Covid-19 Transmission and the Pareto Principle

The Pareto Principle is no doubt over-invoked, and is something of a cliché among business and management gurus. However, the epidemiology of Covid-19 looks to be turning into another case for Pareto true believers.

Related to the law of diminishing returns, the Pareto Principle posits that 80% of outputs come from just 20% of inputs. So, for example, 20% of customers will account for 80% of profits, or 20% of sports players decide 80% of the results.

When it comes to Covid-19, mounting evidence is showing that it is a superspreading disease. In fact, some studies have shown that even less than 20% of infected people cause 80% of new transmissions.

In Hong Kong, 19% of infected people accounted for 80% of SARS-CoV-2 transmissions. In Japan, epidemiologists have put the figure at 10-15%.

The converse of this is that most people infected with Covid-19 in fact spread the virus very little if at all. For instance, one study showed that even spouses of infected people have only a 43% chance of being infected. 

For all co-residents, the rate of infection once another household member is infected is around 15%. In other words, SARS-CoV-2 actually requires super-spreading events if it is to sustain its spread in the general population.

What Causes Covid-19 Super-spreading?

As previously reported on A Rich Life, one of the key drivers is environmental, most particularly spending extended periods of time in close contact with others in rooms with recirculating air. This is confirmed by a Japanese study of 10 clusters, which were tied to gyms, hospitals, a restaurant on a boat, and a “snow festival” in which revellers ate together in small tents.

The same study estimated transmission risk to be 18 times more likely indoors than outdoors. This is so intuitively logical that it shows up the Grinch-like attitudes at play in the media’s obsession with shaming beachgoers.

However, super-spreading also depends on the infected person. Most of this is timing: a Taiwanese research team that traced 2,761 contacts of 100 infected people showed no instances of transmission beyond five days from the onset of symptoms in the infected person. The greater severity of a person’s symptoms also makes them somewhat more likely to pass on the infection. 

With all this in mind, you have to wonder about the NSW government allowing indoor corporate events of up to 300 people, while banning park gatherings of more than 20. It certainly looks like less than 20% of NSW citizens have more than 80% of the lobbying influence on Macquarie Street.