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Johnson & Johnson said Tuesday that people in a closely watched clinical trial who received a booster shot of its COVID-19 vaccine two months after the first dose had significantly higher protection against symptomatic illness.

The one-dose vaccine, which uses technology licensed by Beth Israel Deaconess Medical Center, had been found to be 72 percent effective in preventing moderate and severe disease in the United States, and 66 percent effective against such cases worldwide. That was robust protection, but well below the performance of two-dose vaccines from Pfizer-BioNTech and Cambridge-based Moderna.

But when people received a second J&J shot two months after getting the vaccine, the protection increased to 94 percent in the United States and 75 percent globally, according to the firm. And two doses were 100 percent protective against severe disease that can result in hospitalization or death, regardless of where recipients lived, the company said.

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“We now have generated evidence that a booster shot further increases protection against COVID-19 and is expected to extend the duration of protection significantly,” Paul Stoffels, chief scientific officer of the New Brunswick, N.J.-based pharmaceutical giant, said in a statement.

The results have yet to be published in a peer-reviewed scientific journal, but have been shared with the Food and Drug Administration. The study was conducted largely before the emergence of the highly contagious Delta variant.

The company has not formally applied for permission to offer booster shots to the general public and declined to discuss a potential timetable for such a request in the United States or abroad.

J&J said that when a booster was given two months after the first shot, coronavirus-fighting antibodies rose four to six times higher than those observed after a single shot. When a booster was given six months after a single shot, antibody levels increased ninefold a week later and climbed to twelvefold four weeks after the booster.

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“Scientifically, these new data are very supportive of giving a second shot of the J&J vaccine at any point from two months onward, and the longer you wait for the booster shot, the better the boost is likely to be,” said Dr. Dan Barouch, head of Beth Israel’s Center for Virology and Vaccine Research, which helped develop the vaccine and tested it on animals. He said the company is also testing how well a booster shot performs a year after the initial shot.

On Friday, a panel of scientific advisers to the FDA unanimously recommended the agency clear Pfizer booster shots for people who are at least 65 years old or at high risk of COVID-19. But the committee overwhelmingly rejected Pfizer’s request for a wider rollout to anyone 16 or older as premature.

The Pfizer and Moderna vaccines offer high levels of initial protection from illness, with each two-shot regimen preventing about 95 percent cases of illness in large trials last year. But that protection seems to wane slightly over several months, according to the companies, which are promoting boosters.

Protection from J&J’s vaccine, on the other hand, appears more durable. Researchers released a study last week comparing more than 390,000 vaccinated people to 1.5 million unvaccinated ones. Up to five months after the single shot, the effectiveness of the J&J vaccine against hospitalization held steady at around 81 percent. That study started before Delta emerged and included the period after it became widely prevalent.

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Dr. Paul Offit, a pediatrician at Children’s Hospital of Philadelphia who sits on the FDA advisory panel, said he wasn’t surprised by J&J’s announcement on Tuesday because of previously released data showing that a second dose in laboratory animals increased antibodies. But he wants to see the data that J&J submits for publication in a scientific journal, saying, “We’re in an age of science by press release.”

As the pandemic has continued and health experts have debated the need for third shots of the Pfizer and Moderna vaccines, people who received the J&J shot have sought guidance about whether they’ll need a booster. The new clinical trial, which recruited about 31,000 volunteers around the world, compared people who received one dose of J&J with those who received two doses eight weeks apart.

The FDA cleared the J&J vaccine for emergency use on Feb. 27, about two months after regulators had authorized two-shot vaccines from Pfizer-BioNTech and Moderna. Some health experts had predicted it would become the workhorse in the vaccination campaign in the United States and abroad because it only required one shot and didn’t have the cold-storage requirements of the first two vaccines.

The rollout of the J&J vaccine, however, was hobbled by an 11-day pause imposed by federal regulators in April after reports of a rare risk of blood clots in recipients. Although the FDA lifted the pause after determining that the benefits of the shot outweighed its risks, the vaccine has played a relatively modest role in the campaign in the United States.

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More than 14 million Americans have been fully vaccinated with the J&J vaccine compared with more than 99 million who have received two doses of Pfizer-BioNTech’s vaccine and the more than 67 million who have received two doses of Moderna’s vaccine.

J&J’s vaccine uses a harmless and relatively rare cold virus, adenovirus serotype 26 — or Ad26 — to deliver part of the distinctive spike protein on the surface of the coronavirus into cells to trigger an immune response without making people sick. The Pfizer and Moderna vaccines rely on messenger RNA molecules to stimulate an immune response.



Jonathan Saltzman can be reached at jonathan.saltzman@globe.com.