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The Hard Truth About Antibody Tests

Some think of them as a "quick fix" for getting out of coronavirus quarantine. But there's still a lot we don't know about Covid-19 immunity.

A Covid-19 serological test in France (Sebastien Bozon/AFP via Getty Images)

As President Trump made moves this week to open the economy back up, public health officials warned that there are still not enough coronavirus tests available to make that possible. They cautioned that American health infrastructure—from testing and treating to contact tracing and monitoring those in self-quarantine—needs to be firmly established before gently easing restrictions.

One public health tool that could speed the return to normalcy is serological tests—blood tests that measure antibodies to the virus, in theory determining who may have developed immunity. The blood test has been called “the coronavirus test that might exempt you from social distancing,” “a potential game changer,” the “first step” or the “key” to getting back to normal. The question is whether we’re anywhere close to being able to produce reliable forms of these tests—and whether we even know enough about the virus and immunity in general to be able to draw broad conclusions from them.

Standard coronavirus tests check for active virus particles in the nose and throat, in effect determining who is currently sick and at risk of passing the virus to others. Through a quick, relatively simple finger-prick or blood draw, serological tests can identify people who have already mounted an immune response to the coronavirus—regardless of whether they became ill but have now recovered, or perhaps showed mild or no symptoms at all. These tests look for antibodies that the body creates, often after symptoms have eased—sometimes as long as two weeks after the infection.

Serological tests are important for a number of reasons. Researchers can conduct serological surveys to get a better picture of how and where the virus has spread. For instance, knowing how many children had the virus would affect where and when schools are reopened. Getting a clearer idea of how many people have had the virus is also important in calculating transmission and mortality rates. The tests could help identify hot spots, allowing officials to ramp up testing and contact tracing in hard-hit areas, while easing restrictions in places that seem to be doing okay.

The tests will also play a crucial role in beginning to understand how immunity to this particular virus works—what levels of antibodies may provide protection from the virus, and how long immunity could last. These questions are particularly relevant for health workers and first responders, who could help care for Covid-19 patients without worrying about contracting the virus again or bringing it home to their families. In other disease outbreaks, like Ebola, survivors not only helped researchers learn about their immunological response and aided others in their recovery by donating blood plasma, but also helped care for others who were still sick.

This month, the U.S. Food and Drug Administration (FDA) has granted four emergency authorizations for serological tests. Not all tests need to be authorized by the FDA. A month ago, the agency released guidelines for effective serological testing procedures to aid any company in devising their own tests—part of the FDA’s relaxed rules to encourage the rapid creation of tools quickly in the face of the pandemic.

One such test was developed by Boston Heart Diagnostics. Normally, the company uses blood tests to assess risk for heart disease, liver issues, and signs of diabetes so that patients can make different dietary decisions. Patrick Noland, president of the company, said they have little of that work right now, since patients are avoiding non-emergency visits to their doctors. The company changed direction after the FDA issued guidance last month for developing serological tests. “We realized that, wow, we’ve got everything we need to run Covid antibodies, and immediately started doing that,” Noland told me. They developed and tested their assay, and brought it to market three weeks later. Noland likened their unexpected shift to Dyson, the vacuum cleaner company, switching to ventilator manufacturing. Their Massachusetts lab is also New York State–certified, and they can do 4,800 tests a day, he said.

But serological tests are not a silver bullet. First of all, some tests may not be accurate; researchers and companies creating the tests have struggled to get blood samples from patients who had confirmed cases of the virus, in order to check the accuracy of their serological tests. British officials ordered more than four million serological tests in March that turned out to be faulty; now, working tests may not be available in the United Kingdom until next month. In addition, many of the same issues with testing for the active virus apply to the serological tests, with supply shortages, overstretched laboratories, and a lack of personal protective equipment for the health workers conducting the tests. And some experts worry that tests rushed to the market for commercial consumption and unauthorized by the FDA could create even bigger problemsespecially in the case of false positives, which could lead people to believe they are now immune to the virus when they are not.

