We may soon have a third vaccine. Here’s what you need to know about the Johnson & Johnson shot

Rocky Mountain Regional VA Medical Center investigational pharmacy technician Sara Berech held a dose of the Johnson & Johnson COVID-19 vaccine before it was administered in a clinical trial.Michael Ciaglo/Getty

A new COVID-19 vaccine from Johnson & Johnson that uses technology pioneered by the Center for Virology and Vaccine Research at Beth Israel Deaconess Medical Center in Boston made headlines on Friday when results of its late-stage clinical trials were announced.

The development was hailed as good news, even though the company reported that its one-shot vaccine had a lower efficacy rate than the two-dose vaccines from Pfizer-BioNTech and Moderna, which are already being administered to people across the United States.

Here’s what you need to know:

What did the Johnson & Johnson study say?

The Johnson & Johnson vaccine had an efficacy rate of 66 percent for moderate and severe COVID-19 cases in a late-stage study of 43,783 volunteers across the world, according to the company and the National Institutes of Health.

The results were better in the United States, with the vaccine preventing 72 percent of moderate and severe US cases in the study.

And for the most serious cases, outcomes were even better. Worldwide, the shot prevented 85 percent of severe cases and none of the vaccinated participants in the study were hospitalized or died from COVID-19.

How does that compare with the current vaccines?

The Pfizer and Moderna vaccines prevented about 95 percent of coronavirus cases in large trials.

How is vaccine efficacy measured?

In clinical trials, researchers vaccinate thousands of people and give placebos to thousands of others. The researchers then wait for people to get sick and look at how many of the illnesses are found in the vaccinated group and in the placebo group. They look at the relative difference in the rate of people who got sick in the two groups to calculate the efficacy rate. If the rate of people falling ill in the two groups is the same, the efficacy is zero; the vaccine didn’t work. On the other end of the spectrum, if nobody falls ill among those who have been vaccinated, the efficacy is 100 percent.

What is the difference between efficacy and effectiveness?

Efficacy is a measure of how well a vaccine prevented disease in a clinical trial.

Effectiveness is how well a vaccine performs out in the real world.

In a clinical trial, researchers control who gets the vaccine and how it’s administered. In the real world, it will be given to a wider number and variety of people, including those who have chronic illnesses or take medications. Storage and administration might not always be perfect. All kinds of complications can arise.

As a result, a vaccine’s real-world effectiveness is usually lower than its research-based efficacy.

What are people saying about the Johnson & Johnson vaccine?

Dr. Anthony Fauci, President Biden’s chief medical adviser on the coronavirus pandemic, acknowledged at a White House COVID-19 team news conference that the “first thing people do” will be to compare the US efficacy of 72 percent to the higher numbers for the Pfizer and Moderna vaccines. But he highlighted that the overall efficacy for severe disease was 85 percent and said, “there were essentially no hospitalizations, or deaths in the [Johnson & Johnson] vaccine group.”

Dr. Paul Sax, clinical director of the Division of Infectious Diseases at Brigham and Women’s Hospital, said in an e-mail, “The big plus from the data (limited though they may be at this point) is that the vaccine was extremely good at preventing severe disease. A one-shot vaccine that does not require super cold storage and still prevents severe disease (hospitalization and death) is a real advance, despite the fact that the overall efficacy was lower than with the Pfizer and Moderna vaccines.”

Dr. Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine and an authority on vaccines, said he found the results “really promising.” He added that he suspects that two shots of the Johnson & Johnson vaccine would provide “durable protection at least as good” as the two-shot vaccines from Pfizer and Moderna.

“I hope we can scale production and get this to the American people soon,” he said in an e-mail.

Why did Johnson & Johnson’s vaccine perform better in the United States?

Researchers and public officials believe the efficacy numbers of the vaccine may have been affected by the emergence of new coronavirus variants, particularly in South Africa, one of the places where the clinical trial was held. And they said that fact highlights the need to prepare for the arrival of the variants in the United States.

Dr. Dan Barouch, who runs the virology center at Beth Israel that helped develop the vaccine, said the pandemic has evolved.

“It’s not the same pandemic as it was a few months ago,” he said in a telephone interview. “This is very strong efficacy data against a complex pandemic involving multiple resistant variants circulating globally.”

Fauci and other officials said at a news conference with National Institutes of Health officials said that the Pfizer and Moderna vaccines would likely get lower results now, given the emergence of the South African strain that appears to be stubbornly resistant to immunization.

He said at the White House COVID-19 team news conference that the Johnson & Johnson vaccine and another vaccine developed by Novavax, which also held clinical trials partly in South Africa, handled severe disease “reasonably well.” But, he said, “This is a wakeup call to all of us that we will continue to see the evolution of mutants.”

He said the government and pharmaceutical companies “will have to be nimble to be able to just adjust readily to make versions of the vaccine that actually are specifically directed towards whatever mutation is actually prevalent at any given time.”

Should you get the Johnson & Johnson vaccine even though it’s not as effective as the others?

No vaccine is perfect. The flu shots given to thousands every year are typically only 40 to 60 percent effective, but still are considered worth it.

Early in the campaign to develop a coronavirus vaccine, the US Food and Drug Administration said it would consider authorizing any vaccine with 50 percent or greater efficacy. Doctors were stunned when the Moderna and Pfizer vaccines yielded such outstanding results.

Even though it doesn’t measure up to those, the Johnson & Johnson vaccine’s performance is considered strong. It’s important to remember that this vaccine does a great job of preventing hospitalization and death, even when it doesn’t prevent illness. In the study, none of the vaccinated people needed hospitalization or died from COVID-19.

Material from Globe wire services was used in this report.