[ppod]

Cases are low and falling in New York, Sweden, London, Italy, Spain, despite varying and loosening social distancing measures. How do you explain that? Those places already have herd immunity. Serotesting for antibodies misses t-cell immunity and other forms of resistance, and variation in spreading patterns makes the herd immunity threshold lower than we thought. This is the only plausible explanation and nobody wants to admit it.

[jschwartzi]

Look, what you are saying makes sense until you say "This is the only plausible explanation" at which point I get lost. Just because you can't imagine other explanations doesn't mean that there aren't any. And this also presupposes that there are no hazards to anyone in that case. The fact is this disease remains very dangerous for a lot of people. So while it's plausible that we will have herd immunity sooner rather than later, it also misses the point which is that it's very dangerous to get infected with this virus.

[ppod]

Yes, I admit that the part where I said there were no hazards and that it was safe to get infected with the virus was wrong.

[dragonwriter]

> Those places already have herd immunity.

No, they don't, or they wouldn't have new cases, even without even somewhat looser mandatory controls than other places. What they have is some degree of immunity in the population (not herd immunity) plus (in some subset of those places) some degree of contact tracing backed by targeted mandatory, or at least voluntary, quarantines/isolation of the exposed, and (in large part because of the intense impacts each has had) voluntary general distancing.

[ppod]

You sound very confident. There are some epidemiologists at the University of Oxford who disagree with your definitions.

https://www.medrxiv.org/content/10.1101/2020.07.15.20154294v...

[gridlockd]

> No, they don't, or they wouldn't have new cases, even without even somewhat looser mandatory controls than other places.

That's not what herd immunity means.

The Herd immunity threshold is attained when the R factor drops below 1, assuming otherwise uninhibited spread. The threshold for herd immunity for COVID-19 has been estimated at 50-80%, but that is assuming an R0 that is likely overestimated.

Given that spread is still mitigated by certain interventions, and since we don't know the impact of those interventions on R, nor do we know R0, we don't know if we have herd immunity. However, we do have R below one in many European countries.

[tripletao]

Herd immunity isn't a binary threshold after which zero cases occur. Even in a first-order homogeneous and well-mixed SIR model, you may asymptotically approach 1 - 1/R0 of the population infected without ever getting there. If you do cross that threshold ("overshoot"), then the case count starts to drop, but new people still get infected and die on the downslope. The only case where a disease will naturally burn itself out abruptly is if there was massive overshoot, which would be bad, because it means yet more people died than necessary for natural herd immunity.

And for real some people have many more contacts than others (nurses, police, etc.). They get infected first, with disproportionate harm, but then become immune first with disproportionate benefit. That heterogeneity means 1 - 1/R0 is potentially a significant overestimate of the share of the population that needs to get infected for herd immunity, but there have been very limited efforts to quantify that so far.

It seems like some people believe natural herd immunity (from recovered patients) could work like vaccination does, to effectively eradicate the disease? That's probably false--the most likely natural endgame would be that the coronavirus becomes endemic, always present with some low incidence, with continuing mortality that's very low (because the incidence is low, and because older people probably benefit from immunity from when they were younger and the IFR for young people is <1/100 of older people's) but nonzero.

Finally, herd immunity and interventions (social distancing, masks, etc.) work together. It's possible (and likely I believe) that in hard-hit areas that now show R ~ 1, this is due to the product of both factors, and that either relaxing to their previous lifestyle or applying the same interventions in a naive (100% susceptible) population would show R > 1.

[notahacker]

Cases are lower still or zero in many other places with far too few cases to have achieved herd immunity. The only plausible explanation is that containment measures have some effect.

[zaroth]

Every increase in the percentage of people with immunity reduces R0. At some point, even absent any other measures, the R0 would be below 1 just due to the number of people that are immune; herd immunity.

But in any case, the immunity level of the population reduces the need for other measures in order to stay below an R0 of 1.

There’s a massive spectrum of efficacy in the dozens of different suppression or containment policies that can be applied, combined with demographics and geography of the location being studied.

But in any case, the immunity level of the population is a downward force on R0 that, for example, will naturally keep daily cases lower in New York vs. Florida regardless of policy.

[ppod]

https://www.medrxiv.org/content/10.1101/2020.07.15.20154294v...

The case rate has dropped faster as the restrictions have eased.

[notahacker]

I think you're confusing cause and effect here...