Those who become ill with an infection and recover generally develop antibodies that keep viruses from affecting them in the future — as long as the virus doesn’t mutate beyond recognition for the immune system.

On March 24th, the US Food and Drug Administration (FDA) announced that it will allow the emergency use of plasma from recovered COVID-19 cases for new patients infected with the coronavirus. Clinical trials are also underway to establish the efficacy of convalescent plasma therapy.

Plasma refers to the liquid portion of blood. About 55% of our blood is plasma while the remaining 45% are red blood cells. It is in plasma that antibodies such as white blood cells are suspended and carried to sites of infection.

After a person gets an infection and survives, those antibodies float in the plasma for months, possibly even years.

The notion of infusing a patient with plasma for another person who previously recovered from the same illness can be traced back to the 1890s, during the time of the disastrous 1918 ‘Spanish Flu’ pandemic caused by an H1n1 influenza virus.

A century ago, there are reports of as many as 1,700 patients receiving blood serum from survivors, but it is challenging work to assess the results by today’s standards. These are rather desperate times, though, so authorities are looking to throw everything they have at the new coronavirus.

In 2014, the WHO recommended the use of convalescent plasma collected from patients who had recovered from the Ebola virus in treating currently active cases. A similar recommendation was issued for the SARS outbreak in 2002-2003, which was caused by another coronavirus strain., as well as the MERS outbreak in 2015.

Typically, convalescent plasma or immunoglobulins are used as a measure of last resort in order to improve the survival rate of patients who do not respond to treatment. But its biggest advantage is that it is safe as long as the infused plasma is screened for any sign of viruses or other potentially infectious agents.

In the case of the COVID-19 pandemic, there is no vaccine nor approved treatment (although researchers around the world are conducting accelerated trials on a number of candidates).

This is why the FDA recently approved a century-old approach to treating viral infections, allowing hospitals in the country to perform infusions both as a possible treatment for the sick and as vaccine-like temporary protection for people at high risk of infection.

However, there’s no guarantee this will work.

“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.

This kind of treatment was attempted in China, but the preliminary results suggest only modest success. However, doctors are hopeful convalescent plasma might work if the transfusion is made earlier on in the infection, not in its later phases when the patient is already very sick — which was the condition of most test cases in China.

“Every patient that we can keep out of the ICU is a huge logistical victory because there are traffic jams in hospitals,” Michael Joyner, an anaesthesiologist and physiologist at the Mayo Clinic in Rochester, Minnesota, told Nature. “We need to get this on board as soon as possible, and pray that a surge doesn’t overwhelm places like New York and the west coast.”

According to Nature, New York governor Andrew Cuomo announced that convalescent plasma infusions will begin immediately in order to treat the state’s more than 25,000 cases, and the many more to follow — like any other U.S. state, the number of cases seems to double every two days or so.

Meanwhile, researchers are also starting placebo-controlled clinical trials that will establish the efficacy of plasma infusions at various stages of the coronavirus infection. These are set to take place at hospitals affiliated with Johns Hopkins, the Mayo Clinic and Washington University in St. Louis.

As soon as positive results are reported, doctors could start banking the plasma of survivors for immediate use in hospitals. However, this is an extremely labor-intensive and time-consuming approach. Luckily, it is possible to brew antibodies in the lab. When that happens, be on the lookout for plasma donation announcements — this might be the most important time in our lives to donate blood.