U.S. regulators approved a third coronavirus vaccine in February, giving the country another badly needed tool at a critical time in the pandemic.

The greenlit vaccine, made by Johnson & Johnson, requires only a single shot. It’s easy to use, ship and store, and it did not cause any serious side effects during clinical trials. The nation’s leading medical experts cheered its authorization and urged people to take whichever coronavirus vaccine is available to them.

“You now have three highly efficacious vaccines, for sure, there’s no doubt about that,” Anthony S. Fauci, the nation’s top infectious-disease expert, said the day after the Food and Drug Administration authorized the Johnson & Johnson vaccine. “I think people need to get vaccinated as quickly and as expeditiously as possible.”

How is this vaccine different from the first two?

All in all, the Johnson & Johnson vaccine is less fussy.

For starters, it’s a one-stop shot. No waiting weeks for another appointment. No logistical headaches from keeping track of who needs their second dose and who’s still waiting on their first.

The Johnson & Johnson vaccine does not use the mRNA technology that Pfizer and Moderna used. Rather, it’s what’s known as an adenovirus vector vaccine.

Vector vaccines are the more established approach of employing a harmless cold virus to deliver a gene that carries the blueprint for the spiky protein found on the surface of the coronavirus. The virus enters cells, which then follow the genetic instructions to construct a replica of the coronavirus spike. The immune system uses these replicas to recognize — and respond to — the real thing.

The coronavirus vaccine produced by the University of Oxford and AstraZeneca is also based on adenoviruses, as is a Johnson & Johnson-made vaccine for Ebola, which was approved by the European Medicines Agency last year.

The Johnson & Johnson vaccine “doesn’t give you covid, because it’s not the virus,” Fauci said in a recent public service announcement. “It’s just one protein from the virus that induces your body to make a good response against the whole virus.”

Finally, the shots can be stored for months at refrigerator temperature.

How effective is it?

The bottom line: All three vaccines are “highly efficacious and quite safe,” said Philip J. Landrigan, an epidemiologist and the director of Boston College’s Program for Global Public Health and the Common Good.

The Johnson & Johnson vaccine was 100 percent effective at preventing hospitalization and death, including in South Africa against a more transmissible variant. It was 85 percent effective at protecting against severe cases of illness, and 72 percent effective at preventing moderate illness in the U.S. trials.

Scientists also note that the newest vaccine’s efficacy is far higher than the FDA’s 50 percent requirement.

Advisers to the Centers for Disease Control and Prevention strongly endorsed the vaccine’s effectiveness after its approval.

“Anything that keeps people from getting sick and anything that keeps people from going to the hospital and anything that keeps people from dying is a good thing,” Landrigan said.

Why is the Johnson & Johnson vaccine only one shot?

The Johnson & Johnson vaccine was tested and shown to be effective at protecting people against covid-19 with only one shot, so two doses was not really needed.

In clinical trials, that single-dose was shown to be 100 percent effective against hospitalization and death.

“So ultimately, it’s the data. The data shows that a single shot of this vaccine is sufficient to raise high levels of immune responses and to protect efficiently against covid-19 in humans,” said Dan Barouch, director of the Center for Virology and Vaccine Research at Beth Israel Deaconess Medical Center in Boston, who partnered with Johnson & Johnson to develop the vaccine.

That does not mean the company isn’t working to improve the vaccine. A 30,000 person clinical trial is currently underway in the United States to test a two-dose version of the vaccine. The findings from that study are expected by the end of spring.

“We know a second shot does increase immune responses,” Barouch said. But he said that given the overall effectiveness of the single dose, “there’s not a whole lot of room to improve.”

Barouch acknowledged that, at 72 percent efficacy for moderate cases, there’s still room for improvement. But for keeping us out of the hospital, and keeping us alive, “one shot is sufficient.”

How long does it take for the vaccine to kick in?

A shot in the arm is not like a flip of a switch.

None of the vaccines confer immunity automatically. Instead, think of a ramp that inclines gradually. On day one, your immunity is nil. But as time passes, your body will produce the T cells and B cells necessary to mount a successful defense against the virus.

The CDC estimates this will take “a few weeks” no matter the vaccine, however different people will reach peak immunity at different times. Johnson & Johnson recently reported that after 15 days, a T-cell response was detected in 76-83 percent of the trial participants ages 18 to 55 and in 60-67 percent of participants over 65. By day 29, the company said, 90 percent of the participants had coronavirus-neutralizing antibodies in their blood, regardless of age.

This is a faster average timeline than the Pfizer and Moderna vaccines, which take five and six weeks, respectively, to reach peak immunity after the first dose.

Some experts fear vaccines may be less effective against strains of the coronavirus that were first found in the United Kingdom, South Africa and Brazil. (The Washington Post)

Can I get more than one vaccine?

Current CDC guidance states that the Pfizer and Moderna vaccines “are not interchangeable with each other or with other covid-19 vaccine products.” So, the answer is: No, you shouldn’t get more than one type of coronavirus vaccine, and you shouldn’t mix the two-dose vaccines.

The CDC affixes its advice with the following disclaimer: “Recommendations may be updated when further information becomes available.” Scientists are probably still studying this question, and Landrigan said people should stick to what has been tested and approved.

“There’s always a possibility that there could be a cross-reaction between the two,” he said. “Nobody has data on that, but it’s a possibility.”

Can I choose which vaccine to get?

At the moment, no.

Across the board, vaccine supply is still scarce. Public health experts agree that, as the sports maxim goes, the best ability is availability.

It’s natural to compare medicines, Landrigan said, but at this point in the pandemic, being too choosy could be costly.

“People comparison shop all the time for everything,” he said. “But my advice to people is to get whatever vaccine first becomes available in your community. Don’t delay, because every day you delay is a day you’re unprotected.”

What does a third vaccine mean for the pandemic?

This is good news. Very good news.

The demand for shots currently outstrips the supply, and another approved vaccine means more people will be able to get inoculated faster.

“It will help to finally quell this terrible pandemic,” Landrigan said.

It comes at an important time. When the Johnson & Johnson vaccine was approved, the downward trend in new coronavirus infections had plateaued, perhaps because officials relaxed public health restrictions too soon and more contagious virus variants were becoming more widespread. Experts say a vigorous vaccination effort is key to stamping them out.

At the pandemic’s outset, few would have predicted that Americans would have access to three vaccines within a year.

“It’s extraordinary. I’ve never seen anything like it,” said Landrigan, who spent 15 years at the CDC. “I don’t think any of us expected we would have one, let alone three vaccines, in a matter of 12 months. It’s unprecedented.”

He added: “So long as the virus doesn’t evade us, I think we’re on the right track.”

Angela Fritz, Lena H. Sun, Carolyn Y. Johnson and Laurie McGinley contributed to this report.