On January 10, as soon as the genetic sequence was posted for the new coronavirus then rampaging through China—it didn’t even have the name SARS-CoV-2 yet—Florian Krammer and his lab went to work. Krammer is a virologist and vaccinologist at the Mount Sinai School of Medicine; he specializes in figuring out the bits and pieces of viruses that can teach the human immune system to recognize and fight off germs.

But this time, Krammer wasn’t looking for the soul of a new coronavirus vaccine—at least, not necessarily. He was looking for the sequence for the gene that makes a protein called a spike. Coronaviruses wave it from their outer shells like a cudgel. It’s part of the machinery they use to attach to and invade cells, turning them into factories for making more viruses (and making people sick along the way). Krammer planned to engineer cells to make that spike protein, so he could drag it through a blood sample to see if any antibodies—the action heroes of the immune system—stuck to it. That is, Krammer was building a blood test for Covid-19.

This kind of “serological assay” might lead to the first real treatment for the disease. The idea is called “convalescent serum” or “convalescent plasma,” a century-old idea that uses the blood of people who’ve recovered from a disease to treat people who still have it. Survivors of a disease keep antibodies to that pathogen in their blood; they’ve “seroconverted,” which is to say their blood and immune system have changed. “There are several initiatives right now to find people who seroconverted and use their serum, their plasma, to treat severe cases. That has been done in China, and China also sent serum to Italy,” Krammer says. But to do it right, you need to know who those seroconverted people are and how many antibodies they’re packing. You need a test.

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The nasal-swab snot tests that have been in such short supply can tell if someone is actively infected with the virus. A blood test can’t—at least, not until the infection is far enough along that the immune system has kicked into gear. But a blood test can tell if someone has already been infected and gotten better. (This is why epidemiologists would love to see widespread blood testing too—it’d give them a more accurate idea of the total number of infections out in the world.) And a blood test would work even if that person never displayed any symptoms. “Some people get asymptomatic infections or mild infections and never get detected, but they become, most likely, immune,” Krammer says. “If you have health care workers and can test them to find out they have immunity already, you can strategically use them to deal with Covid-19 patients … You could also think about screening people in the population to see who is immune and can go back to work safely.”

Making the test turned out to be relatively easy. Krammer’s lab published the methods as a preprint—not peer-reviewed, but available for people to try—on March 18. Within three days, 50 labs around the world had requested the fragment of viral RNA he used to make the spike protein and samples of the spike protein itself, which Krammer shipped out. Everyone working on the problem knew that tests were in the pipeline, that Chinese researchers had a serological test but hadn’t released data, and that Singapore was using a test but hadn’t published the recipe. Now that Krammer’s was out there, the hard work could start.

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While Krammer was synthesizing his spike protein, a coalition of more than 100 scientists was assembling around the country to start using convalescent plasma against Covid-19. It’s an approach to fighting disease that predates vaccines and antibiotics, and now—with skyrocketing numbers of people infected with a disease against which they have no natural defenses and no drugs—it may be the stopgap that health care workers are hoping for.