Lilly, which is developing the drug with the biotechnology firm Abcellera, tested three doses of its antibody against placebo in a trial enrolling about 450 patients recently diagnosed with mild to moderate Covid-19. The middle dose, 2,800 mg, met the trial’s primary goal of significantly reducing patients’ levels of SARS-CoV-2 after 11 days.

But other doses of the drug did not meet that goal, including a higher dose of 6,000 mg and a lower one of 700 mg. That could raise the risk that the benefit was due to chance, because normally one expects a medicine to become more effective as the dose increases, known as a dose response.