Africa variant, scientists say early trials suggest.
A new study, not yet peer reviewed, involved about 2,000 people who were on average 31 years old.
But Prof Sarah Gilbert, Oxford lead vaccine developer, said vaccines should still protect against severe disease.
She said developers were likely to have a modified Oxford jab by the autumn to combat the South Africa variant.
Meanwhile, vaccine minister Nadhim Zahawi told the BBC’s Andrew Marr that a booster in the autumn and annual vaccines could be required to combat variants
More than 100 cases of the South Africa variant have been found in the UK.
A study outlining early trials, first reported by the Financial Times, suggests the vaccine offered “minimal protection” against mild and moderate disease caused by the variant, the University of Oxford said.
Prof Gilbert told the Andrew Marr Show that current vaccines “have a reduction in efficacy against some of the variant viruses”.
“What that is looking like is that we may not be reducing the total number of cases but there’s still protection in that case against deaths, hospitalisations and severe disease.”
She added: “That’s really important for healthcare systems, even if we are having mild and asymptomatic infections, to prevent people going into hospital with Covid would have a major effect.”
Mr Zahawi, who is overseeing the vaccine rollout, said: “We see very much probably an annual or booster in the autumn and then an annual (jab), in the way we do with flu vaccinations where you look at what variant of virus is spreading around the world, rapidly produce a variant of vaccine and then begin to vaccinate and protect the nation.”
Meanwhile, he said that the UK government has ruled out issuing so-called “vaccine passports” to enable people who have had the jab against coronavirus to travel abroad.
In a separate statement, AstraZeneca said the jab offered “limited” protection against mild and moderate disease caused by the variant.
A spokesman for the pharmaceutical company said it had not yet been able to properly establish whether the jab would prevent severe disease and hospitalisation caused by the South Africa variant because those involved in the study had predominantly been young, healthy adults.
But the company expressed confidence the vaccine would offer protection against serious cases, because it created neutralising antibodies similar to those of other coronavirus vaccines.
The study, which assessed a two-dose regimen, is due to be published on Monday.
Andrew Pollard, professor of paediatric infection and immunity and chief investigator on the Oxford vaccine trial, said: “This study confirms that the pandemic coronavirus will find ways to continue to spread in vaccinated populations, as expected, 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.”
Prof Shabir Madhi, who has led trials for the Oxford-AstraZeneca vaccine in South Africa, said that a vaccine developed by Janssen showed it stopped moderate to severe disease, rather than all disease, and this kept people out of hospital.
“These findings recalibrate thinking about how to approach the pandemic virus and shift the focus from the goal of herd immunity against transmission to the protection of all at-risk individuals in population against severe disease,” Prof Madhi added.
On Saturday, AstraZeneca said its vaccine provided good protection against the variant first discovered in Kent, which is now dominant in the UK.
Current vaccines were designed around earlier versions of coronavirus, but scientists believe they should still work against the new ones, although it is not yet clear how well they work against different mutations.
Early results suggest the Pfizer-BioNTech vaccine protects against the new variants.
And early results from Moderna suggest its vaccine is still effective against the South Africa variant.
Experts say vaccines could be redesigned and tweaked to be a better match for new variants in a matter of weeks or months if necessary.
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