Both patients and donors had to meet many different eligibility criteria.

After close to 500 volunteers stepped forward to donate their blood plasma to the university, 11 were chosen, giving a total of 20 plasma donations, the university said in a press release.

The plasma had been frozen and stored, awaiting a patient who met the criteria for the plasma therapy. That patient arrived last week, the university says.

Associate Professor in Anaesthesiology and Intensive Care at the University of Tartu Jüri Karjagin, one of the leaders of the study, said: "There are many criteria the patient must meet to receive convalescent plasma."

These would include the individual having to have fallen sick with COVID-19 within the preceding 10 days.

"Within that period, the patient's own level of antibodies is sufficiently low. If we can administer existing antibodies to the patient at that time, it may alleviate the progress of the disease," Karjagin continued.

Other prerequisites include acute dyspnoea, a respiratory rate above 25 breaths per minute and a chest X-ray showing infiltrates in more than 50 percent of the lung field, the university says.

Intensive care patients are excluded, since their own antibodies have developed by the time they would have met the other criteria, however.

The initial drive for donors saw 18–56-year-olds who had recovered from COVID-19 invited, by medical researchers and doctors at the University of Tartu, together with the North Estonia Medical Centre's (PERH) blood center.

Jüri Karjagin said that since ways to cure COVID-19 are limited, doctors and medical researchers opted for the tried-and-tested method of transfusion of blood plasma from people recovered from the infection, a phenomenon known as convalescent plasma, a practice in use for over a century which has recently been utilized during the swine flu outbreak and the SARS-1 pandemic.

Whittling down of 11 candidates from original 500

The 500 initial applicants were screened for eligibility, with 36 women and 19 men meeting the criteria, giving an average donor candidate profile of 37 years of age, weighing 73 kg and with a haemoglobin level of 139.

Most of the successful candidates were from the type 0 or type A blood groups – 16 and 26 people, respectively.

So far as symptoms for the erstwhile sufferers went, fever and a change in taste perception were commonest, with some also reporting coughs, sore throats and respiratory difficulties.

Forty-eight of them had suffered from a mild form of the disease and recovered at home, while three had been hospitalized in a general ward, and two of them in intensive care.

Antibodies from the 55 shortlisted were assessed, with testing conducted in cooperation with the Veterinary and Food Board (VTA) Karjagin said.

Viral strains isolated in Estonia were combined with the plasma, incubated for an hour, then incubated for a further four to seven days at body temperature (37C) after Vero E6 cells (referring to a lineage of cells used in cell cultures – ed.) had been introduced.

Results from this experiment, with samples examined under the microscope, ruled out 35 of the total donors, leaving 20. However, nine of these fell at the last hurdle too, for a variety of different reasons including having been diagnosed with Lyme disease, having recently gotten a tattoo, or having reacted badly to the venous procedure.

Karjagin said he was hopeful that a positive outcome from the first donation will be seen soon.

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