Mayo Clinic researchers reported a strong hint that blood plasma from COVID-19 survivors helps other patients recover, but it's not proof and some experts worry if, amid clamor for the treatment, they'll ever get a clear answer.

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More than 64,000 patients in the U.S. have been given convalescent plasma, a century-old approach to fend off flu and measles before vaccines. It's a go-to tactic when new diseases come along, and history suggests it works against some, but not all, infections.

There's no solid evidence yet that it fights the coronavirus and, if so, how best to use it. But preliminary data from 35,000 coronavirus patients treated with plasma offers what Mayo lead researcher Dr. Michael Joyner on Friday called "signals of efficacy."