[nradov]

These models are fundamentally flawed in that they assume immunity or susceptibility are binary conditions. Based on recent research it appears a significant fraction of the population has at least limited immunity from prior exposure to other coronaviruses. They can still get infected but the immune system clears it more quickly and they tend to suffer fewer symptoms compared to immunologically naive patients.

https://www.jci.org/articles/view/143380

https://pubmed.ncbi.nlm.nih.gov/32978311/

[gpm]

Every model is wrong, some models are useful. Does this flaw make this model useless? On the flip side is it a useful approximation even if it's not completely accurate?

[vannevar]

I believe this model accounts for such individual response:

"Heterogeneity in contact structure and individual variation in infectivity, susceptibility, and resistance are key factors..." (emphasis added)

[AstralStorm]

In general, that's one of those anomalous diffusion problems. Unfortunately, there are too many parameters to estimate without simplifying the model. So the reasonable solution is to take a maximum expected value of each and you can still be wrong.

Important data we do not know is chiefly how effective SARS aerosols are and at what range and time.