fraught time, as Americans are whipsawed by encouraging developments, including sharp declines in nursing home deaths, and concerning news such as the emergence of potentially menacing variants.

A Food and Drug Administration advisory committee on Feb. 26 voted to approve emergency use authorization for Johnson & Johnson’s coronavirus vaccine. (The Washington Post)

With demand for vaccines outracing supply and officials scrambling to get much of the population vaccinated before variants spoil an improving picture, a third vaccine is “really good news,” said Eric Toner, senior scholar with the Johns Hopkins Center for Health Security. The vaccine will not have a big effect initially on supply shortages. Only a few million doses will be shipped to states in the days immediately after authorization, though production will ramp up in coming weeks, with 20 million doses to be delivered by the end of March and 100 million total in the first half of the year, according to the company.

The new vaccine, which is for adults 18 and older, has clear practical and logistical advantages over the first two vaccines — it does not have to be kept frozen, and there is no need for a second round of appointments. That makes it a boon for rural areas and other hard-to-reach communities, and for distribution to community health centers and physician offices that might not have the freezers needed for the other vaccines, public health officials say.

But the Johnson & Johnson shot also has a lower efficacy rate, leading some public health experts and government officials to worry that it will be viewed as substandard compared with the other vaccines. The Johnson & Johnson efficacy rate is 66 percent overall and 72 percent in the United States in preventing moderate to severe cases of covid-19, the disease caused by the coronavirus.

The vaccines by U.S. pharmaceutical giant Pfizer and its German partner BioNTech and by the biotech company Moderna are about 95 percent effective following their two-shot regimens.

Nirav Shah, director of the Maine Center for Disease Control and Prevention, said he is already getting tweets and emails from people expressing reservations. “The gentle ones say, ‘Is it okay if I get J & J, or should I wait for Pfizer?’ " he said. “The strong ones say, ‘I don’t want J & J. I want Pfizer.' ”

Shah said he responds by saying all three vaccines have been shown to be fully effective in protecting against what people fear most — hospitalizations and death. And he notes that the two earlier vaccines were tested months before the emergence of “variants of concern,” including one first detected in South Africa that appears to affect the efficacy of the vaccines, so the results are not an “apples to apples” comparison. He urges people to get whatever vaccine they can.

Shah said he is planning to equip public health nurses and other vaccinators with the Johnson & Johnson vaccine and send them to far-flung corners of his state. “J & J will open up a new wing in our vaccination efforts — not just more people, but harder-to-reach people,” he said.

Jeanne Marrazzo, an infectious-disease doctor at the University of Alabama at Birmingham, placed the newest coronavirus vaccine in the context of vaccines used to thwart other infectious diseases.

“In a normal world, people would be jumping up and down for a vaccine that is more than 70 percent effective,” she said, noting that the FDA’s efficacy requirement is 50 percent. Instead, she said, “Some people are saying, ‘I am going to wait until I get the good vaccine.’ ”

She doesn’t want the residents of Alabama’s rural areas — which tend to be less affluent and have a significant percentage of African Americans — to feel they are getting shortchanged if Johnson & Johnson’s product is deployed there.

“It’s something you have to think about as you make vaccines available because people are making comparisons,” Marrazzo said.

Paul Goepfert, director of the Alabama Vaccine Research Clinic and one of the scientific leaders of the Johnson & Johnson trial, said emerging evidence in the United Kingdom shows that the protection provided by the vaccine is similar to that from a single dose of Pfizer.

“Dose for dose, this vaccine is as good as any of the other ones,” he said. “I think the messaging needs to be this vaccine is comparable to the other two vaccines, and as soon as you have the availability [to take any vaccine], you should take it.”

But some experts say it is important for people to understand there might be differences between vaccines in preventing moderate cases of covid-19, because even those infections could have long-term consequences. At the same time, some doctors say they are encountering people saying they prefer the Johnson & Johnson vaccine because they don’t want to have to get a second shot.

For now, consumers are unlikely to have a choice of vaccines because there simply isn’t enough supply. That is poised to change in coming months. Pfizer-BioNTech and Moderna have committed to delivering a total of 220 million doses by the end of March. They say they have solved manufacturing challenges and are in a position to overcome scarcity that has hampered the nation’s fight against the coronavirus.

