About 17 million Americans received the Johnson & Johnson Covid vaccine, only to be told it was the least protective of the options available in the United States. But new data suggests the vaccine now prevents infections, hospitalizations and deaths at least as well as the Pfizer-BioNTech and Moderna vaccines.

The reasons are unclear, and not all experts are convinced that the vaccine justified itself. But the mounting data nonetheless provides considerable reassurance to recipients of the vaccine and, if confirmed, has broad implications for its rollout in some parts of the world.

In Africa, for example, distributing a single-dose vaccine that can be refrigerated for months is by far the most practical option.

Johnson & Johnson has at least temporarily closed the only factory making usable batches of the vaccine. But Aspen Pharmacare, based in South Africa, is getting ready to stock up large quantities to the rest of the continent. Only about 13 percent of Africans are fully immunized, and only about 1% have received a booster dose.

“In the context of Africa, where we need vaccines to get out quickly, the single dose is very exciting,” said Linda Gail-Bekker, director of the Desmond Tutu HIV Center at the University of Cape Town, who has studied the effectiveness of the vaccine in South Africa.

The Johnson & Johnson vaccine has been touted as an attractive option for communities with limited access to health care, including some in the United States, due to its ease of administration and mild side effects. But he had a bumpy ride.

The vaccine appeared to produce a weaker initial immune response, and more people who received the single-dose vaccine had infectious breakthroughs, compared to those who received two doses of Pfizer or Moderna, the mRNA vaccines.

In April, federal health officials in the United States and South Africa suspended the J.&J. distribution of the vaccine as they reviewed reports of a rare blood clotting disorder in women. Although the two countries resumed deployments soon after, the vaccine’s reputation never fully recovered.

But the idea that the vaccine is inferior has become obsolete, some experts said: More recent data suggests it has more than held its own against competitors.

“We knew that J.&J. was kind of downgraded in people’s minds,” said Dr. Gail-Bekker. But “it punches above its weight for a single-dose vaccine.”

Through last June, cumulative CDC data showed vaccination with the Moderna vaccine resulted in the lowest rates of breakthrough infections; those who got Johnson & Johnson saw the highest rates, with Pfizer-BioNTech somewhere in the middle.

During the summer months, the gaps – especially between J.&J. and Pfizer – began to shrink. Right now, all vaccines seem to work about equally well against coronavirus infections; in fact, Johnson & Johnson seems to be holding up better.

As of January 22, according to the latest available data, unvaccinated people were 3.2 times more likely to be infected than those who had received the single-dose Johnson & Johnson vaccine; they were 2.8 times more likely to be infected than those who received two doses of the Moderna vaccine and 2.4 times more likely than those who received two doses of Pfizer-BioNTech. Overall, the Johnson & Johnson vaccine therefore appeared to be somewhat more protective against infection than the two alternatives.

Among Americans who received booster doses, all vaccines appeared to be about equally effective against the infection. A booster injection added little to Johnson & Johnson’s previous level of protection (although the data does not indicate who received what type of booster injection).

The data was collected by the CDC of 29 jurisdictions, representing 67% of the population.

“The CDC data adds to the growing number of evidence indicating the Johnson & Johnson Covid-19 vaccine provides long-lasting protection against infectious breakthroughs and hospitalizations,” the company said in a statement.

The findings indicate that the J.&J. The vaccine deserves further consideration, said Dr. Larry Corey, a vaccine development expert at the Fred Hutchinson Cancer Research Center in Seattle.

“This vaccine platform may have surprising features that we hadn’t anticipated,” he said. The data “is interesting, provocative, and we should spend more time understanding it.”

Dr Corey said the results were consistent with his experience of HIV research with the adenovirus that forms the backbone of the Johnson & Johnson vaccine. “It has much longer durability than almost any other rig we’ve worked with,” he said.

Scientists are only beginning to guess why the profile of the vaccine is improving over the months.

