i pulled that 20% number out myself, and it does seem to be representative of a kind of inflection point, where community spread perhaps plateaus for a while and then starts to slow down - but understand that that number is increasing while the infections are slowing down, too. something the media always misses in these discussions is that these numbers coming out of studies are lagged. so, that 7% in stockholm was measured in april, leaving the swedish authorities to extrapolate then current infection rates based on independently measured growth curves. that's how they got 20% out of 7%...

but, i'm posting to suggest that you're more likely to find reasons that this number grows higher in some populations due to their living conditions rather than seek it out in genetic factors. one of the examples cited is prisons, where it seems obvious that cramped living conditions have accelerated immunity. i know that we've recently measured high immunity rates in new york city, as well, where the population density makes living conditions not that much different from those in prisons, in many cases.

that doesn't mean that people living in more normal living conditions won't get to the same rates of immunity we're seeing in prisons, it just means that they haven't gotten there yet, because the rate of spread is that much slower - and slowing, due to that 20% threshold being sufficient to minimize spread under normal social conditions.

so, check back in a few months, and those 20%s will be 30%s and 40%s. but, the places that happens will likely not see explosive new outbreaks, but rather a manageable stream of steady cases.

vancouver would appear to not be there yet, but i wonder if toronto is. see, this is why my faith in the numbers, being as low as it is, is frustrating. if we assume that cases are undercounted by a factor of 10, which i think is a low estimate in the specific context of ontario, then we're only at 3%, and vulnerable. but, i suspect that the factor in ontario is closer to 100 than to 10, which has us hovering in that sweet region.

we're going to find out which is true, in ontario; it's not currently clear.

but, let's not pull out the genetics when the issue is obviously environmental. that kind of pseudo-science doesn't help at all - and could lead to creepy deductions.

https://theconversation.com/coronavirus-could-it-be-burning-out-after-20-of-a-population-is-infected-141584