Mass Testing Could Be a Covid Game Changer
Mass Testing Could Be a Covid Game Changer
Until a vaccine arrives, the world has to find a way to live with Covid-19 and without lockdowns. One approach gaining currency is testing entire populations with new tests that deliver near-instant results. Slovakia just used such rapid diagnostic tests on 3.6 million people — two-thirds of its population — in two days.
Now Boris Johnson is piloting a similar program in Liverpool. Starting Friday, the northern English city is offering all residents and workers what is referred to as a “rapid antigen test,” to screen as much of the population for the virus as possible. Individuals can perform their own nasal swab and the result comes in as little as 15-30 minutes. Some 2,000 military personnel are being brought in to help out. If the experiment goes well, the government could distribute millions of the tests to other parts of the country before Christmas.
The stakes for Johnson are enormous. Britain just posted its highest death rate from the virus since May and Liverpool has the country’s highest rate of infection. Many in Johnson’s own Conservative Party are furious about the month-long national lockdown starting Thursday. Rapid testing, the prime minister told Parliament this week, “could be a massive and possibly decisive use to us in this country in defeating the virus.”
The claim isn’t entirely hype. Done properly, mass testing has the potential to bring infection rates down and give people the ability to gather more freely again. But it’s no silver bullet. Rapid diagnostic tests have to be folded into a much stronger program of regular testing, contact tracing and isolation. At the moment, the UK’s test and trace system is failing.
There is also a danger that the government will undermine the program by overselling it. Although the rapid antigen tests being used are cheaper and faster than the gold-standard polymerase chain reaction (PCR) tests, their drawback is accuracy. They rely on the same technology — what’s called a lateral flow immunoassay — that’s used in a basic pregnancy test. But these detect a protein on the surface of the virus, which is trickier than detecting viral RNA and can produce false results. Indeed, the US Food and Drug Administration warned just this week that antigen testing in nursing homes was throwing up false positives (largely, it seems, through failing to follow instructions).
While the test being used in Liverpool — produced by the Chinese company Innova Medical Group, Inc. — is reported to identify those who do not have the disease 99.9% of the time, its ability to identify people with Covid is lower. The company reports a 96% accuracy on this front, but that still leaves a lot of people who are falsely told they are negative. This means a lot of people circulating in the community who have the disease but have been given a Covid-free sticker.
One complaint (including from the Royal Statistical Society) is that the government hasn’t made its evidence base for the testing plans clear. Never one to undersell an initiative, Johnson’s plans seem to be asking more of the test than it was designed for. Innova’s Directions for Use pamphlet says the test is designated for people who have symptoms and who are in the first five days of those symptoms. However, Johnson has encouraged everyone to get tested, whether or not they are symptomatic.
“The whole point of mass screening is to pick up people who are asymptomatic or presymptomatic. And we have no data at all on how well this test picks up those people,” says Jon Deeks, a professor of biostatistics at the University of Birmingham Institute of Applied Health Research.
Innova also warns that false negatives are more likely in areas where the prevalence of Covid is high, which is certainly the case in Liverpool. “If you are using these tests to see whether your rugby team is safe to play with each other and you are missing cases, then you suddenly end up with a rugby team that is sick,” says Deeks.
Some scientists argue that if testing is widespread enough and repeated regularly, lower accuracy is not a problem. The argument is partly that repeat tests will catch some missed infections — it’s a volume game. According to a model published recently by the National Bureau of Economic Research, rapid tests could save tens of thousands of lives in the US and boost GDP even at lower levels of accuracy and even where compliance with isolating is low. But such models rely on assumptions that need testing in community settings.
One thing is clear: Using a standard PCR test to confirm (at least some of) the antigen test results would establish a more accurate picture of who has the disease, especially among asymptomatic people. That would also aid research and modelers, and help the government adjust the testing program accordingly. Unfortunately, there are no plans to employ such an approach in Britain.
If enough people get tested, there’s reason to hope that antigen tests can make a difference in areas like Liverpool and eventually in larger-scale use. But their value as a potential game changer for Johnson will be limited if they are not tied to major improvements to contact tracing and isolating during the course of this lockdown. A more comprehensive strategy is the only way to bring back some normalcy.