The US Food and Drug Administration (FDA) has given emergency authorisation to use plasma to treat Covid patients.
The technique uses antibody-rich blood plasma from people who’ve recovered from the disease and has already been applied to 70,000 people in the US – in trials or for the gravely ill.
The FDA says initial trials indicate it is safe, although more are needed to prove effectiveness.
Several experts have questioned the robustness of studies into its use.
The announcement came a day after President Donald Trump accused the FDA of impeding the rollout of vaccines and therapeutics for political reasons, and on the eve of the Republican National Convention, where he will launch his campaign to win a second term in the White House.
“This is what I’ve been looking forward to doing for a long time,” the president told reporters on Sunday.
“I’m pleased to make a truly historic announcement in our battle against the China virus that will save countless lives.”
Mr Trump described the procedure as a powerful therapy, as he appealed to Americans to come forward to donate plasma if they had recovered from Covid-19.
More than 176,000 people have died from coronavirus since the start of the outbreak in the United States, according to a tally by Johns Hopkins University. Nearly 5.7 million cases have also been confirmed nationwide. The country has had more confirmed cases and deaths than anywhere else in the world.
The agency said it had concluded it was safe after reviewing the results of 20,000 patients who had received the treatment so far.
The FDA said people under the age of 80 who were not on a respirator and received plasma containing high levels of antibodies had a 35% better survival rate a month after the treatment than those who had received plasma with a low level of antibodies.
“It appeared that the product is safe and we’re comfortable with that and we continue to see no concerning safety signals,” said Peter Marks, director of the FDA’s Center for Biologics Evaluation and Research.
In his own comments, Mr Trump did not use such nuanced language, saying instead that the plasma treatment had been “proven to reduce mortality by 35%”.
Several experts, including Dr Anthony Fauci, a member of the White House’s coronavirus task force, have expressed reservations about the robustness of studies so far.