Johnson & Johnson said Friday its COVID-19 vaccine prevented moderate to severe disease in an estimated 72% of U.S. participants in its trial, making it less protective than the Pfizer-BioNTech and Moderna products now in wide distribution.

Yet because it requires just one shot and is simpler to store, the J&J drug would be easier to disseminate, making it a welcome addition to the arsenal against the pandemic, said infectious disease experts who were not involved with the study.

The drug demonstrated an 85% efficacy in preventing severe disease, and its potency appeared to become greater over time. A month after getting the injection, no recipients developed a case of COVID-19 that led to hospitalization or death, the company said.

But the efficacy numbers for the J&J product were lower in some other countries — just 57% protection against moderate to severe disease in South Africa, where a coronavirus variant has mutated to the point that it has partly escaped protection from the vaccine. That suggests the world is in a race against time, before the variant becomes firmly established elsewhere.

It was not immediately clear why the J&J vaccine seemed less protective than the Pfizer and Moderna products, both of which prevented more than 90% of disease in trials, said Paul Offit, a prominent vaccinologist at Children’s Hospital of Philadelphia. But he said the results were nevertheless promising, adding that the company is still studying whether a second dose might boost performance even higher.

“If you can keep people out of the hospital, that is the goal,” said Offit, who was not involved in the study. “Even 72% is also excellent.”

Yet the differing levels of efficacy raise the question of who gets which drug, said Michael LeVasseur, an assistant professor of epidemiology at Drexel University’s Dornsife School of Public Health. He foresaw the possibility of equity issues — if wealthier people have better access to the higher-performing drugs — and also the question of the best medical practice.

“Do we give the less effective one to those who are at lower risk of disease?” he asked.

Still, the performance of the J&J vaccine is better than that of the flu vaccine in most years, said Offit, a co-inventor of a vaccine against rotavirus. Even the 57% figure in Africa is encouraging, despite the mutations that have enabled the virus to evade protection conferred by the vaccine, he said.

“It’s not completely escaping,” he said. “There is still some protection.”

The J&J vaccine works differently from the Pfizer and Moderna products, which consist of genetic instructions in the form of RNA, delivered inside millions of tiny spheres made of waxy molecules called lipids.

The J&J vaccine, on the other hand, consists of genetic instructions that are packaged within another type of virus called an adenovirus, which has been modified so it cannot cause disease.

But in all three cases, the end result is the same. The genetic instructions prompt the recipient’s cells to make a harmless fragment of the coronavirus, thereby allowing the immune system to make customized antibodies and other defenses should the person ever become infected with the real virus.

The J&J vaccine, made by Belgium-based subsidiary Janssen Pharmaceuticals, has another advantage besides requiring just one shot. Compared to the RNA vaccines, it is easier to store.

Company officials said the drug should remain stable for at least three months at standard refrigerator temperatures.

The Pfizer product must be stored at a temperature below negative 90 degrees Fahrenheit, well below the range of standard freezers, though it can be kept in refrigerators for five days right before administration. The Moderna vaccine must be stored at negative 4 degrees, though it can be refrigerated for up to 30 days before administration.

Still other candidates are on the horizon. Earlier this week, drugmaker Novavax said its two-dose vaccine prevented nearly 90% of symptomatic cases in a trial in the United Kingdom, though as with the J&J product, the vaccine appeared less effective against the variant identified in South Africa.