A preliminary South African government study published Thursday showed a booster of the Johnson & Johnson Covid vaccine was 85 percent effective in preventing hospitalization from the Omicron variant, a finding that helps revive the shot's reputation.

The South African Medical Research Council compared 69,000 health care workers who received two doses of the vaccine, based on viral vector technology, against a group of people who were unvaccinated.

The research, which has not yet been peer reviewed, was conducted from November 15 to December 20, a time when the heavily-mutated Omicron variant increased from 82 to 98 percent of Covid-19 cases in the country.

When a booster shot was given six to nine months after the first dose, vaccine efficacy against hospitalization increased over time, from 63 percent at 0-13 days to 85 percent one to two months post-boost.

"This data is important given the increased reliance on the Ad26.COV.2 vaccine in Africa," wrote the authors, using the formal name for the J&J shot.

The result was also hailed by the company. In a statement, J&J scientist Mathai Mammen said it showed the vaccine "remains strong and stable over time, including against circulating variants such as Omicron and Delta."

Around half a million South African health staff have received Johnson jabs as part of clinical trials.

Africa's hardest-hit country, South Africa has recorded more than 3.4 million cases and 90,000 deaths since the start of the pandemic.

 T cells

An earlier South African study in December found that two doses of the Pfizer-BioNTech vaccine prevented hospital admissions by up to 70 percent. Results for three doses of that vaccine aren't yet known.

The news comes weeks after the United States formally recommended mRNA vaccines, made by Pfizer and Moderna, over the J&J shot, because of greater risks related to a rare form of blood clotting.

Preliminary lab studies had also shown that the J&J vaccine triggers fewer neutralizing antibodies -- Y-shaped proteins that block infection -- against the heavily mutated Omicron variant, compared to mRNA vaccines, and it was suggested that it would be less effective in real life too.

The reason it performs better in the real world than some expected could be it evokes a heightened response of another part of the immune system, known as cellular immunity.

A separate, small analysis carried out by the Beth Israel Deaconess Medical Center in Boston on 65 individuals found that a J&J booster on top of two doses of the Pfizer Covid vaccine might provide greater protection than three doses of the Pfizer vaccine.

Though antibody levels surged to high levels after three doses of Pfizer, they also waned within a few weeks, while they rose steadily after two Pfizer shots and a J&J shot, and were at higher levels after four weeks.

Boosting with the J&J vaccine also elicited a strong increase in "killer T cells."

Though they can't stop infection, killer T cells search for cells that have been infected with the virus and destroy them, helping prevent severe illness. They are able to withstand variants far better than antibodies.

Overall, the data might mean that mix-and-match boosting could provide stronger protection than continuing with the same vaccine, but the long term durability of both strategies needs further study, said J&J.