Officials often talk about the coronavirus' all important R number.
This describes the number of people a patient statistically goes on to infect.
Keeping the R below one via social distancing, hand washing and face coverings means an infectious outbreak eventually peters out.
Although useful, it may be overly simplistic, which is where the K number comes in.
This describes the variation in how different patients infect others.
A nurse working on the front-line with insufficient personal protective equipment would be expected to unwittingly infect more people than a vulnerable individual who has been told to self-shield entirely at home.
Understanding the coronavirus’ K number may allow officials to better lift restrictions without compromising safety.