(Bloomberg) -- Trials of a human blood-based Covid-19 treatment that was granted emergency authorization by the U.S. will be expanded to prove whether it’s safe and effective.

Two human studies of convalescent plasma, a component of blood taken from recovered coronavirus patients, will enroll more patient at hospitals and clinics across the country, with $48 million from the Operation Warp Speed program to accelerate drug and vaccine development.

“The evidence on convalescent plasma as a treatment for severe cases of Covid-19 is promising but incomplete,” National Institutes of Health Director Francis Collins said Tuesday in a statement. “We need to carry out rigorous randomized control clinical trials to determine how this therapy can improve outcomes.”

President Donald Trump and Food and Drug Administration Commissioner Stephen Hahn last month announced an emergency-use authorization for convalescent plasma, which is thought to contain factors that control disease, calling it a “milestone achievement.” Hahn misstated the therapy’s impact on patient deaths in a press conference, leading observers like philanthropist Bill Gates to question the agency’s credibility in the pandemic.

One trial launched in April by NYU Langone Health in New York will partner with the University of Texas Health Science Center at Houston and the University of Miami in Florida to enroll about 1,000 participants. A second begun the same month at Vanderbilt University Medical Center in Nashville, Tennessee, will also expand to about 1,000 patients through access to 50 additional sites.

Early studies suggested that convalescent plasma might benefit severely ill and hospitalized Covid-19 patients. In clinical trials, the treatment is compared to placebo and the possibility of bias is minimized to determine whether it’s safe and effective.

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