Melanie James, from Cardiff, received a transfusion of monoclonal antibodies at University Hospital Llandough as part of the Recovery clinical trial.

The BBC reports she was breathless and receiving oxygen but said she felt “much better” after receiving the treatment.

Cardiff and Vale University Health Board said the trial was in its “very early stages”.

The transfusion of monoclonal antibodies is the latest treatment to be added to the Randomised Evaluation of Covid-19 Therapy (Recovery) clinical trial, the world’s largest randomised controlled clinical trial.

The new arm of the trial aims to determine the effectiveness of monoclonal antibodies in preventing Covid-19 from entering the cells of patients infected with the virus, and preventing patients from becoming more severely unwell.

Melanie James said she was “never in any doubt” about taking part in the trial after her health “deteriorated very quickly”.

She said “I started to feel better the day after the transfusion, and only had a small amount of oxygen during that night.

“Although I’m still recovering, I already feel much better than I did a week ago.

“I can’t speak highly enough of the treatment and care that I received from everybody involved, from clinical staff to the cleaners and those who offered me a drink.”

Antibodies could be described as the “warriors” of the immune system.

When coronavirus infects your body, antibodies attach to the spikes of the virus, blocking it from entering your cells.

But we produce many different types of antibodies, the most potent are called neutralising antibodies. So scientists “sieve” through them to find the one that’s best at sticking to the spike.

The chosen antibody is multiplied in the lab, and produced in huge quantities. This is then given to patients, immediately boosting their immune response.

Cardiff and Vale University Health Board, which became the first in the UK to take part in Recovery in March, has recruited more than two hundred and ten people to the trial.

In June the trial found dexamethasone to be the first drug proven to improve survival of Covid-19.

Zoe Hilton, Cardiff and Vale UHB’s research team lead, said “We are delighted that Melanie is feeling better, and wish her all the best with her ongoing recovery at home, however it is important to acknowledge that this arm of the trial remains in its very early stages and the widespread effectiveness of this treatment isn’t yet known.”

Oxford University’s Prof Martin Landray, who is co-leading the Recovery trial, told the BBC earlier this month the monoclonal antibody treatment offered to Donald Trump was based on “established technology, used previously to develop similar sorts of drugs against Ebola”.