Delta infections are surging through populations with low vaccination rates. Dr Rochelle Walensky, director of the US Centres for Disease Control and Prevention, said that’s leading to “two truths” — highly immunised swathes of America are getting back to normal while hospitalisation is rising in other places.

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“This rapid rise is troubling,” she said: A few weeks ago, Delta accounted for just over a quarter of new US cases, but it now accounts for just over 50 per cent — and in some places, such as parts of the Midwest, as much as 80 per cent.

The lab experiments add to real-world data that the 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.

The research has been released as Pfizer seeks US authorisation for a third dose of its COVID-19 vaccine, saying that another shot within 12 months could dramatically boost immunity and maybe help ward off new coronavirus mutations.

On Thursday, Pfizer’s Dr Mikael Dolsten told The Associated Press that early data from the company’s booster study suggests people’s antibody levels jump five- to 10-fold after a third dose, compared to their second dose months earlier.

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But authorisation would be just a first step - it wouldn’t automatically mean Americans get offered boosters, cautioned Dr William Schaffner, a vaccine expert at Vanderbilt University Medical Center. Public health authorities would have to decide if they’re really needed, especially since millions of people have no protection.

“The vaccines were designed to keep us out of the hospital” and continue to do so despite the more contagious Delta variant, he said. Giving another dose would be “a huge effort while we are at the moment striving to get people the first dose.”

AP