Crowds in motion

It's bad science to say that Covid-19 infections create herd immunity

By Graham Lawton

If Covid-19 can circulate freely through the crowd, it can lead to millions of deaths

Shutterstock / R. Classen

ONES I'm writing this, my 19 year old son is isolating himself in his university room with symptoms of Covid-19, waiting for test results. He is doing pretty badly, although there is an overwhelming chance that he will make a full recovery. But I worry that he will be one of the few young adults to get seriously ill, or even die, or die in the end
long-term health problems.

For some, however, his illness is welcome; In fact, they wish all his colleagues. Lockdown measures do more harm than good, according to the signatories of an open letter called the Great Barrington Declaration, and we should open up society and let the virus rip apart.

OK, that's a bit of an exaggeration. The declaration – named after the US city in which it was signed – advocates a strategy called "targeted protection" that allows the most vulnerable and everyone else to "get back to normal immediately". This allows herd immunity to build up.


The statement publicly exposed a scientific disagreement that had simmered for months. On the one hand, there are established scientists who are reluctant to view restrictions on freedom as the only way to keep the pandemic under control while we wait for vaccines. On the other hand are the libertarians, who see the damage to national economies and individual lives as too great.

The mainstream media has picked up on the disagreement narrative but completely overlooked the basic problem of the declaration: its highly dubious claims about herd immunity. This is central to the strategy, but the document is giving science a big stir.

Herd immunity is conceptually simple. If enough people become immune to an infectious agent, the entire herd is protected, as infectious people rarely encounter a non-immunized person and transmission therefore fails.

The level of individual immunity required to achieve 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 with an R number of around 15 and a herd immunity threshold of 95 percent. The numbers for the coronavirus SARS-CoV-2 are 3.5 and 60 to 70 percent.

Herd immunity has so far only been achieved by vaccination. However, the statement advocates naturally acquired immunity. In other words, when two-thirds to three-quarters of the population catch the virus.

There are a number of problems here, not least of which is collateral damage. Even if the death rate is below 1 percent, the virus's release will hospitalized and kill millions.

However, there is one more important scientific detail that is overlooked in the declaration, along with most of the discussions about herd immunity. We cannot assume that widespread individual immunity automatically creates herd immunity.

Herd immunity can only be built if the immune response completely prevents people from ingesting and transmitting the virus. Sometimes it happens, but often it doesn't. In most cases, if we acquire the virus again, an immune response will prevent us from getting sick, but will not prevent it from passing it on. The same goes for vaccines.

We don't yet know if natural immunity to SARS-CoV-2 (or the experimental vaccines) will stop transmission. Until we do, the assumption that herd immunity occurs automatically is unscientific and frankly irresponsible.

There are many other reasons to be skeptical of the explanation. For example, it doesn't even mention the debilitating, permanent effects of "Long Covid". But they are of minor importance to the fundamental hole in his heart: the mysterious and dangerous failure to properly grasp the concept of herd immunity. Get well soon my son.

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