by LaoDan on Thu Sep 24, 2020 2:36 pm
I think that the speculation about herd immunity is primarily wishful thinking, and far too premature. While we do not know the precise number of individuals in any population that have contracted COVID-19 and been unconfirmed (due to being symptomless or otherwise not being tested, etc.), the confirmed cases in the USA stands at ~7 million out of a population of about 330 million. 7/330 = ~2%, very far from what one would expect would be needed to confer herd immunity. If one doubles, triples, quadruples that number to account for the uncertainties, then we are STILL far below the number needed to confer herd immunity in the USA (and the world – see below). I do not have numbers for NYC, but anyone who does can do the calculation for that specific location.
As an example, please consider the following:
If we assume that 50% of the population is, for various reasons (“dark matter”...), protected against developing an illness from the coronavirus (Karl Friston modeling, which should include things like T-cell resistance, etc.) and apply that to the 1918 “Spanish” flu (H1N1) pandemic, then his numbers seem reasonable. It has been estimated that 1/3 of the world population became infected/sick from the 1918 flu outbreak, and like the current pandemic there was no vaccine and few if any prophylaxis.
We can probably assume that the 1918 pandemic ended (in the summer of 1919) when herd immunity was achieved. But ~30% of a population is far below what is now understood to confer herd immunity, so ~30% (sick) + ~50% (asymptomatic; e.g. Karl Friston modeling) = ~80% of the population needing to be immune/resistant in some fashion to confer herd immunity to the remaining ~20% of the population [note that this percentage falls within the expected range of less than 70% to as much as 90% for herd immunity, depending on the specific disease, and since both H1N1 and the coronavirus are fairly high on their contagiousness, this seems reasonable for both].
Note that even though asymptomatic spread has been known since “typhoid Mary” [1907], in 1918 they did not have the diagnostic capabilities that we do today. Therefore the estimated ~30% that caught the disease was probably only counting those who actually developed noticeable symptoms. Today’s statistics are different since most state “confirmed cases” whether or not those cases showed symptoms. This makes comparisons between 1918 and 2020 difficult. However, I am not aware of a better pandemic to use for comparison.
The above would mean that, of the 7.8 billion current world population, ~3.9 billion may have natural immunity/resistance where symptoms are not noticeable or mild enough to not become ill, ~2.3 billion could be susceptible to infection and illness prior to the virus running its course (after achieving herd immunity) and conferring protection for the remaining ~1.6 billion people. We are nowhere close to that at present since ~32 million confirmed worldwide cases (both symptomatic AND asymptomatic but still confirmed) is far below ~2.3 billion. Of course, the coronavirus may have differences from the H1N1 flu, and could be better or could be worse. Also, different locations could have different exposure levels so some locations will achieve herd immunity earlier that other locations. But speculating that the coronavirus may be significantly less contagious than the H1N1 flu of 1918 seems extremely foolish. I expect a similar ~30% of the population showing symptoms if no mitigating factors were in place.
The above numbers assume that the population was not taking significant protective steps (the masks worn in 1918-1919 were relatively useless in stopping the spread of the 1918 flu pandemic). It is probably safe to state that social distancing, reduced travel, and protective mask wearing would reduce the spread of the current disease, and therefore result in a lowering of the necessary herd immunity %. But it is not known how long these protective measures would need to be maintained beyond reaching that reduced % for herd immunity. I suspect that relaxing these measures too soon would allow a resurgence of the outbreak because the population would be increasing the needed herd immunity % value. Once herd immunity has been reached, as long as the protective measures are maintained, the population should be relatively safe and the virus should remain suppressed. All of this fits with the “flattening the curve” ideas that specialists talk about. A “second wave” cannot be ruled out, especially in locations where mitigating factors are reduced too early.
Do the above seem reasonable? Are there any other major factors that I am not accounting for?