South Africa halted the rollout of AstraZeneca's Covid-19 vaccinations after data showed it gave minimal protection against mild infection from one variant, but Britain said the shot still stopped death and serious illness.

The novel coronavirus has killed 2.3 million people and turned normal life upside down for billions, but new variants of the virus have raised fears that the world could be locked in a cat-and-mouse battle for years with the pathogen.

Researchers from the University of Witwatersrand and the University of Oxford said the AstraZeneca vaccine provided minimal protection against mild or moderate infection from the so-called South African variant among young people.

Prof Shabir Madhi, lead investigator on the AstraZeneca trial in SA, said the vaccine's similarity to another produced by Johnson & Johnson, which reduced severe disease by 89%, suggested it would still prevent serious illness or death.

"There's still some hope that the AstraZeneca vaccine might well perform as well as the Johnson & Johnson vaccine in a different age-group demographic [with] severe disease," he told BBC radio.

Andrew Pollard, chief investigator on the Oxford vaccine trial, said the South African study had shown that the virus would, as expected, find ways to continue to spread in vaccinated populations.

"But taken with the promising results from other studies in South Africa using a similar viral vector, vaccines may continue to ease the toll on health-care systems by preventing severe disease," Pollard said.

While thousands of individual changes have arisen as the virus mutates on replication and evolves into new variants, only a tiny minority are likely to be important or change the virus in an appreciable way, according to the British Medical Journal.

The AstraZeneca vaccine prevents death and serious illness and is effective against the main variants of the virus in the UK, though people may have to have a booster shot as it mutates.

"There is no evidence that this vaccine is not effective in preventing hospitalisation and severe illness and death, which ultimately is what we're seeking with these vaccines today," British junior health minister Edward Argar told Sky on Monday.

"The dominant strains in this country are not the 'South African strain' - there are a small number of cases of that. The dominant strains here are the historic one we've had, and then the Kent variant, against which this vaccine is highly effective."

Argar said just 147 people had been known to have been infected with the "South African variant" in Britain, though he allowed that booster shots - such as against the common flu - might be needed in the future as the virus mutates.

"It would be just be normal in a sense as we did with the flu vaccine to update it to catch anything the virus is trying to do to keep ahead of it," he said.

Sarah Gilbert, professor of vaccinology at the University of Oxford, said efforts were under way to develop a new generation of booster shot vaccines that will allow protection against emerging variants.

"This is the same issue that is faced by all of the vaccine developers, and we will continue to monitor the emergence of new variants that arise in readiness for a future strain change," she said.

Among coronavirus variants most concerning for scientists and public health experts are the so-called British, South African and Brazilian variants, which appear to be more contagious than others.

The UK, which has the world's fifth worst official death toll with more than 110,000, has vaccinated more than 12 million people with a first dose. About half a million people have received a second dose.

Remembering Freddy | Grief and gratitude in a time of Covid-19

Subscribe for free episodes: iono.fm | Spotify | iTunes | Player.fm | Pocket Casts