Pfizer and BioNTech said they expect that a third dose will perform well against the Delta strain, and that they would be seeking authorisation in the United States, Europe and other regions in coming weeks.

Initial data from an ongoing trial showed a third shot pushed antibody levels five to 10 times higher against the original coronavirus strain and the Beta variant, first found in South Africa, compared to the first two doses alone, according to a statement.

The companies said they expected similar results for Delta – but added they are also developing a Delta-specific vaccine against the strain.

US regulators said late Thursday they were still studying the need for booster shots.

“Americans who have been fully vaccinated do not need a booster shot at this time,” the Food and Drug Administration and US Centres for Disease Control and Prevention said in a joint statement.

“We are prepared for booster doses if and when the science demonstrates that they are needed.”

Currently only about 48 per cent of the US population is fully vaccinated – and some parts of the country have far lower immunisation rates, places where the Delta variant is surging.

In Brazil, which has the world’s second-highest known Covid-19 death toll after the United States, authorities have said the variant was spreading rapidly in the country’s most populous state Sao Paulo.

WHO urges ‘extreme caution’ in lifting restrictions

And in Africa, the WHO warned the worst was yet to come after the most disastrous week in its history of pandemics.

“The fast-moving third wave continues to gain speed and new ground,” said Dr Matshidiso Moeti, the WHO’s regional director for Africa.

In Russia, where Delta has driven the death rate to pandemic highs among a vaccine-hesitant population, Moscow police have launched criminal probes to crack down on fake inoculation certificates.

Also Thursday, researchers from France’s Pasteur Institute reported new evidence that full vaccination was critical.

In laboratory tests, blood from several dozen people given their first dose of the Pfizer or AstraZeneca vaccines “barely inhibited” the Delta variant, the team reported in the journal Nature. But weeks after getting their second dose, nearly all had what researchers deemed an immune boost strong enough to neutralise the Delta variant – even if it was a little less potent than against earlier versions of the virus.

The French researchers also tested unvaccinated people who had survived a bout of the coronavirus, and found their antibodies were fourfold less potent against the new mutant. But a single vaccine dose dramatically boosted their antibody levels – sparking cross-protection against the Delta variant and two other mutants, the study found. That supports public health recommendations that Covid-19 survivors get vaccinated rather than relying on natural immunity.

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The lab experiments add to real-world data that the Delta variant’s mutations aren’t evading the vaccines most widely used in Western countries, but underscore that it’s crucial to get more of the world immunised before the virus evolves even more.

Researchers in Britain found two doses of the Pfizer vaccine, for example, were 96 per cent protective against hospitalisation with the Delta variant and 88 per cent effective against symptomatic infection. That finding was echoed last weekend by Canadian researchers, while a report from Israel suggested protection against mild Delta infection may have dipped lower, to 64 per cent.

Agence France-Presse and Associated Press