WASHINGTON — The  Food and Drug Administration advisory committee endorsed Johnson & Johnson’s coronavirus vaccine late Friday afternoon, with panel members saying the shot will be a crucial third option to vaccinate millions of Americans.

The vaccine appears widely effective against the virus and is especially good at preventing severe cases, according to data presented by FDA staff and Johnson & Johnson at the meeting. And because the vaccine is given as a single dose and is easy to ship, it presents an appealing profile for public health officials struggling to vaccinate hard-to-reach or skeptical populations — groups of people increasingly in the sights of state leaders launching outreach efforts.

The FDA panel voted 22-0 in favor of authorizing the shot. Tje FDA is not bound by the group’s recommendations but often follows them.

If the FDA acts on the recommendation — and it’s expected to do so quickly — the vaccine will become the third authorized for use in the United States. It will be the first one-shot vaccine, and one that can be stored in regular refrigerators, making it easier to distribute than the Pfizer/BioNTech and Moderna vaccines, which are more delicate and require two doses.

The vaccine, made by Johnson & Johnson’s vaccine arm, Janssen, was tested in advanced clinical trials in more than 44,000 people in the US, South Africa and Latin America. Globally, it showed an overall efficacy of 66% in preventing moderate-to-severe disease. In the US, this efficacy was 72% and it was 85% effective in preventing severe disease. No one who got the vaccine died of Covid-19 during the trial.

Top U.S. health officials have said vaccination is the best way to bring the coronavirus pandemic under control. The virus has infected more than 28 million Americans and killed more than half a million.

“We urgently need more vaccines under EUA to protect the millions of Americans who remain at risk,” Dr. Greg Poland, head of the Mayo Clinic’s Vaccine Research Group, told the meeting.

“We need a vaccine that can be quickly mass produced,” Poland said at Friday’s meeting. “We’d want to see a reasonable duration of efficacy and protection,” he added. “The Janssen vaccine candidate checks nearly all the boxes.”

How the vaccine works

Members of the committee said they had few doubts about their decision.

“I think it’s a relatively easy call. It clearly gets way over the bar and it’s nice to have a single-dose vaccine,” Dr. Eric Rubin, editor in chief of the New England Journal of Medicine and a professor at the Harvard TH Chan School of Public Health, said after the vote.

Several members stressed the urgency of fighting a pandemic that has killed more than half a million Americans.

“We are still in the midst of a deadly pandemic. There is a shortage of vaccines that are currently authorized and I think authorization of this vaccine will help meet the need at this moment,” said Dr. Archana Chatterjee, vice president for medical affairs at Rosalind Franklin University in Chicago.

“There’s an urgency to get this done. We’re in a race between the virus mutating, new variants coming out that can cause further disease and stopping it,” added Dr. Jay Portnoy, a professor of pediatrics at the University of Missouri Kansas City school.

“We need to get this vaccine out. I do believe the evidence supports its safety and effectiveness.”

Janssen’s vaccine is different from the Pfizer and Moderna vaccines, which are made using a new technology that employs genetic material called messenger RNA or mRNA. Janssen’s vaccine uses a common cold virus called adenovirus 26. It has been genetically engineered so that it cannot replicate in the body. Instead, it infects a few cells in the arm when injected, carrying the genetic code for a small part of the coronavirus. Those cells then produce material that looks like pieces of the virus to the immune system, stimulating an immune response.

It’s an older vaccine technology. “The military has used adenovirus vaccines a lot,” Dr. Arnold Monto, an epidemiologist at the University of Michigan’s School of Public Health who chairs the VRBPAC, said during the meeting.

Although the Johnson & Johnson vaccine does not appear to have the astonishing 94% to 95% efficacy of the Pfizer and Moderna vaccines, researchers told the VRBPAC committee that it was tested at a different time — when more virus was circulating and after new variants had arisen that have the ability to evade some of the immune response. Although they were not able to show that these variants were responsible for the apparent lower efficacy of Johnson & Johnson’s vaccine, they told the committee it could have been a factor.

What happens next

If the FDA issues an EUA, it could come as soon as Friday night. It’s up to the US Centers for Disease Control and Prevention to decide whether the vaccine should actually be given to Americans. The CDC’s Advisory Committee on Immunization Practices (ACIP) has scheduled a meeting for Sunday to take up the question. ACIP will vote on Sunday afternoon on whether the CDC should OK the vaccine’s distribution and may also designate priority groups.

The CDC director, Dr. Rochelle Walensky, will then give the final go-ahead.

The US federal government says it’s ready to distribute about 4 million doses of the vaccine immediately.

Dr. Anthony Fauci, the chief medical adviser to the White House and director of the National Institute of Allergy and Infectious Diseases, has said Americans should not try to pick and choose vaccines but rather, get whichever vaccine is available.