Covid-19 patients with severe symptoms may be best donors for plasma therapy: Study

Covid-19 patients who are sick enough to be hospitalised are likely to have high levels of antibodies that can protect against the disease, and may be the best donors for convalescent plasma therapy, according to a study.

The research, published on Monday in the Journal of Clinical Investigation, also found that older males who have recovered from Covid-19 after having been hospitalised are strong candidates for donating plasma.

Doctors have been using infusions of plasma -- the part of blood that contains antibodies -- from recovered Covid-19 patients to treat Covid-19 patients and also as a possible prophylaxis to prevent the disease, the researchers said.

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Clinical trials of convalescent plasma treatment against Covid-19 are ongoing, and doctors until now haven't had guidance for selecting survivors who are likeliest to have strong antibody responses, they said.

"We propose that sex, age, and severity of disease should be used to guide the selection of donors for convalescent plasma transfer studies because we found that these were significant patient characteristics that not only predicted the amount of antibody but the quality of that antibody," said Sabra Klein, a professor at Johns Hopkins Bloomberg School of Public Health in the US.

The researchers tested the blood of 126 Covid-19 survivors and found high variability in their antibody levels and their antibodies' ability to neutralise the Covid-19-causing coronavirus, SARS-CoV-2.

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Three factors were associated with stronger antibody responses: having been sick enough with Covid-19 to be hospitalised, being older, and being male, they said.

Initial studies of recovered Covid-19 patients have revealed a significant variability in their antibody responses to the virus -- some survivors having very weak responses that would almost certainly be ineffective in helping new patients.

The researchers in the new study looked for factors that might help explain some of that variability and guide clinicians to the patients most likely to have high levels of SARS-CoV-2 neutralizing antibodies.

They examined samples of plasma from the 126 recovered patients using several tests.

These included tests of the plasma's ability in cell cultures to neutralise cell-to-cell infection with SARS-CoV-2, as well as commercial tests for levels of antibodies to the coronavirus's spike protein that studs the surface of coronavirus particles and allows the virus to attach to and infiltrate human cells.

Consistent with several prior studies, the researchers found considerable variability among the subjects in their spike-protein antibody levels and plasma coronavirus-neutralisation potency.

On average, the plasma of survivors who had been hospitalised with Covid-19 had markedly more anti-spike protein antibodies and neutralised the virus more effectively -- suggesting that disease severity prompts a stronger immune response.

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"We know that the magnitude of antibody responses correlates with disease severity in other infectious diseases, such as active tuberculosis," Klein said.

Older age and male sex, which prior studies in both China and Europe have shown are associated with more severe Covid-19, were also associated with stronger antibody responses, though these links were weaker than for hospitalisation status.