COVID19 update, June 30, 2020: droplet and aerosol transmission; herd immunity requires as little as 20%?

Been a very busy day at work, but let me just share with you two things:

(a) a long essay that Jeff Duntemann drew my attention to:

Aerosols, Droplets, and Airborne Spread: Everything you could possibly want to know by Justin Morgenstern MD, an emergency physician located in the greater Toronto area.

The essay is long but very much worth your while.

(b) Coronavirus: could it be burning out after 20% of a population is infected? We pointed earlier to a preprint that showed that, if susceptibility to the infection isn’t assumed to be all or nothing, that this leads to a second-order mathematical model that predicts much lower herd immunity thresholds than the common first-order model. See also (h/t: masgramondou): https://judithcurry.com/2020/05/10/why-herd-immunity-to-covid-19-is-reached-much-earlier-than-thought/ 

But it is unlikely that lockdowns alone can explain the fact that infections have fallen in many regions after 20% of a population has been infected – something that, after all, happened in Stockholm and on cruise ships. 

That said, the fact that more than 20% of people have been infected in other places means that the T-cell hypothesis is unlikely to be the sole explanation either. Indeed, if a 20% threshold does exist, it applies to only some communities, depending on interactions between many genetic, immunological, behavioural and environmental factors, as well as the prevalence of pre-existing diseases. 

Understanding these complex interactions is going to be necessary if one is to meaningfully estimate when SARS-CoV-2 will burn itself out. Ascribing any apparent public health successes or failures to a single factor is appealing – but it is unlikely to provide sufficient insight into how COVID-19, or whatever comes next, can be defeated.