Measure of Dispersion (k)


The measure of dispersion is defined by the k number and is how variable the infection is i.e how it spreads whether in a steady manner or in big bursts.  Although the R number gives information about a region as a whole it is an average and hides many differences between individuals and their impact on transmission. The smaller the k number the lower the number of people who are spreading he disease to others. A low k number means that there are more super-spreaders.  

Covid-19 is passed on through people shredding the virus particles from their bodies. How infectious someone is depends on:

  • how long they have been infected for

  • the dose of the original infection

  • the severity of their symptoms 

However, it is their actions when they are at their most infectious that really drives super-spreading events. A choir practice, a visit to a pub or nightclub or going to work in a cold meat packing factory are all events indoors for a prolonged period of time and can result in a larger number of people who get infected. Super-spreading clusters of Covid-19 almost always occur in poorly ventilated, indoor environments where many people congregate over time especially where there is loud talking or singing without wearing masks.

It was thought that Spanish flu had a K number of 1 and perhaps 40% of infected people might not pass the virus on to anyone else. When social distancing measures are not in place, Covid-19 is thought to have a K as low as 0.1 and the proportion of people not infecting anyone rises to 70% .

Understanding the k value can help governments control the virus by understanding and limiting super-spreader events. Currently most governments conduct forward tracing where they look at an infected persons contacts after they became infected. By conducting backward tracing and identifying who infected that person and then trace foward the contest of the infecting person at super-spreading events a lot more cases will be found helping in the control of the spread of the virus.