Britain said six cases had been detected of the “P.1” variant identified in the Brazilian city of Manaus, against which current vaccines appear to be less effective. Two were in South Gloucestershire in England and three in Scotland.

But a sixth individual in England, one of more than a million people tested on Feb. 12-13, had yet to be identified.

“If you’ve had a test on the 12th or 13th of February, (and) haven’t had your results back, please get in touch,” vaccination minister Nadhim Zahawi told Times Radio. “We are working with the postal system to try and locate them.”

The P.1 variant shares significant mutations with a variant first detected in South Africa known as B.1.351, which has raised similar concerns.

Prime Minister Boris Johnson told broadcasters: “We don’t have any reason at the present time to think that our vaccines are ineffective against these new variants of all types.”

But Adam Finn, of the UK Joint Committee on Vaccination and Immunisation, told the BBC: “The evidence we have suggests that certainly the South African variant and potentially this Brazilian variant, which is somewhat similar - the vaccines we have at the moment are less effective at reducing at least mild disease and possibly transmission.

“We are optimistic that the vaccines will continue to prevent severe disease, but the evidence for that is still fairly limited.”

Manufacturers are working on measures to adapt their vaccines to mutations of the virus.

Johnson said the government would continue with a “cautious but irreversible” relaxing of lockdown restrictions.

One of the cases in the household in South Gloucestershire had flown from Brazil via Zurich to London, arriving on Feb. 10.

All three Scottish cases were identified as people who had flown from Brazil to Aberdeen via Paris and London and had self-isolated for the required 10 days.

By Sunday, over 20 million people in Britain had been given a first dose of COVID-19 vaccine - just over 30% of the population - while around 800,000 had had a second dose.