A treatment for COVID-19 that has already been approved in the U.S. helped a test group of infected patients get better in one to three days, a new study has found.

Known as convalescent plasma (CP) therapy, the treatment uses blood from recovered patients. Ten COVID-19 patients in China who were severely ill were given a dose of plasma donated from survivors of the virus, which had the antibodies necessary for their immune system to fight off the virus.

“All symptoms in the 10 patients, especially fever, cough, shortness of breath, and chest pain, disappeared or largely improved within 1 [day] to 3 [days] upon CP transfusion,” said the study, published in a respected journal called the Proceedings of the National Academies of Sciences.

The study was conducted in Wuhan, China – where the coronavirus emerged in December – and was led by Kai Duan of China’s National Biotec Group Co. Ltd.

The therapy is not new: It was first used a century ago during the 1918 Spanish flu pandemic. The process is approved in the U.S., and many physicians think it can help those already infected with SARS-CoV-2. A vaccine is 12-18 months away, most experts say, but the CP therapy is cheap and readily available as it simply uses blood from those infected and cured. CP therapy has also been used to battle SARS and MERS, two similar coronaviruses, and Ebola.

“The recovery criteria were as follows: 1) normality of body temperature for more than 3 [days], 2) resolution of respiratory tract symptoms, and 3) two consecutively negative results of sputum SARS-CoV-2 by RT-PCR assay (1-[day] sampling interval). The donor’s blood was collected after 3 [weeks] postonset of illness and 4 d postdischarge,” the study said.

There are nearly 1.5 million confirmed cases in the world, with more than 83,000 deaths, according to the Johns Hopkins Center for Systems Science and Engineering.

Scientists are also working quickly on a simple blood test to find out who has already had the virus — and therefore has antibodies and is immune.

“Some of you could have had it already,” Dr. David Agus, a professor of medicine and engineering at the University of Southern California, said Tuesday on “The Howard Stern Show.” “There will be a test soon to see who has immunity, who had the virus. That test should come out over the next several days, and that will be key to understanding what’s happening in the country.”

The Centers for Disease Control and Prevention (CDC) has already begun preliminary studies to try to find out how many Americans have already been infected with the virus.

“The first, which has already begun, will be looking at blood samples from people never diagnosed as a case in some of the nation’s Covid-19 hot spots, to see how widely the virus circulated,” STAT reported on Saturday. “Later, a national survey, using samples from different parts of the country, will be conducted. A third will look at special populations — health care workers are a top priority — to see how widely the virus has spread within them. Bresee said the CDC hopes to start the national survey in the summer; he gave no timeline for the health workers study.”

The Food and Drug Administration (FDA) has also authorized the first blood test, known as a serology test, to look for antibodies in the blood. Cellex Inc., a medical device company based in North Carolina, says the test could help physicians determine how widespread the virus is and the duration of immunity for people after they recover.

“Based on the totality of scientific evidence available to FDA, it is reasonable to believe that your product may be effective in diagnosing COVID-19,” FDA chief scientist Denise Hinton wrote in a letter to James Li, CEO of Cellex. “The known and potential benefits of your product when used for diagnosing COVID-19 outweigh the known and potential risks of your product.”

Having antibodies in the blood means that that person likely — though not definitely — had the virus and, for at least a period of time, will have some immunity. That means testing for antibodies could help identify who can safely return to the workforce.