The 60% HIT (Herd Immunity Threshold) is based on a simple model with R0=2.4. With this model, the disease spread goes into a declining stage once the HIT is reached, and eventually about 90% of the population gets infected (similar to hitting the brakes on a vehicle, it is going to travel further before reaching a complete stop).
I had assumed that more sophisticated models would yield higher HITs with the same R0. This does not seem to be the case. Models which account for heterogenity in the population in terms of infected folks spreading the disease, actually show much lower HITs. For the case with R0=2.4, with the heterogenity estimated from various parts of the world, it seems the HIT could be like 15% to 20%, or even lower. This implies that eventually about 20% to 30% of the population would get infected, if the disease had unfettered access to do its thing.
Places like northern Italy (Robbio - 22% infected), NYC (21% infected), Gangelt, Germany (15% infected) or even NY state (10% infected?) might actually be at or close to the herd immunity threshold. Not to mention the Diamond Princess (21% infected eventually). My earlier thought on this phenomenon was, that many infected folks were not developing antibodies. That could still be the case, but there could be this competing explanation of heterogenity as well.
I'm reasonably sure that Sweden would show a similar infection rate (20% to 25%) when they get down to doing antibody testing, they seem to be well in the wind-down phase right now. I'm of course willing to eat crow on this, that is the nature of the game. Please note: I'm not recommending any change of policy WRT lockdowns, I'm not qualified to do that. Just pointing out that these assumptions about HITs and eventual infected fractions might be off after all.
We will only know in subsequent waves. The irony might be that places like NYC, northern Italy, Spain, or badly hit parts of Germany might be among the safest places to be in, come September.https://www.medrxiv.org/content/10.1101 ... qus_threadhttps://www.nicholaslewis.org/why-herd- ... n-thought/
Would love to hear from folks familiar with epidemiology on the above, the math part of the second link seemed ok to me, but that's the limit of my knowledge.