JOHANNESBURG (BLOOMBERG) - Covid-19 vaccines will have to be adjusted to improve their protective power against mutations such as the one that's now prevalent in South Africa, researchers said.

A study into a vaccine from Novavax published on Thursday showed that vaccines may be somewhat less effective against new variants.

Johnson & Johnson released similar results on Friday (Jan 29) from its trial.

While the Novavax vaccine was 89.3 per cent effective in preventing symptomatic Covid-19 among 15,000 volunteers in Britain, a trial of 4,400 people in South Africa showed that the vaccine was 60 per cent effective in those who were HIV negative and for those who were HIV positive, the efficacy was even lower.

Most cases of the virus seen in that trial had the South African mutation.

"It's wonderful that we have a vaccine that's more than 60 per cent effective," said Glenda Gray, president of the South African Medical Research Council. However, vaccines will need to be adjusted "as these variants take hold at a global level," she said.

The more transmissible South African variant called B.1.351 has been found in almost 30 countries, including the US, and is likely to be circulating in many others, according to the World Health Organisation.

It attaches itself to human cells more efficiently but does not cause more severe illness.

"The more people that are infected and the longer that goes on, the more likely you are going to get more variants," Helen Rees, chairwoman of the WHO's African Region Immunisation Technical Advisory Group.

"Speed is of the essence in vaccination."

The Novavax results were encouraging because "everyone has been concerned" that Covid-19 vaccines may not work against the new strain, said Shabir Madhi, a professor of vaccinnology and lead researcher for the local arm of the Novavax trial.

In an earlier interview this month, Madhi said that adjusting vaccines may take five to six weeks.