The latest Covid conundrum is why so many people seem to be currently testing positive on their instant Lateral Flow Test and then test negative when they take their PCR. Tom Chivers unpacked the maths on these pages earlier this week, and Dr Oliver Johnson has confirmed in an excellent Twitter thread that the disparity between the testing methods seems to be widening recently.
I’ll leave the probability calculations to superior brains, but I wanted to throw in one piece of anecdotal evidence that I don’t feel gets talked about enough. (To repeat: I am aware this is anecdotal!)
I had Covid back at the start of August — for the first time as far as I know, and after two Pfizer vaccinations. It was hardly the worst week of my life (I watched the entirety of Call my Agent, which was a saving grace) but I was ill enough to feel somewhat shortchanged by Messrs Pfizer and BioNtech. I had all the classic symptoms — loss of taste and smell, high fever, fatigue etc — and it felt pretty much how I would expect it to feel in an otherwise healthy 39 year old.
Anyway, I took a lateral flow test every day of the experience. The photographic record of them is above. Notice how the first positive test was just a faint little second line, and then, as I became more sick, it got darker and darker. The worst days roughly correlated with the darkest lines. As I got better, the second line got thinner and thinner, until eventually — finally! — I started testing negative, as I have ever since.
Over the course of the past two years I reckon I have taken over a hundred LFTs. There was a period when we were doing them every day in the office. At one point in this strange era, people made guests do them upon arrival at dinner parties. Every time I took a plane, or had any kind of cold symptoms, I would be doing LFTs.
I have never, not once, had anything even half-suspiciously looking like a second line except when I had Covid; furthermore the perfect increase of the strength of that second line as I got more sick, up to a peak, and gradually back down to nothing, suggests to me that the tests are extraordinarily accurate. There was no noise, no real jumps backwards and forwards. Others who have had Covid or who have been testing their children report the same gradual increase and decrease. If they were such sketchy, rough and ready tests, it feels like this would not be happening?
So since my Covid experience I have a newfound respect for the accuracy of these little Chinese-made devices. We may all abhor them by now, and the hideous swabbing experience that goes along with them, but it seems hard to deny that they are highly clever — and highly accurate.
(Incidentally, this accuracy makes a mockery of the obsession with vaccine passports. Now that we know how easy it is to get, carry and pass on Delta Covid post vaccination, if you did have a scenario that you were very anxious to keep Covid-free — a wedding with lots of oldies, for example, or a hospital ward — it would surely be much safer to test everyone with LFTs on arrival than ask people for vaccine status?)