How much could pre-existing immunity affect the dynamics of COVID-19? To answer this question, it's useful to look at two similar but subtly different concepts - the 'reproduction number, R' and the 'basic reproduction number, R0'... 1/
R is the average number of secondary infections generated by a typical infectious person, so reflects both social behaviour *and* susceptibility within a population. In contrast, R0 is expected number of secondary infections if everyone in population were fully susceptible. 2/
There's more on the relationship between the two in my talk here (from 27:00): newton.ac.uk/seminar/202007…. But why is this distinction important? 3/ 
If we're calculating the impact of immunity, R0 is the crucial value. Suppose, hypothetically, that 50% of a population isn't susceptible to an infection. If observed R is 3, it would mean the R0 is much higher (because R is 3 despite only half population being susceptible). 4/
So if we are calculating how much effect pre-existing or acquired immunity might have on an epidemic, we have to first estimate what R0 would be (i.e. in the absence of any immunity). If we use the observed value of R instead, we risk double counting effects of immunity. 5/
Punchline: if you think some groups are less susceptible to SARS-CoV-2, then it's important to adjust for this and use the larger R0 value when discussing hypothetical dynamics of immunity. 6/6
