Results of two trials of immunotherapy for patients with severe influenza have surprised researchers by drawing a blank for influenza A, while providing some hope for treatment of influenza B.

The double-blinded trials, published simultaneously in the Lancet Respiratory Medicine, are the first major studies testing the efficacy of an intervention based on the convalescent plasma treatment used in the deadly 1918 flu pandemic.

Doctors reported then, and smaller studies have shown since, that blood or plasma from patients recently recovered from the infection can be helpful in treating others who were seriously ill.

In one of the new trials, known as FLU-IVIG, 308 people hospitalised for flu in 45 hospitals across nine countries, including Australia, were randomised either to a single 500mL infusion of anti-influenza hyperimmune intravenous immunoglobulin or saline placebo.