Your test will correctly identify 921,500 of them. And 970,000 people will not have had the disease. Your test will correctly identify 28,500 of them. Of those million, about 30,000 will actually have had the disease. Now let’s imagine that you test a million people, completely at random. Imagine that 3% of people in this country have had Covid-19, which probably towards the upper end of most reasonable estimates at the moment. Unless you know what percentage of people in the population have had it, then you simply don’t have enough information to answer the question. How likely is it that you’ve had the disease? So you are given the test, and it comes back positive. The US FDA has just issued emergency approval for a test with reported numbers pretty much like that (to be precise, 94% and 96% respectively). Now, imagine that you have a test that is 95% accurate - that is, if you use it on someone who has had the virus, it’s 95% likely to say they have had the virus if you use it on someone who hasn’t, it’s 95% likely to say they haven’t”. Let’s assume that people who have had the disease really are immune. But using it to establish individual immunity is very different and potentially much more dangerous. I think antibody testing is absolutely vital - for understanding the spread of the disease, at a population level. But I wanted to sound a note of extreme caution. Immunity passports sound like a good idea. Tony Blair seems to be talking about something similar when he writes, in the foreword to a report for his Institute for Global Change, that unless we can “track who has the disease and who has had it”, we will remain locked in a situation “in which everyone has to be isolated so that we don’t miss the small number of cases which need to be isolated”. This is the idea behind mooted “immunity passports” for people who’ve tested positive. If they’re immune to getting it again, they can’t spread it, so they should be allowed back out into the world, to help get the economy started again. It’s vital for understanding how far the disease has spread throughout the population without knowing that, we can’t know how deadly it is, or exactly how alarmed we should be.īut other people have been making another point: that if someone has already had the disease, then - hypothetically, hopefully - they ought to be immune to getting it again. I’ve been banging on about antibody testing in recent pieces. The Government wants to build testing programmes that can give a reliable answer from a pinprick test in under 20 minutes. There are two main kinds of test for Covid-19: PCR tests which tell us who has the disease right now and antibody or serological testing, which can tell us who’s had it in the past. Everyone agrees with that - the question is how it can be done. This article was written by Tom Chivers and originally posted on UnHerd here.
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