US health experts said Tuesday they are halting clinical trials of convalescent blood plasma in patients with mild to moderate Covid symptoms after preliminary results showed no benefit from the treatment.

The trial that began in August involved just over 500 people who went to emergency rooms but did not need to be hospitalized.

The people chosen for the study also had risk factors such as obesity, high blood pressure, diabetes or heart trouble.

Some of them received the treatment -- blood plasma from people who had Covid and overcame it -- and others got a placebo.

Doctors looked at how many needed additional care or outright hospitalization, or who died, in the 15 days after undergoing the treatment.

An independent group of experts determined that while the convalescent plasma intervention caused no harm, it was unlikely to benefit patients, the National Institutes of Health said.

Convalescent plasma is the liquid part of blood from a Covid patient that recovered. It contains anti-bodies produced by the body after being infected.

In late October a study carried out in India and published in the medical journal BMJ said the treatment offered limited effectiveness.

It did not reduce mortality or keep people with moderate Covid symptoms from developing a serious case.

More than 100,000 people have received the treatment in the US since the start of the pandemic and many more elsewhere in the world, the NIH said.

In late August, at the persistent request of then president Donald Trump, the US Food and Drug Administration granted emergency authorization for transfusions of blood plasma from recovered Covid patients to people who were hospitalized with the disease.

The then head of the FDA, Stephen Hahn, acknowledged an error at a press conference with Trump, saying he had cited figures that overestimated the benefits of the treatment.

The FDA recently restricted its emergency authorization to use of plasma only with a high concentration of antibodies and for patients hospitalized with early stages of Covid or with a limited ability to produce antibodies.