Convalescent plasma does not reduce the risk of intubation or death in Covid-19 patients, according to a study which found that people receiving the therapy experienced more serious adverse events than those getting standard care.
Intubation is a procedure where a tube is inserted into the windpipe to make it easier to breathe.
The research, published in the journal Nature Medicine, also found that the antibody profile in the blood of people who have had the virus is extremely variable and this may modify the response to the treatment. Convalescent plasma therapy uses blood from people who have recovered from an illness to help others recover.
India had dropped convalescent plasma therapy (CPT) from its national clinical management protocol for Covid-19 in May itself.
The Canadian researchers also found that patients receiving convalescent plasma experienced significantly more serious adverse events than those receiving standard care.
The majority of those events were an increased need for oxygen and worsening respiratory failure, they said. However, the rate of fatal incidents was not significantly different from the control group of patients who did not receive the blood.
The clinical trial, called CONCOR-1, included 940 patients at 72 hospitals in Canada, the US, and Brazil.
The trial found that convalescent plasma had highly variable donor antibody content due to the highly variable immune response to the virus.
Different antibody profiles in the convalescent plasma were observed to significantly impact whether or not patients experienced intubation or death.
Unfavourable antibody profiles, meaning low antibody titres, non-functional antibodies or both, were associated with a higher risk of intubation or death.
“These findings may explain the apparent conflicting results between randomised trials showing no benefit, and observational studies showing better outcomes with higher titre products relative to low titre products," said study co-principal investigator Jeannie Callum, an associate scientist at the Sunnybrook Research Institute in Canada.
"It appears that it may not be that high-titre convalescent plasma is helpful, but rather that low-titre convalescent plasma is harmful," Callum said.
The researchers noted that the harm may come from the transfusion of convalescent plasma containing poorly functioning antibodies.