Thanks for this Adam! When R is affected by this variation outbreak sizes become more variable - a stochastic phenomenon. I'm also being asked how this relates to the variation that deterministically lowers herd immunity thresholds medrxiv.org/content/10.110…
I'll try to explain.
I'll try to explain.
When variation affects not only the number of secondary infections an infected individual causes but also the propensity that individual had to become infected in the first place a form of selection will be operating.
Those individuals who are more prone to infection (either because they are more exposed or more susceptible) are removed from the susceptible pool earlier in the epidemic. As a result the mean susceptibility (or exposure) in the susceptible pool decreases over time.
The decrease in mean susceptibility as more cases occur leads directly to deceleration in epidemic growth and to herd immunity thresholds being lower for greater variation.
