The case for quick and dirty tests for the coronavirus

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Tim Harford

Published7 hours ago

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Fast, cheap tests don't need to be perfect and will relieve the weariness associated with hopes for a return to normality

FINANCIAL TIMES – Screwtape, C.S. Lewis’ unforgettable devil, has this advice for crushing people who are facing a test of endurance. “Feed him with false hopes… Exaggerate the weariness by making him think it will soon be over.”

Thanks to the coronavirus pandemic, we are starting to learn all about weariness and false hopes.

It seems endless. And since a highly effective vaccine remains an uncertain prospect, is there any way we might get back to normality without one?

I think there is. The image I can’t shake off is that of the Ready Brek advertisements that have run since I was a child in the 1970s. They show children walking to school in wet and gloomy British winters shielded by a warm orange glow because they ate their porridge-adjacent breakfast.

So indulge me for a moment of science fiction. What if everyone who was infectious glowed bright orange? The virus would be extinct in humans within a month.

The coronavirus is an information problem. A few people are infected, but we don’t know who. We are forced to assume that anybody might be – and, as the economist Joshua Gans observes in his forthcoming book, this is extraordinarily costly.

It would be worth a lot to know who is and who is not infectious, and the obvious way to approach the orange glow scenario is to produce tests so cheap, so plentiful and so easy to administer that everyone could test themselves frequently, by spitting onto a strip of special paper.

Such an idea has been discussed for months in certain circles, including in a campaign by Nobel laureate economist Paul Romer.

But it came to the foreground in Britain recently when – against the backdrop of a faltering testing system – Prime Minister Boris Johnson promised millions of tests a day.

A promise from Mr Johnson might well have negative value. He and his ministers have repeatedly set and missed targets for the scale of testing and the speed at which tests are processed.

Self-administered Covid-19 testing in England. The writer says false positives are a disaster if we react wrongly, but a shoddy test used well can nudge the odds in our favour. PHOTO: AGENCE FRANCE-PRESSE

And while industry press statements have suggested that cheap, accurate tests are just around the corner, independent experts such as Birmingham University professor Jon Deeks point to a history of overpromising and a lack of trustworthy evidence that new tests are effective.

Even if a test that is as cheap as a cup of coffee could be produced and distributed in sufficient quantities, problems remain.

The most obvious is the issue of false positives. The Office for National Statistics estimated that one in 2,000 people were infected with the coronavirus in England and Wales late last month. With a test that has a false-positive rate of just 1 per cent, you would wrongly flag up 20 uninfected people for every genuine case. The rarer the virus, the worse this problem becomes.

No wonder the experts are sceptical – not just that a cheap test will be produced, but that it could be useful for mass screening if it was. Yet, while false positives are a disaster if we react to them in the wrong way, even a shoddy test used well can nudge the odds in our favour.

 

 

Imagine a quick and dirty test which takes five minutes to conduct, but produces a false positive rate of 10 per cent. Two hundred false positives might be flagged for every genuine case. That is disastrously high if we apply current British rules, in which the tested person and everyone in their household has to self-isolate for more than a week.

But even the bad test produces some information: The person with a positive test is 10 times more likely to be infected than a randomly selected person.

So what about the following rule?

If you take the test at the school gate and test positive, you must go home and try again tomorrow. If you take a positive test at the theatre entrance, you will need to leave and your ticket will be refunded. If you take it on arrival at Heathrow Airport, you’ll have to do a more accurate swab test and isolate until results arrive.

These are all irritating scenarios for the 200 out of 201 who do not actually have the virus. But they are not nearly as irritating as no school, no theatre, no flights and everyone back in lockdown by Christmas.

Fast, cheap tests don’t need to be perfect to help contain the virus. They don’t even need to be nearly perfect. Cheap and quick is enough – provided we use the data wisely.

We can’t shut down a school or an office block because one person tests positive on a ropey test: The risk of false positives is too great. But we can ask them to stay at home instead and book a more accurate test.

I have long believed that we undervalue two things. First, when it comes to technology, we undervalue quick-and-dirty relative to expensive-and-perfect. Vaccines get vastly more attention than the prospect of spitting onto a paper strip to produce a result that is probably wrong.

Second, we undervalue data. We spin it, make league tables out of it, turn it into targets, lie about it and disbelieve it.

But data, even noisy data, about who is infectious is information that could save both livelihoods, and lives.

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