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A lab technician shows a bag of frozen blood plasma from a donor who has recovered from Covid-19 at the Blood and Tissue Bank Foundation in Palma de Mallorca, Spain, on October 5. Jaime Reina / AFP / Getty Images

Blood from the most seriously ill Covid-19 patients may be the best drug for convalescent plasma therapy, according to a new study from Johns Hopkins University.

Doctors have used plasma, the liquid part of blood that contains antibodies that can fight the virus, from recovered coronavirus patients to treat Covid-19.

The researchers examined plasma samples from 126 Covid-19 survivors and found great variability in antibody levels and their ability to neutralize the coronavirus, the Johns Hopkins team wrote in their report published in the Journal of Clinical Investigation.

Age and gender could also be important. Older men who have been hospitalized with Covid-19 and have recovered are “strong candidates” for a plasma donation, according to the analysis.

“Three factors have been linked to stronger antibody responses: COVID-19 was sick enough to be hospitalized, being older, and being male,” the report said.

The sicker a person is with the virus, the stronger the antibody response, studies have shown.

“We suggest that gender, age, and disease severity should be used as a guide in donor selection for convalescent plasma transfer studies, as we have found these are significant patient characteristics, including not just the amount of antibody, but also the patient’s Predict the quality of this antibody. Sabra Klein, a professor in the Department of Molecular Microbiology and Immunology at the Johns Hopkins Bloomberg School of Public Health who led the study team, said in a statement.

With clinical trials of convalescent plasma therapy ongoing, researchers said doctors had few guidelines on choosing the best Covid-19 survivors with the strongest antibody responses for treatment.

Several other studies have also found that survivors whose illness was severe enough to warrant hospitalization have more antibodies to the coronavirus spike protein, a surface protein that allows the virus to attach to and enter human cells.