TORONTO -- Convalescent plasma treatments that offered some hope early in the pandemic are ineffective in seriously ill COVID-19 patients, a new Canadian study suggests.

The Canadian-led international study looked at using plasma from recovered COVID-19 patients in other patients with COVID-19 in hopes that it would prevent intubations and deaths. However, the results of the study suggest that the treatment does neither.

“It has been thought that the blood plasma of COVID-19 survivors would help those seriously ill from the virus but, unfortunately, it does not,” Dr. Donald Arnold, co-principal investigator of the study, hematologist and professor of medicine at McMaster University, said in a press release.

What the researchers found instead was that patients receiving the convalescent plasma therapy often had more negative outcomes than those receiving standard care for COVID-19. While deaths among the two groups were similar, the negative events that plasma therapy patients experienced most often included needing oxygen and worsening respiratory failure.

The trial ended enrolment to the study early as the data showed a lack of benefit from the therapy, and in the end included 940 patients in 72 hospitals in Canada, the United States and Brazil, according to the press release.

The study also found that the antibodies in the donor convalescent plasma varied widely, which the researchers suggest is a result of the varying immune responses in COVID-19 patients. The different antibody levels in the plasma appeared to have a significant impact on patient outcomes in those receiving the therapy.

Those who received donor plasma with low levels or non-detectable antibodies had a higher chance of a worse outcome, according to the study.

“It appears that it may not be that high-titre convalescent plasma is helpful, but rather that low-titre convalescent plasma is harmful,” Jeannie Callum, co-principal investigator, associate scientist at the Sunnybrook Research Institute, and professor at Queen’s University and the University of Toronto, said in the press release.

The reason for this is still unknown, but one of the researchers offered a hypothesis.

“One hypothesis is that those dysfunctional antibodies could compete with the patient’s own antibodies and could disrupt the mounting immune response. This phenomenon has been observed previously in animal models and in human studies of HIV vaccines,” co-principal investigator Philippe Bégin said in the press release.

While more research is needed on the topic, the researchers said that there are many similar ongoing studies worldwide that combined will assist in treating COVID-19 patients.

“This information from Canada’s largest clinical trial on convalescent plasma and COVID-19 may be analyzed together with the results of several similar studies going on in the world to provide more robust information and insight that will guide clinical practice and health policy globally,” Bégin said.