Even as you read this, researchers are feverishly working to uncover potential treatments for COVID-19, the highly infectious illness caused by the SARS-CoV-2 coronavirus. One of the treatments being studied is the use of plasma from people who’ve recovered from COVID-19 on currently sick patients. It’s called convalescent plasma therapy.

If you had COVID-19 and recovered, your body developed natural defenses against the disease called antibodies, which are located in the liquid portion of your blood called plasma. Convalescent plasma, which is donated by people who recovered from COVID-19, contains disease-fighting COVID-19 antibodies. In theory, once convalescent plasma is transfused into sick patients, the antibodies battle the active virus and help the patients get better.

The use of convalescent plasma therapy is not a new concept. It dates back to at least the time of the Spanish flu of 1918. It has also been used to fight measles, severe acute respiratory syndrome (SARS), Ebola and other lesser known diseases.

There are several risks associated with convalescent plasma therapy. The treatment is given by transfusion, so blood-borne germs can be transmitted to the recipient, which can cause infections. Another risk is transfusion-related acute lung injury, or TRALI, in which the transferred antibodies damage blood vessels in the lungs.

Transfusion-associated circulatory overload, or TACO, is another risk of convalescent plasma therapy. TACO occurs when your body can’t handle the added blood volume from the transfusion, which can be up to half a liter of fluid. Both TRALI and TACO lead to difficulty breathing and death. Fortunately, these risks are rare.

Several studies have been conducted across the globe to determine the safety and effectiveness of convalescent plasma therapy for the treatment of sick COVID-19 patients. Doctors in China began experimenting with the therapy in January 2020.

In one of their studies, published in the Proceedings of the National Academy of Sciences in April, the Chinese doctors reported that 10 out of 10 patients who received convalescent plasma improved, whereas three out of 10 patients with the same characteristics who didn’t get the therapy died. Other Chinese studies, as well as small studies from Italy and elsewhere look promising as well.

In the US, a comprehensive nationwide study, the results of which were posted on a public server called MedRxiv May 14, found convalescent plasma therapy safe for use on COVID-19 patients. A team of more than 5,000 doctors from more than 2,000 hospitals and laboratories tested the experimental therapy. Of the 5,000 seriously ill patients treated as part of the study, fewer than 1 percent experienced serious adverse events.

The study’s principal investigator, Dr. Michael Joyner, a physiologist at Mayo Clinic, said researchers will start receiving data on the treatment’s effectiveness in the next few weeks, and he expects to have a “much clearer picture” within the next several months.   

“We’re very encouraged that the treatment is safe,” Dr. Joyner says. “That was really the first hurdle for us. And showing that it’s safe gives us confidence to move on and begin to try to understand the efficacy.”

A study from Houston Methodist Hospital published in the American Journal of Pathology May 26, confirmed that convalescent plasma therapy is safe for COVID-19 patients. In that study, 19 out of 25 patients improved with the treatment and 11 were discharged from the hospital. Houston Methodist was also the first academic medical center in the nation to transfuse plasma from recovered COVID-19 patients into two critically ill patients.

With no adverse side effects caused by the convalescent plasma transfusions, the Houston Methodist study concluded that the treatment is a safe option for patients with severe COVID-19 disease.

While these initial studies are promising, randomized controlled clinical trials (RCTs) are necessary to truly gauge the overall safety and effectiveness of convalescent plasma therapy for the treatment of sick COVID-19 patients.

RCTs are clinical research studies in which participants are allocated at random to receive either the new treatment being studied or a control treatment. The control may be the current standard or care, a placebo or no treatment. RCTs are the most effective way to study new treatments without bias.

These clinical trials, which will provide more definitive answers on convalescent plasma therapy for sick COVID-19 patients, are currently being conducted in the US.