Just as we have got our heads around the R number, now we must consider the K number.
The reproductive value, or R0, is used to track how many people, on average, will be infected for every one person who has the disease. Lockdown and social distancing has helped maintain an R number to below 1.0. So, were R0 be 0.8, for every 10 people with the disease a further eight would be infected – and the disease eventually dies out. Where R0 rises above 1.0, the disease spreads. What is now known about the transmission of Covid-19 is that the likelihood of any individual infecting another varies significantly. This is termed overdispersion and is numerically represented by the value K, the overdispersion parameter.
Now, the lower the K value, the smaller the percentage of the population are responsible for spread of the disease. Models based on R values of two to three predict SARS-CoV-2 has a K value of just 0.1. This means that the majority of secondary transmission may be caused by a very small fraction of individuals (80% of transmissions caused by ~10% of the total cases), the so-called super-spreaders. If our track-and-trace system ever kicks in, let us hope it can identify the hot spots and isolate accordingly. Without this localised response to the low K number, I fear our friend R will rise again.
As we go to press, let us hope for a cessation of the violence currently rife across the US. Lives should always trump property and shop windows. Thankfully, violence is not something we expect from our law keepers. That said, we should all question why, according to a report by Liberty Investigate,1 BAME people in England are 54% more likely to be fined under coronavirus rules than white people. Have they not heard of the Cummings Interpretation?
References
- https://libertyinvestigates.org.uk/articles/bame-people-disproportionately-targeted-by-coronavirus-fines/