J&J’s booster shot provides strong protection against severe disease from Omicron, a study says.

A Johnson & Johnson booster shot provided strong protection against the Omicron variant, greatly reducing the risk of hospitalization, according to a clinical trial in South Africa.

The study, which compared more than 69,000 boosted health care workers with a corresponding group of unvaccinated South Africans, found that two shots of the vaccine reduced the risk of hospitalization from Omicron by about 85 percent. In comparison, another study in South Africa found that two shots of the Pfizer-BioNTech vaccine reduced the risk of hospitalization by about 70 percent.

Although the U.S. Food and Drug Administration has authorized the Johnson & Johnson vaccine as a booster shot, the Centers for Disease Control and Prevention recommended that other vaccines be preferred. The C.D.C. raised concerns about rare but life-threatening blood clots that have been linked to the Johnson & Johnson vaccine.

But the authors of the new study, which was published on a preprint server and has not yet been peer-reviewed, said that the results were important for vaccination efforts in Africa, where the Johnson & Johnson vaccine is a mainstay of Covid public health efforts. As the continent braces for a wave of Omicron cases, a second dose of the vaccine could prevent a surge of hospitalizations.

In a different clinical trial that ended in September, when Delta was still the dominant variant worldwide, Johnson & Johnson found that a second dose of its vaccine given eight weeks after the first greatly increased its efficacy. In the U.S. arm of the trial, efficacy against mild to severe Covid-19 rose to 94 percent, compared with 74 percent for one shot. Across trial sites in 10 countries, the vaccine protected all volunteers against severe disease.

Those results prompted South Africa to launch a trial in November among health care workers who had already received one dose of the vaccine between six and nine months earlier. When the Omicron variant began surging across South Africa in late November, the researchers running the trial began tracking how boosted health care workers fared against the variant, finding that it worked well.

This result was somewhat surprising, given that antibodies taken from people who had received one dose of the vaccine failed to block Omicron from infecting cells in laboratory experiments.

It’s possible that the booster shots raised antibodies to protective levels. And while antibodies help the body fend off infections, they are just one of many parts of the immune system.

Certain immune cells help fight Covid by attacking virus-infected cells. In a study posted online on Tuesday, South African researchers found that immune cells taken from people who received Johnson & Johnson vaccines recognized Omicron-infected cells almost as well as they recognized cells infected with other variants.

It’s possible that in addition to raising antibodies, Johnson & Johnson booster shots also increase the army of immune cells that can wage war on Omicron.