How many COVID-19 cases have gone undetected? And are those who had mild cases of the disease—perhaps so mild they dismissed it as a cold or allergies—immune to new infections? If so, they could slow the spread of the burgeoning pandemic.
Answering those questions is crucial to managing the pandemic and forecasting its course. But the answers won’t come from the RNA-based diagnostic tests now being given by the tens of thousands. They look for the presence of viral genes in a nose or throat swab, a sign of an active infection. But scientists also need to test a person’s blood for antibodies to the new virus, known as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Such tests can detect active infections, too, but more importantly, they can tell whether a person has been infected in the past because the body retains antibodies against pathogens it has already overcome.
Labs and companies around the world have raced to develop antibody tests, and a few have been used in small studies and received commercial approval, including several from China. But so far, large-scale data from such tests—for example showing what fraction of people in the hard-hit city of Wuhan, China, might now be immune—is still lacking or at least not public. Scientists hope that will soon change as more tests become available.
A new recipe could offer labs an alternative to waiting for or buying commercial tests. Florian Krammer, a virologist at the Icahn School of Medicine at Mount Sinai, and his colleagues posted a preprint yesterday describing a SARS-CoV-2 antibody test they have developed, and directions for replicating it. It’s one of the first such detailed protocols to be widely distributed, and the procedure is simple enough, he says, that other labs could easily scale it up “to screen a few thousand people a day,” and quickly amass more data on the accuracy and specificity of the test. Together with increased availability of commercial tests, that means some important answers about immunity to COVID-19, the disease caused by the novel coronavirus, may be available soon, he says.
To create the test, the researchers began by designing a slightly altered version of the “spike” protein on SARS-CoV-2’s outer coat. (The alterations made the protein more stable for use in the lab.) That protein helps the virus enter cells, and it is a key target in the immune reaction against the virus, as the body churns out antibodies that recognize the protein and tag the virus for destruction. They also isolated the short piece of the spike protein called the receptor-binding domain (RBD), which the virus uses to attach to cells it tries to invade. They then used cell lines to produce large quantities of the altered spike proteins and RBDs.
Those labmade molecules provided the basis for an ELISA test, in which antibodies in a sample of blood or plasma trigger a color change when they recognize a target protein—here an RBD or the spike protein. Initial tests of four blood samples from three confirmed COVID-19 patients and from 59 serum samples banked before the start of the outbreak showed that the test worked, as antibodies to SARS-CoV-2 bound to the test’s proteins. It showed positive results only for the COVID-19 patients and not for any of those controls.
The control blood samples came from people between the age of 20 and 70, many of whom had previously been infected with other viruses. Among them was a different coronavirus, NL63, which causes cold symptoms. Its spike protein uses the same receptor on human cells to infect them, so scientists had worried that antibodies to that virus might cross-react and cause false-positive tests. “Across the board, the controls look very negative,” Krammer says—which is good news.
The fact that antibodies to NL63 don’t also react to SARS-CoV-2 proteins is encouraging for another reason, he adds. Some viral diseases, such as dengue, can cause more serious symptoms if a person has been previously exposed to a related strain of the virus and already has partial immunity. Existing antibodies can react to the related invader and trigger a dangerous overreaction, a phenomenon known as an antibody-dependent enhancement (ADE). Some researchers have suggested ADE might explain why the virus is more deadly in the elderly and less so in children, who have had less exposure to other coronaviruses.
Krammer says he and his colleagues are already using their test in their New York City hospital to better understand how quickly COVID-19 patients start to develop antibodies to the virus. In the future, it could also help identify recovered patients who could then donate their SARS-CoV-2 antibody-rich serum to help treat critically ill patients. Another key application, Krammer says, would be to identify people who have developed likely immunity to the virus. They might be able to treat patients safely or take on other front-line jobs during the pandemic.
Widespread antibody testing could also provide key data for efforts to model the course of the pandemic. Current predictions vary so widely, causing some scientists to question the need for severe containment methods such as lockdowns and social distancing. By indicating how much of the population is already immune because of mild infections, antibody data could offer a key to how fast the virus will continue to spread.
Such data could inform practical issues such as whether and how to reopen schools that have been closed. Relatively few cases have been diagnosed among children, but it isn’t clear whether that’s because they don’t get infected or because their infections are generally so mild that they go unnoticed. Testing children for SARS-CoV-2 antibodies should resolve that.
Longer term antibody tests will also help researchers understand how long immunity to the virus lasts, a key issue for any future vaccine. For other coronaviruses, Krammer notes, immunity after an infection is strong for several months, but then begins to wane. Doctors in Germany are now testing COVID-19 patients from the small cluster of cases in Bavaria in January. One month after infection, antibody levels remained high, says Clemens Wendtner, an infectious disease specialist at the Schwabing Clinic.
Krammer’s team is eager to test as many blood samples as possible, but as the outbreak takes hold in New York City, it is forcing work in his laboratory to slow down. He has told lab members to avoid commuting. “Everyone who isn’t within walking or biking distance is staying home.”