news releases announcing those deals got none of the flashy media attention that the billion-dollar contracts for Covid-19 vaccines did when they arrived later in the summer. And the government did not disclose how much it would be investing.

American Red Cross and America’s Blood Centers since last April.

“The convalescent plasma program was intended to meet an urgent need for a potential therapy early in the pandemic,” a health department spokeswoman said in a statement. “When these contracts began, treatments weren’t available for hospitalized Covid-19 patients.”

As spring turned to summer, the Trump administration seized on plasma — as it had with the unproven drug hydroxychloroquine — as a promising solution. In July, the administration announced an $8 million advertising campaign “imploring Americans to donate their plasma and help save lives.” The blitz included promotional radio spots and billboards featuring Dr. Fauci and Dr. Hahn, the F.D.A. commissioner.

provided access to its advertising agency, which created the look and feel for the Fight Is In Us campaign, which included video testimonials from celebrities.

although he later corrected his remarks following criticism from the scientific community.

the Infectious Diseases Society of America recommended that plasma not be used in hospitalized patients outside of a clinical trial. (On Wednesday, the society restricted its advice further, saying plasma should not be used at all in hospitalized patients.) In January, a highly anticipated trial in Britain was halted early because there was not strong evidence of a benefit in hospitalized patients.

narrowed the authorization for plasma so that it applied only to people who were early in the course of their disease or who couldn’t make their own antibodies.

Dr. Marks, the F.D.A. regulator, said that in retrospect, scientists had been too slow to adapt to those recommendations. They had known from previous disease outbreaks that plasma treatment is likely to work best when given early, and when it contained high levels of antibodies, he said.

“Somehow we didn’t really take that as seriously as perhaps we should have,” he said. “If there was a lesson in this, it’s that history actually can teach you something.”

pandemic exceptionalism” — had drained valuable time and attention from discovering other treatments.

“Pandemic exceptionalism is something we learned from prior emergencies that leads to serious unintended consequences,” she said, referring to the ways countries leaned on inadequate studies during the Ebola outbreak. With plasma, she said, “the agency forgot lessons from past emergencies.”

While scant evidence shows that plasma will help curb the pandemic, a dedicated clutch of researchers at prominent medical institutions continue to focus on the narrow circumstances in which it might work.

Dr. Arturo Casadevall, an immunologist at Johns Hopkins University, said many of the trials had not succeeded because they tested plasma on very sick patients. “If they’re treated early, the results of the trials are all consistent,” he said.

found that giving plasma early to older people reduced the progression of Covid-19. And an analysis of the Mayo Clinic program found that patients who were given plasma with a high concentration of antibodies fared better than those who did not receive the treatment. Still, in March, the N.I.H. halted a trial of plasma in people who were not yet severely ill with Covid-19 because the agency said it was unlikely to help.

With most of the medical community acknowledging plasma’s limited benefit, even the Fight Is In Us has begun to shift its focus. For months, a “clinical research” page about convalescent plasma was dominated by favorable studies and news releases, omitting major articles concluding that plasma showed little benefit.

the website has been redesigned to more broadly promote not only plasma, but also testing, vaccines and other treatments like monoclonal antibodies, which are synthesized in a lab and thought to be a more potent version of plasma. Its clinical research page also includes more negative studies about plasma.

Nevertheless, the Fight Is In Us is still running Facebook ads, paid for by the federal government, telling Covid-19 survivors that “There’s a hero inside you” and “Keep up the fight.” The ads urge them to donate their plasma, even though most blood banks have stopped collecting it.

Two of plasma’s early boosters, Mr. Lebovits and Ms. Berrent, have also turned their attention to monoclonal antibodies. As he had done with plasma last spring, Mr. Lebovits helped increase acceptance of monoclonals in the Orthodox Jewish community, setting up an informational hotline, running ads in Orthodox newspapers, and creating rapid testing sites that doubled as infusion centers. Coordinating with federal officials, Mr. Lebovits has since shared his strategies with leaders in the Hispanic community in El Paso and San Diego.

And Ms. Berrent has been working with a division of the insurer UnitedHealth to match the right patients — people with underlying health conditions or who are over 65 — to that treatment.

“I’m a believer in plasma for a lot of substantive reasons, but if word came back tomorrow that jelly beans worked better, we’d be promoting jelly beans,” she said. “We are here to save lives.”’

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