I'll chalk it up to an innocent typo, but you got the terminology wrong. R_0 means R(t) where t=0. In other words, it by definition means the basic reproduction number which does NOT account for any level of immunity.
R, or as I prefer to call it, R(t), is the number that factors in how many have immunity. So you mean herd immunity is when R(t) < 1.
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Anyway, I was having trouble understanding exactly what you were saying about Sweden, but to be clear, Sweden has hit herd immunity, unless you think that the utter absence of COVID mortality at this point is due to seasonality.
What you need to understand is that simplistic models view the population as homogenous, both in terms of mixing (social connections) AND susceptibility/transmissibility. Neither are true. Essentially, the people who tend to get infected sooner in the pandemic are the ones who tend to spread more, so the first, say, 10% of antibody prevalence is worth a lot more than the next 10%.
There's also the presence of pre-existing T-cell cross-reactivity; exposure to other hCoVs is protective against SARS-2. It seems that it does not protect infection, but it does make the disease course a non-issue: this probably explains the high degree of asymptomatic infection (with part of it just being explained as an artifact of the PCR cycle threshold fuckery). Technically these findings are already implicitly factored into the estimates we have for R_0, etc. (BTW, initial studies were claiming R_0 of 5.2, 5.5, etc, but now with more data it seems it's around 2, give or take)
Many, and I am in this category, believe that the true herd immunity threshold is somewhere around 20% of the population. That explains what we've seen in places like New York, Sweden, etc. This is lower than what the classic herd immunity formula would predict, because that formula does not account for a non-homogenously-mixed population (as I mentioned earlier), nor does it account for certain individuals having an innate genetic resistance that makes it less likely that they get infected.
Ssshhhhhh.... Sweden can only be used as an example when you compare it to complete fuck-ups like the US or UK, or if you add in countries that were hit early before we knew how virulent covid-19 was and that it could be spread asymptomatically. If you put it into context with its similar neighbors then the failure of the herd immunity plan is laid bare.
So bad form to reply to one's own comment, but am I the only one who finds it quite ironic that today the Swedish PM was warning of a growing second wave there because people were not following social distancing suggestions and it turns out that they were not even close to achieving herd immunity.
Sure, the PM didn't know what they are talking about, but an ill-informed rando on HN who clearly has no expertise in either medicine, epidemiology, or statistics is the one we should pay attention to... Case numbers are rising. Death rate still far in excess of Norway, Finland, or Denmark. Well, at least we get a no-name comic tweeting pictures of life in Sweden to prove our point. This whole conversation is such a waste of time.
R, or as I prefer to call it, R(t), is the number that factors in how many have immunity. So you mean herd immunity is when R(t) < 1.
---
Anyway, I was having trouble understanding exactly what you were saying about Sweden, but to be clear, Sweden has hit herd immunity, unless you think that the utter absence of COVID mortality at this point is due to seasonality.
What you need to understand is that simplistic models view the population as homogenous, both in terms of mixing (social connections) AND susceptibility/transmissibility. Neither are true. Essentially, the people who tend to get infected sooner in the pandemic are the ones who tend to spread more, so the first, say, 10% of antibody prevalence is worth a lot more than the next 10%.
There's also the presence of pre-existing T-cell cross-reactivity; exposure to other hCoVs is protective against SARS-2. It seems that it does not protect infection, but it does make the disease course a non-issue: this probably explains the high degree of asymptomatic infection (with part of it just being explained as an artifact of the PCR cycle threshold fuckery). Technically these findings are already implicitly factored into the estimates we have for R_0, etc. (BTW, initial studies were claiming R_0 of 5.2, 5.5, etc, but now with more data it seems it's around 2, give or take)
Many, and I am in this category, believe that the true herd immunity threshold is somewhere around 20% of the population. That explains what we've seen in places like New York, Sweden, etc. This is lower than what the classic herd immunity formula would predict, because that formula does not account for a non-homogenously-mixed population (as I mentioned earlier), nor does it account for certain individuals having an innate genetic resistance that makes it less likely that they get infected.