WASHINGTON (AP) — Hospitals are gearing up to test if a century-old treatment used to fight off flu and measles outbreaks in the days before vaccines, and tried more recently against SARS and Ebola, just might work for COVID-19, too: using blood donated from patients who’ve recovered.

Doctors in China attempted the first COVID-19 treatments using what the history books call “convalescent serum” — today, known as donated plasma — from survivors of the new virus.

Now a network of U.S. hospitals is waiting on permission from the Food and Drug Administration to begin large studies of the infusions both as a possible treatment for the sick and as vaccine-like temporary protection for people at high risk of infection.

“We won’t know until we do it, but the historical evidence is encouraging,” Dr. Arturo Casadevall of Johns Hopkins University’s school of public health told The Associated Press.

Casadevall drew on that history in filing the FDA application. The FDA is “working expeditiously to facilitate the development and availability of convalescent plasma” a spokesman said.

It may sound like “back to the Stone Age,” but there’s good scientific reason to try using survivors’ blood, said Dr. Jeffrey Henderson of Washington University School of Medicine in St. Louis, who co-authored the FDA application with Casadevall and another colleague at the Mayo Clinic.

When a person gets infected by a particular germ, the body starts making specially designed proteins called antibodies to fight the infection. After the person recovers, those antibodies float in survivors’ blood — specifically plasma, the liquid part of blood — for months, even years.