A booster of US coronavirus vaccine Johnson & Johnson was 85% effective in preventing serious illness in areas where Omicron was dominant, a South African trial study indicated Thursday.

The South African Medical Research Council conducted the study on health workers from November 15 to December 20, but it has still not been peer-reviewed.

It found the top-up jab to largely protect staff in a country where the highly transmissible strain is now behind most Covid cases.

"Data from the... study confirm that the Johnson & Johnson COVID-19 booster shot provides 85% effectiveness against hospitalization in areas where Omicron is dominant," Johnson & Johnson said.

"This adds to our growing body of evidence which shows that the effectiveness of the Johnson & Johnson Covid-19 vaccine remains strong and stable over time, including against circulating variants such as Omicron and Delta," it said.

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.

An earlier South African study in December found the Pfizer/BioNTech vaccine to be less effective overall against Omicron, but still prevented hospital admissions by up to 70%.

© 2021 AFP