No such thing as an average infected person

I am very struck by this quote from a paper measuring the concentration of corona virus (aka SARS-CoV-2) in swabs taken from infected people

Initial SARS-CoV-2 viral load is widely distributed ranging from 3 to 10 log copies/ml …

Jacot et al, medRxiv 2020

Note the log in the first sentence, the range is not from 3 to 10 — about a factor of 3 — it is from 10³ to 10¹⁰ viruses per millilitre — a range where the top end is 10 million times the bottom end. In other words, some people at some times during their COVID-19 infection have ten million times as much virus as others do. On a log scale, the average is 10^6.5 ~ 3 million viruses per millilitre but some infected people have thousands of times more, while others have thousands of times less.

Ten million is about the population of Greece, so a combined total of COVID-19 sufferers about the population of Greece, with a viral load at the bottom end of this range, would have about as much virus in their mucus as one single person at the top end.

Those with much more virus in their mucus must be more infectious. This may be part of the reason for what are sometimes called “superspreading events”, such as the one where we think one person infected between 30 and 50 others, after they attended a choir practice. Other factors may result in one person infecting more than 30 others while another may not pass it on to a single other person, but in any event there is no such thing as an average infected person.