When a new virus like the novel coronavirus appears and starts infecting people, one critical asset in the fight against it is blood from people who were sick and then recovered. These blood samples can help scientists understand how the immune system responds to it, and can help in the search for therapies to treat the disease.

That’s why, this week, the Vaccine Research Center at the National Institutes of Health put out a call looking for blood donations from people who had COVID-19 and are now healthy.

Analysis of blood samples can give researchers information about if and how people develop protective antibodies after an infection. The immune system usually produces antibodies, which can bind to and deactivate viruses, during and after a viral infection. Those antibodies provide an infected person a level of protection from the virus in the future — they’re unlikely to be infected again because their body’s new antibodies will stop the virus.

“The initial steps are to follow individuals who have recovered and have a way to measure their antibody response,” says Darrell Triulzi, director of the Division of Transfusion Medicine at the University of Pittsburgh Medical Center.

Scientists want to understand how strong the immune response to a novel coronavirus infection is, and how well that response protects people against the virus in the future. They also need to know how long the protection lasts. Long-term research on SARS patients, for example, shows that the protective immune cells were no longer present in some people six years after they were sick.

Preliminary research (that hasn’t yet been published) on COVID-19 patients shows that they do produce high levels of antibodies, which virologists say is a sign people wouldn’t get sick from the virus a second time. Another still unpublished study on monkeys found that they developed antibodies after they were infected with the novel coronavirus, and they didn’t get sick a second time if they were exposed to the virus again. “It indicates that infection results in protective immunity against SARS-CoV, at least in the short term,” Angela Rasmussen, a research scientist at the Center for Infection and Immunity at the Columbia University Mailman School of Public Health, wrote in an email to The Scientist.

But more research is still needed to understand if the antibodies actually provide therapeutic benefit, Triulzi says.

Scientists also turn to the blood of patients who have recovered from COVID-19 as a possible stopgap treatment for the most at-risk people. Because their blood plasma is presumably full of protective substances like antibodies, if it’s injected into sick people, it may help them fight off disease. It’s an old strategy and dates back as far as the 1918 Spanish flu outbreak in the United States, when doctors reported that it helped reduce the number of deaths in seriously ill patients. Recently, it’s been used on an experimental basis to treat people with MERS, H1N1, and Ebola.

“In Ebola it seemed effective, and all we have are case reports right now for COVID-19,” Triulzi says. At a press briefing, Stephen Hahn, commissioner of the Food and Drug Administration, said that the agency was evaluating it. “It’s a possible treatment,” he said.

The treatment is risky, though, and there are always concerns that the use of plasma could make any subsequent infection with the virus in question worse. It would likely only be a temporary measure until more refined treatments became available. But the benefits may outweigh the risks for health care workers or older people who are more likely to become seriously ill if they were infected with the virus.

Just because blood can help researchers study COVID-19, though, doesn’t mean that the blood of people who are or have been infected is generally dangerous, Triulzi noted. An infectious virus wouldn’t linger in someone’s blood for a long time after they’re sick, so it’s not a concern for blood banks or people receiving blood transfusions. “Transmission by blood has not been reported and is unlikely,” he says.

And during a pandemic like this one, it’s more important than ever for healthy people to donate blood if they’re able. “Donation drives have been cancelled left and right. Yet the need for blood continues,” he says. “There isn’t a risk of getting the coronavirus from the donation process. And we still need the blood.”