A team of UK doctors have reopened a clinical trial to explore the benefits of blood plasma, from people both infected and vaccinated against COVID, to help treat people with weakened immune systems.

The therapy involves transfusing plasma -- the pale yellow liquid in the blood that is rich in antibodies -- from people who have recovered from COVID-19 into patients who have leukaemia, lymphoma or other blood cancers and are hospitalised with the viral infection to accelerate their disease-fighting response.

The use of convalescent plasma initially touted as the holy grail against COVID has been disbanded by several health experts including in India. It was also granted emergency use authorisation by US Food and Drug Administration (FDA) in 2020 -- the first year of the pandemic.

But in August 2021, a study led by the US National Institutes of Health (NIH) stated that early administration of convalescent plasma does not prevent disease progression in a high-risk group of COVID-19 patients. In May, the Indian Council of Medical Research (ICMR) dropped plasma therapy as COVID-19 treatment, citing no significant benefit.

And in December 2021, the World Health Organisation advised against the use of convalescent plasma as it shows no improvement in survival and other important measures, and said that it is not recommended for patients with COVID-19.

But one of the trials, conducted in the UK to assess the benefits of the therapy, found that plasma with the highest levels of antibodies might help the immunosuppressed, the Guardian reported.

"It's really important because this is a group of patients who are still dying from COVID. This is something that could be beneficial to them," Prof Lise Estcourt, head of NHS Blood and Transplant's clinical trials unit and chair of the new trial was quoted as saying.

Using the therapy, doctors successfully treated the first UK patient, while more than 15 hospitals across the UK have agreed to take part in the trial.

According to the doctors, the plasma will be taken from patients who have been both infected with COVID and vaccinated against the virus, as this produces the highest concentrations of antibodies with the broadest effectiveness against different COVID variants.

The highest level of antibodies seen so far in the reopened trial was in plasma donated by a man in his 20s. Tests found that his plasma contained more than 100,000 units of antibody per millilitre, about 100 times more than was seen in the first wave of the pandemic, the report said.

If the trial finds that the plasma works, it would be a valuable treatment, Estcourt said, because some immunosuppressed people do not respond to the vaccine and "monoclonal antibody" treatments, which are often given to patients, can be less effective against new variants.

"It could also be of particular use in the developing world, where access to more expensive treatments is limited," she said.