Blood from people who have recovered from COVID-19 may help treat or prevent the disease in others.

Scientists are preparing to see if injecting patients with the clear liquid part of blood, known as plasma, from patients who have recuperated from coronavirus infection can halt the virus.

It's an age-old treatment, used for more than a century against various outbreaks, from the 1918 influenza pandemic to measles in the 1930s to Ebola in 2014.

FILE - A worker checks bottles with blood plasma derivatives at the Octapharma company in Dessau, Germany, Oct. 7, 2009.

When patients recover from a disease, their blood contains antibodies to the germ that caused it. Antibodies are like the immune system's warning flags. When a germ infects a patient again, those warning flags draw the immune system's attention to the invader, and the body can fight it off.

Injecting patients with convalescent serum can pass on that protection.

"If you can harvest those antibodies and give them to people that either have been exposed [to the germ] and therefore are at risk, or are early on in the disease, then you have a good chance of making a difference," said Dr. Shmuel Shoham, an associate professor of medicine at Johns Hopkins University School of Medicine.

Not a sure thing

It doesn't always work, however.

Shoham did a study to see if plasma from recovered influenza patients would help people with moderate to severe flu.

It didn't.

But Shoham hasn't given up hope that convalescent serum could help fight COVID-19, in part because the patients in his influenza study may have been too far along in their illness.

"When you're trying to treat a disease that's later on, when it's too deep into the tissues, other parts of the immune system are important," he said. "So maybe in the influenza patients we were getting it too late. They already had disease inside of them."

Shoham says it may work as prevention for people at high risk, such as health care workers. Vaccines work by triggering the immune system to produce antibodies against a weakened or dead version of a germ. Plasma would provide those antibodies, but only temporarily. Patients receiving the plasma aren't producing their own antibodies, so protection would eventually wear off.

Chinese physicians treated some patients with convalescent plasma, but not in controlled clinical studies, so it's not clear how well it worked.

Call for plasma

Chinese authorities have called for more citizens who have recovered from COVID-19 to donate plasma.

Passengers arrive from a domestic flight at Beijing Capital Airport, March 27, 2020. China will cut international flight routes and bar entry to returning foreigners based in the country to stem the spread of COVID-19, officials said.

"I hope everyone can do his share to help end the epidemic as soon as possible," said Xiong Fei, a bus driver in Wuhan, who caught the virus while treating his mother, who died of the disease, according to China's state-run broadcaster, CCTV.

Blood banks routinely collect plasma for use in surgeries and emergency rooms.

Spanish health care company Grifols has signed an agreement with the U.S. government to collect plasma at the company’s U.S. blood donation centers. It is also planning to purify coronavirus antibodies from plasma for further clinical testing against the disease.

FDA approval

The U.S. Food and Drug Administration this week authorized doctors to use convalescent plasma on an emergency basis.

"Now, this is a possible treatment. This is not a proven treatment. Just want to emphasize that," FDA Commissioner Stephen Hahn said at a press conference.

More testing is needed to find out if treating patients with convalescent serum is safe and effective.

VOA's Brian Padden contributed to this report.