Manufactured by Johnson & Johnson, the one-shot coronavirus vaccine provides strong protection against serious illness and death from Covid-19, and can reduce the spread of the virus through vaccinated people. This emerges from new analysis published online by the Food and Drug Administration on Wednesday.

The vaccine had an overall effectiveness rate of 72 percent in the US and 64 percent in South Africa, where a highly contagious variant emerged in the fall and is now driving most cases. The effectiveness in South Africa was seven points higher than previous data published by the company.

The vaccine also showed 86 percent effectiveness against severe forms of Covid-19 in the US and 82 percent against serious diseases in South Africa. This means that a vaccinated person has a far lower risk of being hospitalized or dying from Covid-19.

The analysis confirmed that Americans are likely to soon benefit from a third effective coronavirus vaccine, developed in less than a year, as the demand for vaccines significantly exceeds supply. The FDA could approve the vaccine as early as Saturday, pending a vote by its vaccine advisory board on Friday after discussing the newly released documents.

“With a J&J vaccine, we can accelerate the adoption of vaccines for our country and for the world,” said Dan Barouch, a virologist at Beth Israel Deaconess Medical Center in Boston who led much of the vaccine’s early research last year.

The Johnson & Johnson vaccine can be stored at normal refrigerated temperatures for at least three months, which makes it much easier to distribute than the approved Moderna and Pfizer-BioNTech vaccines, which require two doses and must be stored at cold temperatures.

However, access to the new vaccine could initially be severely restricted. Dr. Richard Nettles, US vice president of medical affairs at Janssen Pharmaceuticals, the drug development arm of Johnson & Johnson, told lawmakers Tuesday that nearly four million doses would be ready to ship after the FDA approves the vaccine, well below the 12 million had originally promised to give it to the federal government by the end of February.

White House officials had an even lower estimate on Tuesday, telling governors on a weekly phone call that around two million doses would be available next week, according to a government official.

Dr. Nettles said Tuesday that a total of 20 million cans would be ready by the end of March. The company has a contract to supply 100 million cans by the end of June.

Updated

Apr. 24, 2021, 8:18 p.m. ET

The Johnson and Johnson vaccine has a lower rate of effectiveness than the Moderna and Pfizer-BioNTech vaccines, which are both around 95 percent.

In South Africa, the Johnson & Johnson vaccine is the clear winner so far. Novavax’s shot had an effectiveness of 49 percent in South Africa. And a small study in South Africa with the AstraZeneca-Oxford vaccine found that it doesn’t offer much protection at all. The negative results prompted the South African government to abandon its plan to give a million doses of AstraZeneca vaccines to healthcare workers. Last week the government started giving Johnson & Johnson’s vaccines instead and has given more than 32,000 to date.

The newly released documents, which included the FDA’s first technical analysis for the company’s 45,000-person study, showed the vaccine was safe, with significantly milder side effects than Pfizer and Moderna vaccines, and with no reports of severe allergic reactions such as Anaphylaxis.

The protection of the vaccine was the same among black, Hispanic, and white volunteers, and among different age groups. The study found a lower effectiveness of 42.3 percent in people over 60 years of age with risk factors such as heart disease or diabetes. However, that number came with a great deal of statistical uncertainty, the FDA found.

Although multiple vaccines can protect people from contracting Covid-19, it’s unclear whether the gunshots can also prevent people from becoming infected and passing the virus on to others, leading to a debate about how quickly society will change after the start Vaccinations can return to normal.

Moderna’s study found some evidence that vaccinated people were less likely to develop an infection without symptoms. And AstraZeneca found that its vaccine reduced asymptomatic infections by about half.

Johnson & Johnson looked for asymptomatic infections by checking for coronavirus antibodies 71 days after receiving a vaccine or placebo. The new analyzes estimate that the vaccine has a 74 percent effectiveness rate against asymptomatic infections. However, this calculation was based on a relatively small number of volunteers, and the FDA noted that “there is uncertainty about the interpretation of this data and no final conclusions can be drawn at this time.”

“I think it will add to the growing evidence that the vaccines actually prevent infection and disease,” said Dr. Barouch.