As stay-at-home orders, social distancing, and the other disruptions to everyday life necessitated by COVID-19 continue, antibody testing has emerged as a watchword for politicians seeking a return to normal or at least as normal as life can be in the wake of such a massive crisis.
But what, exactly, is an antibody test? What makes them different from the tests we have been using for months? And how do they fit into the broader fight against COVID-19? Here’s what you need to know.
What is an antibody test?
Antibody tests measure proteins the body produces to help fight off a viral infection. For many viruses, this immune response means that the body can’t become infected again — at least not soon after the initial infection. Unlike polymerase chain reaction tests that detect the presence of a virus, antibody (or serological) tests detect the presence of antibodies, markers of immune response that show up in blood after the onset of symptoms. An antibody test won’t tell you if you currently have COVID-19, but it will tell you if you have antibodies present that indicate that you had the virus before.
Why are antibody tests important?
It’s very likely that someone who tests positive with a serological test will be immune from the virus for a certain amount of time, as is the case with other coronaviruses. Harvard epidemiologist Marc Lipsitch wrote that, based on the available studies, his “educated guess” is that “most individuals will have an immune response” that will last at least a year. So far, researchers have shown that antibody levels don’t decline for at least two months after infection, according to a new study of 177 COVID-19 patients lasting that amount of time. The study was published on the pre-print website MedRxiv and has not yet been peer-reviewed.
A positive antibody test could, therefore, be a source of great relief to someone who isn’t displaying any symptoms of COVID-19, suggesting that they were at a far lower risk for infection. And widespread testing could reveal a critical mass of such people who might be able to go back to school or work even as others remain at home.
Serological tests could also provide a better picture of how widely the virus has spread, including whether or not enough people have been infected to establish herd immunity. Once herd immunity has been achieved, large sections of the economy could begin to reopen safely.
Antibodies also hold promise for the development of therapies to treat COVID-19, and several studies that depend on antibody testing are already underway.
One such study identified a neutralizing antibody that, in the words of one of its authors, “has potential to alter the course of infection in the infected host, support virus clearance or protect an uninfected individual that is exposed to the virus.” The study goes on to say unequivocally that the antibody, 47D11, will be useful in the development of future serological tests for SARS-CoV-2. It also stands to reason that, if this antibody does fulfill its potential, then tests that reveal its presence or absence in blood will be useful to physicians.
What does a positive antibody result mean?
Antibody tests can’t tell you whether or not you are infected or contagious. Rather, a positive antibody test can mean you’ve had the virus and recovered and aren’t contagious or that you still have it and are contagious.
“Just because you’ve got good antibodies doesn’t mean you can’t spread the disease,” says Janis Weeks, a biologist at the University of Oregon.
There’s also the risk of false negatives. Preliminary data suggests it could take 10-15 days to mount a detectable antibody response to the coronavirus, so if your blood is tested just a few days after contracting SARS-CoV-2 it might not show up in a test.
However, a true negative antibody test does not mean that you haven’t been infected by the coronavirus. About 2% to 8.5% of people with COVID-19 do not develop antibodies even weeks after being infected, according to the new MedRxiv study.
With all of these caveats, the Centers for Disease Control (CDC) recommendation is essentially an abundance of caution. Anyone who does test positive for the virus should self-isolate for one week and then remain six feet away from others for an additional three days. The CDC says that the chances of someone remaining contagious after this 10-day period are low.
Are antibody tests reliable?
The FDA waived its normal regulatory process for coronavirus antibody tests, allowing tests to go to market without FDA approval as long as they’re validated by the developer, the FDA is notified of that validation, and the tests are labeled to make clear that they are not to be used as the sole basis for diagnosis.
In practice, this means that many of the tests that are available aren’t necessarily reliable, as there is no external review of the tests’ efficacy.
That’s a problem because this is a tricky test. Other kinds of coronavirus — the four that cause a big chunk of common colds, SARS, and MERS — produce similar antibodies that might trigger a false positive that could mean an unprotected person, assuming they’re immune, exposes themself to the virus.
The FDA has authorized nine developers of serological tests under the terms of its emergency use authorization process, which does include independent review. It’s also collaborated with the NIH to enable that body to evaluate tests submitted by developers. A test that has made it through either of these processes should give “greater confidence in test performances” to folks who take it than one that has not.
