Two doses of Johnson & Johnson's Covid-19 vaccine provided up to 85 percent protection against hospitalization from the Omicron variant, researchers in South Africa reported on Thursday.

The study from the South African Medical Research Council evaluated a second booster shot in 69,092 health care workers from November 15 to December 20. Researchers observed that effectiveness preventing hospitalization rose from 63 percent to 84 percent within 14 days and then 85 percent one to two months post-boost.

The results mark the first evidence of the effectiveness of such a vaccine boost while Omicron is circulating and are important considering the increased reliance on the J&J vaccine in Africa, where the virus strain was first observed, the researchers wrote. The study has not been peer-reviewed.

The Food and Drug Administration authorized J&J boosters in October as part of a broader expansion of the country's booster campaign heading into the fall and winter. All J&J recipients who are at least two months past their shot may get a booster, a recognition of that vaccine's lower efficacy compared to the Pfizer-BioNTech and Moderna Covid shots. The one-dose J&J vaccine is authorized for people 18 and older.

J&J's shot has had limited uptake in the U.S. since the federal government paused its use in April over concerns about blood clotting post-inoculation. The company's U.S. production was also interrupted for months by problems at a contractor's plant.