Statisticians and scientists said that Dr. Hahn, in saying at the news conference that 35 out of 100 sick Covid-19 patients would have been saved by receiving plasma, appeared to have overstated the benefits…

The publicly released data from the Mayo Clinic shows that, among the larger group of more than 35,000 patients, when plasma was given within three day of diagnosis, the death rate was about 22 percent, compared with 27 percent when it was given four or more days after diagnosis.

Dr. Eric Topol, a professor of molecular medicine at Scripps Research in La Jolla, Calif. and a longtime expert in clinical trials, said that convalescent plasma has not yet shown the benefit that Dr. Hahn described — and that he should issue a correction.

“He needs to come out with that, and until he does, he has no credibility as an F.D.A. commissioner,” Dr. Topol said.