COVID-19 Variable "K" Explained

Stan Adams
Oct 8, 2020 3:56:35 PM

You may have heard of "R0" (pronounced R-naught) to describe, on average, how many people a person infected with COVID-19 can infect, but have you heard of the variable "K"?

I must admit, I had not heard of the variable "K" until reading The Atlantic's recent article "This Overlooked Variable Is the Key to the Pandemic" by Zeynep Tufekci. I definitely recommend taking the time to read this article, but if you're short on time, I'll break it down here.


Starting with "R0", R0 represents the average number of people an infected person goes on to infect. If R is larger than one, this means that the number of people with the disease is increasing. When R is lower than one, the number of people with the disease is decreasing, or, on average, an infected person spreads the disease to fewer than one person. 

R0 doesn't tell the whole story. R0 tells us the average number of individuals an infected person will infect, but the true number can vary greatly. Some people won't infect anybody else, some will infect 10 people, for example. Six months into the COVID-19 pandemic, we've seen how "super-spreader" events play into spreading COVID-19.

As we get further into this pandemic, it is clear that loud talking/singing and poor ventilation play a role in creating an environment that allows COVID-19 to easily spread. SARS-CoV-2 is an overdispersed pathogen, meaning that it tends to spread in cluster. Think back to early in the pandemic, when a single infected person arrived at a choir practice and infected 87% of the other attendees. Multiple studies have suggested that as low as 10 or 20% of infected people are responsible for as much as 80-90% of transmission.

The variable "K" captures this kind of behavior—the variability of transmission patterns. COVID-19 has a low K value, which means that COVID-19 is spread by a relatively small number of infected people. 

What kind of implications does this have for how we respond to the COVID-19 pandemic? 

Two primary takeaways:

  1. A super-spreading event can lead to new clusters and many new cases in an area where cases are otherwise low
  2. We can take mitigation steps to prevent the conditions that allow for super-spreading events to occur (indoors, poor ventilation, prolonged time, singing/loud talking, no masks)

Until next time, stay vigilant, and let's do everything we can to stop the spread.

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