The Pfizer-BioNTech vaccine is effective against the UK and South African variants of coronavirus, according to a study.
A separate study in South Africa showed the Oxford-AstraZeneca inoculation failed to prevent mild and moderate cases of Covid-19, prompting the country to suspend its vaccination campaign.
But Pfizer and BioNTech said their vaccine was able to neutralise the spike mutations in the variants, which make them more infectious.
The companies cited the results of a peer-reviewed study published in the Nature Medicine medical journal on Monday which found the vaccine was still capable of inducing antibodies against the variant.
It was slightly less effective against the South African variant, but Pfizer said the difference was small and unlikely to lead to a notable reduction in efficacy.
But scientists acknowledged the study of 20 people was small, and that the engineered viruses did not include the full set of spike mutations found in the English or South African variants.
Pfizer and BioNTech said they were “prepared to respond” if there was further evidence a new variant could evade vaccine-induced immunity.
Meanwhile, UK Vaccines Minister Nadhim Zahawi urged people to keep faith in the Oxford-AstraZeneca vaccine.
"While it is right and necessary to prepare for the deployment of an updated vaccine, we can take confidence from the current rollout and the protection it will provide all of us against this terrible disease," he wrote in The Telegraph.
“We need to be aware that even where a vaccine has reduced efficacy in preventing infection there may still be good efficacy against severe disease, hospitalisation and death.”
Prof Shabir Madhi, who led the Oxford-AstraZeneca vaccine trial in South Africa, said there was still hope that the vaccine could prevent severe disease.
The average age of the participants in the study was 31, an age group at lower risk of serious illness from Covid-19.
Prof Shabir Madhi said the Johnson & Johnson vaccine, which uses similar technology to the Oxford shot, was moderately effective at preventing severe illness.
"Extrapolating from that, there is still some hope the AstraZeneca vaccine might well perform as well as the Johnson & Johnson vaccine in a different demographic against severe disease," he told BBC Radio 4's Today programme.
“I think the answer will come pretty soon.”
Prof Peter Openshaw of Imperial College London said “nobody should hesitate to have the vaccine”.
“I am unfettered in my enthusiasm for the vaccines we’ve got in the UK,” he said.