As patients with COVID-19 continue pouring into emergency departments and intensive care units across the nation, an old treatment that has been adapted for a new disease is being tested in New York.

In the past few days, the Mount Sinai Hospital System has injected more than 20 very sick coronavirus patients with a "convalescent serum" based on the blood plasma of people who have recovered from the disease.

One of the first recovered patients to donate, Danny Riemer, 37, of New Rochelle, New York, said he and his wife feel "blessed" that they are now healthy and can volunteer their plasma to help others. "And despite the fact that we did have the virus, our thoughts are really with others, the people who are still fighting the virus, the people who have had much more serious cases than us," he said.

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Dr. Jeffrey Jhang, medical director of clinical laboratories and transfusion services for the Mount Sinai Health System, said: "We have so many patients who are sick. We are crossing our fingers that this will be a game changer and really accelerate the recovery of these patients."

As first reported in a series of stories by NBCNews.com, Dr. Arturo Casadevall of the Bloomberg School of Public Health at Johns Hopkins University in Baltimore has spearheaded a nationwide effort to develop a plasma treatment.

The National COVID-19 Convalescent Plasma Project is a consortium of more than 40 of the nation's top health institutions across 22 states looking to collect plasma from recovered COVID-19 patients to assist in treating those currently ill with the disease or to perhaps prevent others, like front-line medical workers, from getting the virus at all.

Sheera Riemer, who contracted the coronavirus in March, is still waiting for hear whether her antibody count is high enough to donate plasma. Courtesy Riemer family

Convalescent plasma therapy, which involves collecting antibodies from the blood of recovered patients, is nothing new. It was used to treat the 1918 and 1957 flu epidemics, as well as SARS, H1N1 and Ebola and, most recently, some COVID-19 patients in China.

But in recent weeks, Dr. Florian Krammer, a microbiologist, and his team at Mount Sinai's Icahn School of Medicine have made a breakthrough in COVID-19 antibody testing.

Jhang said: "While other tests measure whether the antibody is there or not, our assay can also measure how much of the antibody is there. And that's important, because we can identify those donors with a high amount of antibody who would most likely benefit the patients receiving this plasma."

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The more antibodies, it's thought, the greater the boost to the treatment's efficacy.

Jhang said that the hospital is using an Emergency Use Authorization from the Food and Drug Administration to conduct the tests but that the agency has discussed issuing a formal protocol so more hospitals can use the test.

On Friday, the FDA issued an updated set of guidelines reminding health care providers that more clinical trials must take place before "routinely administering" convalescent plasma to COVID-19 patients can begin.

When it comes to executing the practice on a wide scale, Jhang said, part of the challenge lies in waiting for enough patients to fully recover and develop immune responses. Then they must be screened. Each donor provides the equivalent of four doses of plasma. At Mount Sinai, two doses are being given to the sickest patients. In the past few days, Mount Sinai has given more than 20 very sick patients the plasma of those who recovered.

Jhang said that when Mount Sinai put out the call for plasma donors — contacting recovered patients and emailing its 40,000 staff members — nearly 10,000 people volunteered within a couple of days. While it turns out that not all who volunteered actually had the disease, the hospital staffers are sorting out the able from the willing, and the response was heartening.

Danny Riemer was one of the first patients to answer the call.

"I think that for people who have recovered from the virus and are thinking about what they could do to help, this is probably the least heroic way possible to be a hero," Riemer said. "It's almost as if a survivor is able to pass the life raft on to the next person who needs it."

Riemer and his wife, Sheera, 35, got a notice in early March that they may have been exposed to a person who tested positive for the deadly virus. The state Health Department told them to self-isolate, and within days they began to develop symptoms. They both tested positive but experienced only "mild symptoms" that required no hospitalization. They recovered at home with their three kids — none of whom got sick.

The Riemers heard about the project at Mount Sinai through a family friend. Danny Riemer said that the moment they were released from quarantine, he and his wife went to get tested for the program.

After getting the news that Danny Riemer's COVID-19 test showed that he was no longer sick and that his antibody test had resulted in a high return, he was able to donate plasma late last week.

"It was simple," he said. "There was a needle put into my arm. The machine did its job. And I caught up on work and just kind of relaxed there."

Sheera Riemer initially was not eligible, as her nose swab was still coming back positive for the disease. This week, on her fourth attempt, she was found to be free of COVID-19, but she is waiting to hear whether she has enough antibodies to be a donor.

The researchers and practitioners who make up the National COVID-19 Convalescent Plasma Project are working to determine which patients would be the best candidates to receive the convalescent plasma, said Dr. Michael Joyner, one of the project leaders, who is a professor of anesthesiology at the Mayo Clinic Alix School of Medicine.

"Historically when products like this have been used for other diseases, it's best to give them relatively early in the course of the disease, versus late in the course of the disease," Joyner said, adding that both intensive care and non-intensive care hospital patients are also being considered.

"So I tell people we're trying to help the front-line people put a finger in the dike," he said. "And then we're waiting also for the biotech and vaccine cavalry to arrive."

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Just last week, doctors at the Mayo Clinic, in Rochester, Minnesota, conducted their first convalescent plasma donation from a person who had recovered from COVID-19. The plasma is most likely headed to a patient they hope to keep out of intensive care.

Joyner said the Mayo Clinic is in a unique position. "The pandemic is a little late to get to us," he said. "As opposed to a week or five days away from the peak here, we're probably two or three weeks away from the peak. So we have time to plan."

But Joyner said the treatment was already being implemented at record speed given the demand for an innovation that could mitigate the effects of infection.

On Friday, the FDA approved a clinical trial allowing researchers at Johns Hopkins to prophylactically test how convalescent plasma could be used to boost the immune systems of those like front-line health care workers who are at a high risk of exposure to COVID-19.

"This is a process — done under normal times, would probably take about 18 months from start to finish, as things are done in series," he said. "Because we're doing them in parallel, we've been able to do them somewhere between eight and 18 days."