The Food and Drug Administration said Friday evening that it will “rapidly work” to authorize the Johnson & Johnson single-shot coronavirus vaccine, shortly after an expert committee unanimously recommended the vaccine.

In a statement, the agency said it has notified the company and federal officials involved in vaccine distribution so that they can prepare to ship the vaccine shortly. The FDA may issue an emergency use authorization as soon as Saturday, with the first few million doses of a shot that is relatively simple to store, handle and administer distributed next week.

On Friday, the agency’s vaccine advisory committee strongly endorsed the Johnson & Johnson vaccine, which will be the third coronavirus vaccine in the United States and the first to require just a single dose.

The third vaccine will arrive as the United States this week marked the grim milestone of 500,000 deaths at a crucial moment in the pandemic. After weeks of declining new cases, the downward trend has stalled — a change that makes many experts uneasy that officials are relaxing restrictions and people are letting their guard down just when variants capable of spreading faster or slipping by some aspects of immunity are poised to take off.

The Johnson & Johnson vaccine was 85 percent effective at protecting against severe cases of illness, and there were no deaths or hospitalizations a month after participants received the vaccine. The vaccine was less effective at preventing moderate cases of illness, and some experts have feared there could be an erroneous perception among the public that they should wait for other vaccines, which could prolong the pandemic and leave people unprotected.

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)

“It’s a relatively easy call; it clearly gets way over the bar, and it’s nice to have a single-dose vaccine,” said Eric Rubin, an infectious-diseases specialist at the Harvard T.H. Chan School of Public Health and a member of the advisory committee. “It’s a bit challenging about how to use it clinically right now, but the demand is so large, it clearly has a place.”

Adding another vaccine will help bolster the nation’s limited supply of the two authorized shots, one made jointly by Pfizer and its German biotech partner, BioNTech, and the other from Moderna. It should help accelerate the vaccination campaign, although the initial impact of the Johnson & Johnson vaccine is expected to be modest as the company ramps up manufacturing over the next month, with the first 20 million doses expected to be delivered by the end of March.

“There are limited options to control the virus,” said Gregory A. Poland, director of the Vaccine Research Group at the Mayo Clinic, who spoke to the FDA advisers as part of the Johnson & Johnson presentation and is a paid expert consultant to the company. “We need vaccines that are effective and well-tolerated, and — importantly — ones that are simple to deploy.”

Poland argued that a one-dose vaccine could speed up and simplify the effort to reach individual and herd immunity, making mass immunization campaigns that could end the pandemic more logistically feasible.

Johnson & Johnson applied to the FDA for emergency use authorization in early February, submitting clinical trial data involving 44,000 participants in eight countries. The data showed the vaccine was 72 percent effective in preventing cases of moderate to severe covid-19 in the United States, where variants of concern have only recently begun to be detected.

In South Africa, where a variant capable of evading some parts of immunity became dominant late last year, it was 64 percent effective against moderate to severe illness. Across countries, it was completely effective in preventing hospitalization and deaths a month after study participants received the shot.

During the presentation to FDA advisers, company officials underscored the strong protection the vaccine provided against severe bouts of illness, the outcome that is of greatest concern. That protection was just as robust even in regions of the world where concerning variants have arisen.

Johnson & Johnson, like other companies, is working to create a second generation of the vaccine tailored to the variants as a precaution. An updated version of the vaccine would be ready to be tested in people in a small study before summer, in case it is needed, said Johan Van Hoof, global therapeutic area head of vaccines for Janssen, the Johnson & Johnson division that developed the vaccine.

The emergence of the variants has raised a debate among experts about when and whether vaccines might need to be updated — and whether they protect not only against illness, but infection.

“In the long run, we have to stop infection in order that we don’t give the coronavirus the opportunities to mutate and adapt. … We know that nobody’s going to be safe until we are all able to shut down the virus replication,” said A. Oveta Fuller, an associate professor of microbiology and immunology at the University of Michigan Medical School.

Compared with the first two vaccines, Johnson & Johnson’s one-shot vaccine was tested during a more complicated phase of the pandemic, when a variant capable of slipping by some immunity had emerged.

The United States backed the development of the vaccine and has ordered 100 million doses through $1.5 billion in contracts. Federal health officials have said 3 million to 4 million doses could be shipped next week, if the vaccine is authorized, with 20 million doses total expected in March. The remaining doses are scheduled to be delivered by the end of June.

“During the last two months, we have all seen it is critically important to manufacture and distribute vaccines quickly and efficiently. And Janssen’s vaccine offers logistical and practical advantages to help simplify the distribution and expand vaccine access,” Van Hoof said.

There was some discussion among committee members about another trial testing two doses that is expected to report results later this year. Van Hoof said the company settled on a one-dose vaccine trial first because there was good evidence to suggest that it would be highly effective and that such a vaccine would be the best tool to get the pandemic under control and to reach more people.

The first coronavirus vaccines in the United States were authorized in December. They are being distributed nationally in an unprecedented drive to stem covid-19.

The advisory committee, which includes pediatricians, infectious-disease doctors and biostatisticians, heard presentations by the FDA, the Centers for Disease Control and Prevention and Johnson & Johnson. The stage was set for the committee’s positive recommendation on the vaccine by the FDA’s upbeat assessment, issued Wednesday.

The efficacy rate was lower, about 42 percent, in preventing moderate to severe illness in adults 60 and older who had diabetes, obesity or other chronic medical conditions. But the FDA said there was a high degree of statistical uncertainty regarding the data, which might reflect a smaller number of participants in that category.

The FDA, in deciding whether to clear a product, usually follows the nonbinding advice of its outside experts. An emergency use authorization is not a full approval. It is a temporary clearance that can be granted relatively quickly during a public health emergency and requires a lower level of evidence. But the FDA decided last year to use criteria for coronavirus vaccines that were nearly as rigorous as what is used for approvals.

A CDC advisory committee is scheduled to meet after the FDA’s expected authorization to recommend who should get the vaccine.

All the vaccines in late-stage development work by introducing the immune system to versions of the spike protein that dots the outside of the coronavirus. But the Johnson & Johnson vaccine adds a different technology to the mix, one that takes a more established approach than the Pfizer-BioNTech and Moderna products.

The new vaccine uses a harmless cold virus that has been genetically tweaked so that it cannot make copies of itself. That cold virus acts as a vector to deliver a gene encoding the spike protein to cells. The Pfizer-BioNTech and Moderna vaccines, in contrast, deliver a strip of genetic material called messenger RNA that carries the instructions to build the spiky protein found on the outside of the virus.

In all three vaccines, cells follow genetic instructions to construct the spike and the immune system learns to recognize the real thing from these replicas.