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AS I write this, my 19-year-old son is self-isolating in his university room with symptoms of covid-19, awaiting test results. He is quite poorly, though overwhelmingly likely to make a full recovery. But I worry that he will be one of the few young adults who get seriously ill��or even die, or end up with
long-term health problems.

To some, however, his illness is��welcome; in fact, they wish it on��all of his peers. According to the��signatories of an open letter called the , lockdown measures are doing more harm than good and we should open up society and let the virus rip.

OK, that is a bit of an exaggeration. The declaration – named after the US town where it was signed – advocates a strategy called “focused protection” under which the most vulnerable people shield and everybody else “should immediately be allowed to resume life as normal”. This will then allow��herd immunity to build up.

The declaration publicly exposed a scientific disagreement that has been simmering for months. On one side are mainstream scientists who reluctantly see restrictions on freedom as the only way to keep a lid on the pandemic while we wait for vaccines; on the other, the libertarians who see the damage done to economies and individual lives as too high a price.

The mainstream media lapped up the disagreement narrative, but completely missed the fundamental problem with the declaration: its extremely dubious claims about herd immunity. This is central to the strategy, but the document badly fluffs the science.

Herd immunity is conceptually simple. If enough people become immune to an infectious agent, the entire herd is protected because infectious people rarely encounter a non-immune person, and so transmission fizzles out.

The level of individual immunity required to attain herd immunity depends on how infectious the virus is, as measured by R, the average number of people that each infectious person infects. The classic example is measles, which has an R number of around 15 and a herd immunity threshold of 95 per cent. The numbers for the coronavirus SARS-CoV-2 are about 3.5 and 60 to 70 per cent.

Herd immunity has only ever been attained by vaccination. But��the declaration advocates naturally acquired immunity. In��other words, letting between two-thirds and three-quarters of the population catch the virus.

There are a number of issues with this, not least collateral damage. Even if the death rate is under 1 per cent, letting the virus��run free will hospitalise and��kill millions.

But there is another crucial scientific detail that the declaration – along with most discussions of herd immunity – misses. We can’t take it for granted that widespread individual immunity will automatically create herd immunity.

Herd immunity can only be built if the immune response totally prevents individuals from picking up and transmitting the virus. That sometimes happens, but often doesn’t. A lot of the time,��an immune response stops us from falling ill if we reacquire the virus, but doesn’t prevent onward transmission. The same is��true of vaccines.

We don’t yet know whether natural immunity to SARS-CoV-2 (or the experimental vaccines) will��halt transmission. Until we do,��assuming that herd immunity will automatically appear is unscientific and, frankly, irresponsible.

There are many other reasons to��be sceptical of the declaration. It��doesn’t even mention the debilitating, lasting effects of “long covid”, for example. But they are of secondary significance to the fundamental hole at its heart: the mystifying and dangerous failure to properly grasp the concept of herd immunity. Get well soon, son.��