Some experts expect the United States will reach an inflection point by late spring, with more shots available than people who want to get them. A vaccine developed jointly by the University of Oxford and AstraZeneca and another from Maryland biotechnology company Novavax are also in the late stages of testing, and the United States has contracts for 400 million doses, if the vaccines are successful.

The addition of the Johnson & Johnson shot will diversify the nation’s portfolio of vaccines, adding one that works through a different scientific technology.

The Pfizer-BioNTech and Moderna vaccines employ an approach that had never been used in people outside of clinical trials. They deliver a strip of genetic material called messenger RNA carrying the instructions to build the spiky protein found on the outside of the virus.

In contrast, the Johnson & Johnson vaccine uses a harmless cold virus to deliver a gene encoding the spike protein to cells. The approach is more established. In both cases, cells follow genetic instructions to construct the spike, and the immune system learns to recognize the real thing from these replicas — and to respond.

The latest vaccine will also add a new supply stream to help alleviate shortages or manufacturing delays. A different process and supply chain are used, which could help bolster confidence that if any of the vaccines runs into production difficulties, there will still be supply.

“It’s always better to have that — if one vaccine gets in trouble for some reason or isn’t being produced at a high enough rate. That’s the most common problem we see when a new vaccine is produced: shortages,” said Nancy M. Bennett, professor of medicine and public health sciences at the University of Rochester School of Medicine and Dentistry. “The more the merrier.”

The advantages of each vaccine against the pathogen are still being untangled. Although a single shot of the Johnson & Johnson vaccine didn’t trigger antibody levels to soar as high as a two-dose regimen, it offered good protection against variants. Scientists think that could be because the vaccine robustly triggered another arm of the immune system, T cells, that may provide a different layer of protection.

Johnson & Johnson’s Janssen Pharmaceuticals unit applied to the FDA for emergency use authorization for the vaccine Feb. 4, submitting clinical trial data involving 44,000 participants in eight countries. On Wednesday, the agency released its analysis that the vaccine was safe and effective. On Friday, the agency’s outside vaccine advisers voted unanimously to recommend authorization.

A vaccine advisory panel to the Centers for Disease Control and Prevention is scheduled to have an emergency meeting Sunday to review the safety and efficacy data and to recommend who should get the vaccine.

Meanwhile, vaccine makers in recent days announced progress on work designed to counter viral variants.

Moderna said it has made a new version of its vaccine targeting the variant first identified in South Africa. A small amount of vaccine has been sent to the National Institutes of Health for a trial to determine whether boosting humans with the modified vaccine will stimulate a strong immune response, the company said.

Pfizer and its partner BioNTech said they have started testing a third dose of their original coronavirus vaccine to see how well it protects against variants. They are also doing work aimed at variants of concern.

For scientists who have long worked on vaccines, the scale and velocity of the coronavirus research has been stunning. Goepfert, an HIV researcher who worked on the Johnson & Johnson trials, said the HIV vaccine trials he has worked on might have had as many as 5,000 participants, but the Johnson & Johnson study recruited 44,000 people in about 2½ months.

“What’s fascinating about covid vaccine studies is at first we were worried about enrolling the right kind of people at risk. But everybody’s at risk. You just needed to enroll people,” Goepfert said. “I think that’s sadly what helped us get these vaccine end points.”

The Johnson & Johnson study also might help elucidate some questions that are beginning to arise from real-world use of the vaccines. Goepfert said there were health-care workers in the trial who received the Johnson & Johnson vaccine but then became eligible for an authorized vaccine midway through and received one of the messenger RNA vaccines. Scientists have continued to collect safety data on those people, which should help show whether there are obvious risks from the mixing and matching that may begin to occur in the future, particularly as companies begin testing additional booster doses as a way to protect against variants.

For Helen Boucher, an infectious-disease doctor at Tufts Medical Center in Boston, the events of the past several weeks have been both devastating and exhilarating.

“We have hit this unfathomable number of people who have died and just thinking about it is overwhelming,” she said. At the same, she noted, hospitalizations and deaths are declining and vaccinations are increasing.

“I feel like there is light ahead,” she said. “I think the data supports it.”

Isaac Stanley-Becker contributed to this report.

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