A booster of coronavirus vaccine Johnson & Johnson was 85% effective in preventing serious illness in areas where Omicron was dominant, a South African trial study indicated on Thursday.
The South African Medical Research Council conducted the study on health workers from 15 November to 20 December, but it has still not been peer-reviewed.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.
Around half a million South African health staff have received Johnson jabs as part of clinical trials.
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%.