If you look to China to South Korea, countries on the other side of the COVID-19 curve tend to have one thing in common: a clear picture of who has and who hasn’t been exposed to the virus, says Mark Slifka, an immunologist at Oregon National Primate Research Center. With meticulous contact tracing and (in China’s case) surveillance of people’s location, life is returning to some semblance of normal.
For now, there are too many unknowns for antibody testing to make an immediate difference in our day-to-day lives. But the new development is still important, Slifka says. Studying who has and who hasn’t had the virus will help scientists better understand who’s spreading it, how many people are infected but asymptomatic, and whether people do tend to develop immunity. Already, the National Institutes of Health (NIH) is running antibody tests on up to 10,000 volunteers to try and answer some of these questions.
This type of data can help nudge us back to normal, Weeks says. “As more and more data pours in, everything’s going to come into stronger focus.”
Who should get an antibody test?
If you’re currently displaying symptoms, an antibody test won’t do you much good, as it takes time for antibodies to appear in blood. A PCR test is the more relevant test in this situation.
At the moment, there aren’t enough reliable serologic tests available to say that everyone should take one, which would be ideal. Instead, tests should be reserved for people like healthcare workers who can most benefit from the knowledge that they are likely immune from the disease. As more tests undergo scrutiny from the FDA and other government agencies, more people should take them, and states like New York along with the federal government are ramping up programs to test hundreds of thousands of people per day. Unless you’re part of a higher priority group, you should sit tight until antibody tests are available to everyone.
In fact, one Fatherly editor had an antibody test administered in a clinic in Texas last week and the results were negative for the presence for antibodies. However, the doctor administering the test cautioned that the majority readily available today, most from manufacturers in China, had not been vetted and the results were wildly unreliable. He’d had patients who had previously tested positive for COVID-19 whose antibody came back as negative, for example.
Does having antibodies mean you’re immune?
Let’s say you test positive for COVID-19 antibodies, then wait 10 days. Are you in the clear to stop social distancing? The answer is a resounding no. We just don’t know if coronavirus antibodies make you immune to reinfection. “It isn’t a guaranteed thing,” says Ryan Demmer, an epidemiologist at the University of Minnesota.
Immunity isn’t something you have or you don’t, says Mark Slifka, an immunologist at Oregon National Primate Research Center. It’s more like a sliding scale. For some viruses, including measles, antibodies grant immunity for life, Slifka said. For others, including chicken pox and Dengue, you absolutely can become infected a second time — and subsequent infections are even worse. Until we know where COVID-19 falls on that scale, we should continue to socially distance, regardless of whether we’ve tested positive for antibodies, Slifka said.
“The worst thing you can do is to tell somebody that they’re immune and you’re wrong. Then they have this false sense of security, like they’re bulletproof,” Slifka says.
Research on human immune responses to other coronaviruses can lend some insight into how our immune systems respond to COVID-19. One study followed 176 patients who recovered from SARS-CoV-1, the coronavirus that caused the SARS pandemic, and found that they maintained their immune response for an average of two years. Another study found that one year after recovering from a bout of the common cold, two-thirds of patients re-exposed to that same coronavirus developed another infection — but it was less severe. Based on this data, it’s likely that developing coronavirus does make you immune to a second infection, but only for a year or two, Slifka says.
However, case studies and preliminary data from the novel coronavirus outbreak paint a more complicated picture. Researchers from Fudan University in Shanghai studied 175 patients with mild coronavirus and found that 10 of them failed to develop detectable antibodies. These patients were generally young and healthy. This research, which is published on MedRxiv and hasn’t yet been peer-reviewed, suggests that some people who develop coronavirus might never develop immunity to it, even in the short-term.
What’s the deal with immunity passports?
In the future, knowing who does and doesn’t carry antibodies could help relax social distancing requirements. Some experts in the U.S are batting around the idea of immunity “passports” — certificates that would allow people with antibodies to return to school, work, and social events.
In some parts of the world, immunity passports are already becoming a reality. On Monday, Chile became the first country to implement an immunity passport system. As Italy begins to recover from its outbreak, the Italian government is planning to use the passports, contact tracing apps, and mandatory antibody tests to prevent a second wave of infections.
But experts say it’s too soon to entirely rely on the concept. To begin, the rapid roll-out of the antibody tests has introduced some quality control issues, Weeks said. It’s unclear how accurate they are. And given that immunity to the virus will likely vary from person to person and decline over time, it’s unclear what a license would even mean, Slifka said.
“There are a lot of unknowns to it. We don’t want people to be reckless because they have that immunity card.”