We’ve known it since the early days of the pandemic: To stop the spread of Covid-19, we need more and better testing. But with the crisis marching toward what could be a very dark winter, this old and sensible idea has lately been recast as a fantasy solution. If our Covid tests were only cheap enough to make, and prompt enough in delivering results, according to Harvard epidemiologist Michael Mina, then we could “quickly contain and end this terrible plague.” Let’s call it the Theory of the Magic Testing Cure: Give the masses inexpensive, instant, at-home diagnostics, and everyone who is infectious will know to put themselves in quarantine. “The tests actually become the intervention,” says Mina.
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His theory has been gaining both attention and adherents since July. “To get out of this pandemic, we need fast, easy coronavirus testing that’s accessible to everyone,” The New Yorker’s Atul Gawande wrote earlier this month. Now even governments are signing on: The UK has lately worked up a $130 billion “moonshot” scheme to implement rapid testing on a massive scale; in the US, a similar plan appears to be on the table in Maryland. But all this enthusiasm may be premature, at best. The Magic Testing Cure is based on several key assumptions that may not pan out, and a dangerously narrow understanding of the pandemic.
Let’s start with the basics: The cheap test for which Mina and others have been advocating is called a rapid antigen test, and it looks for proteins on the surface of the virus that causes Covid. Like an over-the-counter pregnancy kit, it’s cheap, quick, and easy to use in the privacy of your own home. The downside is that antigen tests are far less accurate than PCR tests, which have so far been the standard means of diagnosing the disease.
Mina argues that this imprecision is not a problem. Sure, the antigen tests will miss some people who are harboring the virus, but he claims that they excel at catching the most infectious cases. If you take a random group of people who would be Covid-positive on the standard PCR test and give them an antigen test instead, he says, only something like 15 or 20 percent of them would be flagged as infected. “That sounds really scary and terrible,” but he argues that it’s not, because the antigen tests “have a very high sensitivity—say 95 or 98 percent—to detect people when they’re most transmissible.” The tests might turn up negative in people shedding smaller amounts of the virus, but according to Mina, these would be mostly non-infectious cases and not a threat to public health. PCR tests, on the other hand, may continue to be positive even late into someone’s course of infection, when it’s much less important to keep them isolated.
The argument sounds convincing, but it involves “a huge logical jump,” says Benjamin Mazer, pathologist at Johns Hopkins Hospital. For one thing, while it’s reasonable to suggest that people who harbor more of the virus are more infectious, this isn’t a given. Your chances of passing the virus on to other people isn’t just a product of how much you’re secreting, it also depends on your behavior, Mazer says. We still don’t know which combinations of factors lead to the most contagious situations. “Is it the person with high viral load who’s breathing? Or the person with a low viral load who’s sneezing a lot?” These are very hard questions to study, Mazer says, and it’s taken decades to answer them for other infections.
It’s also misleading to suggest that viral load is the only factor that determines whether a given test will be positive. “If you have a bad sample from someone who’s very infectious, the antigen test will miss that,” Mazer says. Lots of little things could affect the results: someone could produce too much saliva for the test, or maybe not enough; they might contaminate their test with food or drink; or else they might fail to follow instructions for how to store the test. Theories are great, but real-world test performance cannot be overlooked, for any kind of tests.
False negatives aren’t the only problem with the Magic Testing Cure, there’s also the issue of false positives. Even as we reach 200,000 deaths in the US, the prevalence of Covid-19 remains low in many places. It’s basic math that when you give a screening test to people who have a low probability of having the virus, the number of false positives will far outnumber the true positives. That means a lot of your positive results will turn out to be mistaken, and lead to people missing work or school, or sacrificing in other ways, for a quarantine that’s unwarranted.
“What we’re getting now is a real-world lesson in what testing can and can’t do.”
Michael Osterholm, Center for Infectious Disease Research and Policy, University of Minnesota
Keep in mind that these tests are being developed and tested with people known to be sick. It’s not clear how they will perform on asymptomatic or presymptomatic people, says Harvard physician and bioethicist Aaron Kesselheim. Mina’s plan “sounds like a wonderful idea, but it needs to be properly validated before we can be confident that it works,” says Steven Woloshin, codirector of the Center for Medicine and Media at the Dartmouth Institute. “What we need is tests of testing strategies.”
But let’s imagine for a moment that all the assumptions about the feasibility and accuracy of these rapid, at-home tests are correct. Even if all those hurdles could be overcome, we’d only be halfway to a solution. It’s what happens after the results are in that make the difference.
Central to the Magic Testing Cure is the premise that people who test positive will self-quarantine to halt transmission. The problem is that a lot of people in the US do not have the means to enter a two-week lockdown, including some of those who are most affected by the virus. Still others do not take the threat of Covid seriously.
When I asked Mina about this, he implied that people who tested positive would behave responsibly enough. The full, 14-day quarantine is recommended out of an abundance of caution, he says; and even just four or five days’ worth of isolation would reduce most transmission. (Never mind that there are many people in America who would lose their jobs or income if they stayed home even that long.) Anti-mask activists and “people who pretend to believe it’s a hoax” might still use the tests, he says, if they could do so privately. “Nobody has to know what your result is or why you’re not going to the bar that night.”
Not everyone shares Mina’s optimism. If a subset of the US population doesn’t believe this pandemic is real, “how are you going to get them to take a test every day?” asks Michael Osterholm, an epidemiologist and director of the Center for Infectious Disease Research and Policy at the University of Minnesota. Among those who comply, Osterholm worries that repeat testing could end up encouraging unsafe behavior. If someone eased up on social distancing, while their daily tests stayed negative, they might start to feel invulnerable. “We saw this with HIV/AIDS. People would engage in high-risk sexual behavior and then they’d get tested, and be negative, and then that’s reinforcing that what I’m doing is not high-risk,” says Osterholm. In fact, “it’s just that they haven’t encountered that infected individual yet.”
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We've already seen examples of how, you know, actual behavior can be ignored amid planning for widespread Covid tests. That's what happened over the summer, when college campuses attempted to reopen. A paper published in July in JAMA Network Open ran through different scenarios for screening college students and concluded that testing students every two days with a test that only catches 70 percent of infections would be enough to control the virus. The catch? The model assumed that the student population would wear masks and practice social distancing, and that positive cases would be isolated within eight hours. Turns out, that’s the hard part. “What we’re getting now is a real-world lesson in what testing can and can’t do,” Osterholm says.
You can run all the models you want, but unless they take into account human behavior, they’re not going to hold up. The University of Illinois at Urbana–Champaign has instituted one of the nation’s most ambitious testing schemes, involving upwards of 10,000 people per day. Even so, they had more than 400 new positive cases in the first weeks of school. As UIUC chemist Martin Burke explained to Nature, “We didn’t model that people would go to a party if they tested positive.”
Testing remains crucial to identifying where the virus is spreading, and there’s no doubt that early lapses in this area worsened the pandemic in the U.S. But it’s human behavior, not tests, that ultimately stop the chain of transmission. For now, there is no shortcut out of this pandemic.
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