Perhaps more importantly, it’s not clear what kind of immunity people develop to this virus, even after they’ve recovered from it. Based upon studies of other coronaviruses, like SARS and MERS, some researchers expect immunity for a couple of months, maybe even a year or two. But some preliminary research has indicated that this coronavirus may not follow the expected pattern. A preprint study (meaning it has not been peer-reviewed or published in an established scientific journal) from Chinese researchers found that nearly one-third of patients who had recovered from the virus had lower levels of antibodies than expected, with five percent showing no antibodies at all. Experts believe this could have to do with the severity of the disease; it’s possible that milder cases provoke fewer antibodies.

Researchers in Germany conducted a serological survey in one of the hardest-hit towns in the country, testing 500 hundred people in the western municipality of Gangelt, which drew thousands of visitors in February for its carnivals and festivals. About 14 percent of the people they tested had some level of antibodies, they found (the study is in German), in addition to two percent of the population with currently active cases, detected through coronavirus tests. That’s a far cry from the 60 percent or so needed to establish herd immunity.

The serology tests are also limited to detecting antibodies in those who fought off infections weeks or months ago. It doesn’t provide a picture for where the virus is currently spreading. A serology test “tells you where you’ve been,” Scott Gotlieb, former commissioner of the FDA and a fellow at the American Enterprise Institute, said. “It doesn’t tell you where you’re going.” Only testing of the virus itself can do that.

Furthermore, many serological tests are still being rationed for use by those who need it most: acutely ill patients, health workers, first responders, police officers and other essential workers. “We need to make sure that our first responders are well and taken care of, so they can keep the rest of us safe,” Noland said. “We need to make sure our hospital workers are well, and can continue to care for us.” He suspects he had Covid-19, but he still hasn’t run the test on himself. “That’s not where our early capacity should go,” he said. Serological tests also cannot diagnose coronavirus cases, Noland cautioned. “If you have a patient that has just contracted the disease, the patients’ body has not started to generate the antibodies. And a false negative is really detrimental to the system.”

What actually stops the virus from being spread, experts say, is all of the measures we’re currently taking: staying at home, physically distancing, self-quarantining if symptoms develop. These tests just help fill in some of the gaps in knowledge. “None of these tests are perfect,” Noland said. “You could have a sampling error with the swab. Or you could swab too late; the patient’s already had it. You could do the blood test too early.” The key, he said, is employing all of the public health tools for clinicians to “get their arms wrapped around” the pandemic response.

Caitlin Rivers, a senior associate at the Johns Hopkins Center for Health Security, highlighted the importance of established public health measures like testing for the virus itself and tracing contacts, as well as protecting health workers. Widespread serological testing is an “interesting idea,” she told me, but she questions about how long and how robust immunity is for this virus need to be answered before it becomes a significant part of the U.S. strategy for stopping the virus. “We’ve never used serology in this way,” she told me. There are also significant issues around “immunity certificates” based upon regular serological testing, recently proposed by some U.S. and international officials. “How would you keep track of people’s immune status? What are the ethics around using this [testing] for individual decision making?” Rivers asked. “There’s a lot left to be done.”

Rivers and others have urged massive improvements to the U.S. health infrastructure in order for Americans to begin returning to work and school when the appropriate time comes. They call this period of shelter-in-place orders “phase one,” and say that officials should be using this time to put other measures in place—from hiring more people who can conduct contact tracing to manufacturing more personal protective equipment and medical supplies. “If we wait to scale up our public health capacity, and if we don’t continue to push on making [coronavirus] testing widely available, then we’re going to be stuck in phase one for longer,” she said.

For those longing to get back to normal, or some version of it, this news can be frustrating, to say the least. But trying to leapfrog over the next steps could send us back to square one, experts say. Continuing to stay home whenever possible and practice physical distancing will afford officials and researchers the time they need to learn more about the virus. Despite the understandable lure of a quick fix, building up health care capacity in every way—more ventilators, more protective gear, more coronavirus tests, more research on the serological tests, more workers to conduct tests and trace cases—is probably a better bet, and more likely to lead to a lasting solution.