The Indian virus mutation shows signs of being a more contagious form of Covid-19, the World Health Organisation said, as it classified the strain as a "variant of concern".
The announcement came after France said on Monday that 20 cases of the Indian variant had been detected in the country.
Scientists in France warned of a race between variants and vaccinations as they said a fourth wave of the pandemic could be unmanageable because of the summer heat and exhaustion of medical staff.
The Indian variant is feared to be responsible for the country’s explosive second wave, and the WHO said it may also have some resistance to antibodies.
Maria Van Kerkhove, the WHO’s technical lead on Covid-19, pointed to early studies “suggesting that there is some reduced neutralisation”.
She said there was “some available information to suggest increased transmissibility of B.1.617”, the technical name for the mutation.
“As such, we are classifying this as a variant of concern at the global level,” she said.
The WHO said it was too early to say whether the variant might have more resistance to vaccine protections.
Ministers in Britain and Germany cited fears of vaccine resistance when they imposed travel bans on India in order to keep out the variant.
Britain has identified 520 cases of the strain which it labelled as a "variant of concern" last week, and 270 cases of two related strains.
UK Health Secretary Matt Hancock said on Tuesday that travel restrictions and contact tracing should keep the variant at low levels.
“We of course are maintaining a very close eye on the Indian variant - there isn’t any evidence yet that the vaccine doesn’t work against it,” he told Times Radio.
France poised to ease lockdown but scientists raise concerns
France, where the domestic lockdown is due to be significantly eased next week, imposed a 10-day quarantine for arrivals from India last month.
French Prime Minister Jean Castex said on Monday that France was “nearing the end” of the crisis, but a government scientific council warned in a briefing paper that the situation remained perilous.
“The coming weeks remain uncertain and there is a risk of a revival of the epidemic,” the scientists said.
“In the medium term, and ahead of the summer period, the scenarios for the virus’s evolution remain open and have to be anticipated.
“In an unfavourable scenario, in the case of a fourth wave, the exhaustion of medical staff means they will not have the same capacity to provide treatment, especially in intensive care.
“If a fourth wave arrived during a summer heatwave, these difficulties would be increased.”
Such a scenario could be avoided if France could provide vaccinations to 35 million people by the end of June, the paper said.
About 18 million people have had one so far, with the vaccination campaign ramping up quickly after a slow start.
The scientists said there was a critical period between early May and late June, in which many people would still be unvaccinated but France would be vulnerable to new variants.
“These variants represent a major risk for France in the very short term,” they said.
The French scientists warned that there are five million people over 60 who are still unprotected against Covid-19.
They called for a “prudent and controlled” re-opening of French society as the third lockdown imposed by President Emmanuel Macron in April is gradually lifted.
Mr Macron faced criticism after admitting he waited until the last minute to impose a new lockdown to fend off the third wave.
France’s infection rate is now averaging about 18,000 new cases per day, about half the level of early April when the lockdown was imposed.
Mr Castex painted a more optimistic picture on Monday, telling Le Parisien that France was "emerging on a long-term basis" from the crisis.
“I say it in the clearest way possible: we are finally in the process of emerging on a long-term basis from this health crisis,” he said.
“Obviously this exit will take place in a progressive, careful and supported way. But the trend is clear. We are nearing the end and it's good news.”
Restaurant terraces will open on May 19, Mr Castex said, but will be limited to half their normal capacity.
Shops will open on the same date with restrictions on the number of people they can admit, with limits set to be lifted on June 30.
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