When studying rates of infections, notably with measles, researchers have seen that in cases where a sufficiently large proportion of a population has been immunised or developed protection against a disease then infection rates also reduce in unprotected members of the population. In other words, the unprotected members appear to receive some indirect protection from the protected members of the population. This effect is known as herd immunity and is a tantalising goal for reducing the prevalence of epidemics.

While appealing in theory this article in Nature provides Five reasons why COVID herd immunity is probably impossible. These include:

Uncertainty around whether vaccinations prevent transmission. For example, herd immunity works when diseases can't be passed between protected members of a population to the unprotected. With COVID-19, vaccines seem to reduce symptoms but may still allow transmission.

It's also challenging to get vaccines evenly to all areas. These combined with complications brought by new variants, immunity fading before widespread vaccination is achieved, and people changing their behaviour — say, mixing more widely — once they've received some protection, means COVID-19 may be more likely to be something we live alongside rather than eradicate.

Herd immunity

Despite all our best intentions, accidents happen. Analysis of accidents in large complex systems such as power stations or plane crashes led to an understanding that "no one failure, human or technical, is sufficient to cause an accident. Rather, it involves the unlikely and often unforeseeable conjunction of several contributing factors arising from different levels of the system."

James Reason's Swiss Cheese Model is a memorable visual metaphor that illustrates how each safeguard may contain a latent flaw, or hole, and that an unfortunate circumstance, may result in these holes lining up to disastrous effect. It's also a nice reminder that multiple layers of defence will be more effective, but even with our best efforts, there's still potential for something to go wrong.

Excerpt from Revisiting the "Swiss Cheese" Model of Accidents (pdf), Reason, Hollnagel and Paries, 2006.

Coronavirus example from the Cleveland Clinic.

The Swiss Cheese Model

How small changes in customer demand can produce considerable disruption upstream in the supply chain. For instance, a sudden surge in the buying of toilet paper may cause retailers to put in larger orders to make sure they can keep up with demand. Manufacturers, in turn, may put in even larger orders for raw materials to make sure they don’t get caught out. In the meantime, customers had bought all the toilet paper they ever needed and demand suddenly dropped causing retailers to cancel all their orders, manufacturers to halt production and suppliers to be left with big stockpiles of unwanted raw materials.

I learned about the bullwhip effect from Tomas Tunguz where he considers economic effects of the coronavirus. He explains it with a brewery example in his article on proxy metrics for startups.

Sketchplanations tagged with coronavirus