Antibody levels spike in the first few weeks after vaccination, but then decline rapidly. The J.&J. The vaccine may produce antibodies that decline more slowly than those produced by other vaccines, some research has found. Or these antibodies may become more sophisticated over time, through a biological phenomenon called affinity maturation.

According to some researchers, the vaccine may have offered a more robust defense against the Omicron variant, responsible for the huge increase in infections in recent months. And studies have shown that the vaccine trains other parts of the immune system at least as well as the other two vaccines.

Not everyone is convinced that the Johnson & Johnson vaccine is catching up. It may only seem effective now because many recipients had breakthrough infections early on, gaining extra immunity, said Natalie Dean, a biostatistician at Emory University. “They may have a different immunity profile,” she said.

The infection rate is now lower in people who received the Johnson & Johnson vaccine but did not receive a booster. Still, the death rate is slightly higher than that of those who received the Pfizer-BioNTech and Moderna vaccines, Dr. Dean noted.

But the differences weren’t huge and disappeared among those who received booster shots. CDC statistics on deaths only last through Jan. 1, and the benefit of the Johnson & Johnson vaccine may only show up in data from February or March, said Dan Barouch, a virologist at Beth Israel Deaconess Medical Center at Boston which collaborated with Johnson & Johnson in the development of the vaccine.

Deaths tend to delay infections, often by weeks or months, “because many deaths occur after extended hospital stays,” he said.

Dr. Dean said that for a clearer comparison of vaccines, she would like to see data with information about individual factors, like previous infections and other high-risk conditions, rather than the overall numbers adjusted for age. age provided by the CDC.

“It’s too bad we don’t have a more direct study of outcomes in people who received J&J,” she said. That’s partly because fewer people have received the vaccine than mRNA vaccines, she said, but also “because we rely on other countries to generate data.”

Some of this information comes from South Africa. In a test, called sonkeDr. Gail-Bekker and her colleagues evaluated one dose of the Johnson & Johnson vaccine in nearly 500,000 healthcare workers and two doses in about 240,000 of those people.

In the first part of the trial, researchers matched recipients to a control group on age, gender, Covid risk factors, socioeconomic status and prior Covid infection. When the Delta variant was dominant in the country, they found that the vaccine was about 67% effective against hospitalization and about 82% effective against death. Protection against the beta variant was similar.

“Certainly during beta and delta waves, the single dose worked very well for severe illness and death,” Dr. Gail-Bekker said.

When the Omicron variant began circulating in South Africa, researchers offered a booster of the same vaccine to participants. Less than half of them agreed.

“It was extraordinary, the pushback,” said Dr. Gail-Bekker. At this point, the perception was that the Pfizer-BioNTech and Moderna vaccines were superior. “There was again the feeling that we were offering a very poor option,” she recalls.

Yet data so far suggests that two doses of J.&J. vaccine had approximately 75 percent efficacy against hospitalization with the Omicron variant, comparable to the protection of the Pfizer-BioNTech vaccine. The researchers presented the results last month at the Retroviruses and Opportunistic Infections Conference in Denver.

Although the trial only involved people who received two doses of the Johnson & Johnson vaccine, it suggests that the vaccine could be an excellent booster for people who initially received two doses of an mRNA vaccine, have said experts.

But in December, the CDC recommended mRNA vaccines on Johnson & Johnson for all adults, citing a risk of rare side effects like blood clots and Guillain-Barré syndrome. The agency found four cases of blood clots per million people vaccinated; women between the ages of 30 and 39 had the highest incidence, at about 11 per million.

mRNA vaccines have also been associated with infrequent side effects. They are thought to cause around 11 cases of myocarditis, or inflammation of the heart, per 100,000 vaccinated men aged 16-29.

More data on different vaccine combinations could clarify which is safer and more effective in the long term, Dr. Dean said. The arrival of new variants can also give some vaccines an edge over others, she said.

“I keep a very open mind about what might become the best vaccine schedule for the future,” she said.