Five truths about covid that defy our intuitions

19th August, 2021

It’s striking how much Covid confusion still reigns. Some of the informational miasma is deliberate — there’s profit for some in the bewilderment of others — but much of it stems from the fact that epidemics defy our intuition. So, here are five counterintuitive Covid truths that easily slip beyond our understanding:

1. If a large share of hospitalised people are vaccinated, that’s a sign of success. It has been common to see headlines noting that a substantial minority of people who have been hospitalised or even killed by Covid have been fully vaccinated. These numbers suggest vaccine failure is alarmingly common. The fallacy only becomes clear at the logical extremes: before vaccines existed, everyone in hospital was unvaccinated; if vaccines were universal, then everybody in hospital would be vaccinated. Neither scenario tells us whether the vaccines work.

So try this. Imagine that 1 per cent of the unvaccinated population will end up in hospital with Covid over a given time period. In a city of a million people, that would be 10,000 hospital stays. Now let’s say that 950,000 people get fully vaccinated, that the vaccine is 95 per cent effective against hospitalisation, and that the vaccine doesn’t reduce transmission (although it does). Here’s the arithmetic: 500 of the 50,000 unvaccinated people end up in hospital. A total of 9,500 of the vaccinated people would be at risk of a hospital visit, but the vaccine saves all but 5 per cent of them. These unlucky 475 still go to hospital. The hospital contains 500 unvaccinated and 475 vaccinated people — almost half and half — which makes it seem as though the vaccine barely works. Yet when 95 per cent of people take a 95 per cent effective vaccine, hospital visits fall from 10,000 to fewer than 1,000.

2. Herd immunity isn’t the end of a pandemic. In the simplest epidemiological models, herd immunity is the moment when so many people are immune — either because of vaccines or prior infection — that the epidemic begins to die away of its own accord. The keyword here is “begins”. An epidemic has momentum, like a train. Herd immunity is the beginning of an uphill gradient, initially very gentle. If the train is moving at top speed as it begins to climb the hill, it will keep travelling for a long way before it stops. The difference between hitting herd immunity during a raging epidemic — yes, I am thinking of the UK — and hitting it through vaccination during a lull could be millions of unnecessary cases.

3. Masks matter, but not for the reason you think. Microbiologist Ravi Gupta has called the end of mask mandates in England “criminal”, while radio presenter Julia Hartley-Brewer has said she will boycott the bookshop Waterstones if they politely suggest wearing masks. What is it about masks that ignites such rhetoric? This is all about the social stakes involved. While you can’t see who’s been vaccinated or who has ignored a ping from the contact tracers, you can see who’s wearing a mask. I think it is considerate to wear a mask, an act that evidence suggests may protect me, probably protects others and certainly reassures them. For most people, wearing a mask is only a minor annoyance, so why not do it? Only our innate tribalism can turn mask-wearing from a simple, promising precaution into the dividing line between the saintly and the damned.

Cabinet ministers have boasted about removing their masks as soon as possible, which feels like boasting about farting in a lift: it might be a relief but it’s a strange thing to advertise. Once one realises this is about signalling tribal loyalty, it makes more sense.

4. Lockdowns also matter less than you think. It is understandable that we have focused so much on lockdowns. The radical social distancing ministers have imposed has been an unprecedented shift in the way we live, but it has saved millions of lives. What we overlook is that much of this social distancing would have happened anyway. Many people “locked down” before lockdowns themselves, out of fear or out of consideration for others, or both. The most famous study of this by economists Austan Goolsbee and Chad Syverson estimated that around 90 per cent of the reduction in consumer traffic was voluntary.

One need not believe the precise number to accept that people have often acted by choice, through fear or altruism. The flip side is also true: a lockdown that isn’t widely supported is neither effective nor tenable. All this matters because it is easy to think that everything revolves around the rules. More important is social solidarity, clear information and prominent people setting a good example. Alas, we’ve had to settle for social solidarity alone.

5. Covid was a near miss. After the disruption to life, love, education and commerce, and after more than four million confirmed deaths around the world — with many more unconfirmed and many still to come — it may seem strange to say so. But this could have been far worse. It could have been as contagious (or more) as the Delta variant from day one. It could have been as deadly as Mers, which has killed a third of the people confirmed to have contracted it. It could have attacked children rather than the very elderly. And it could — like HIV — have defeated efforts to create a vaccine. So while we count the cost, we should also count our blessings — and dramatically strengthen our preparedness for the next pandemic.

Written for and first published in the Financial Times on 23 July 2021.

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