This may be a truth wasted on @mattyglesias, but over here in science, we generally draw a line between 'knowing' and 'assuming'. Asymptomatic transmission of the #coronavirus causing #COVID19 is not as simple as most think. (thread) https://twitter.com/mattyglesias/status/1246637795352944640

One, for respiratory droplet infections, the infectious pathology significantly enhances infectiousness. If you don't cough, sneeze or otherwise create and expel droplets, you're only infectious through exchange of bodily fluids.

Two, because our way of detecting viral load was premised on RT-PCR, and samples were extremely heavily weighted towards progressed disease, asymptomatic transmission was not widely documented. Things evolved way too fast for anyone's testing capacity to do extensive tracing.

The first reliable paper on asymptomatic transmission was Bai et al. (published on 21 February, but I'm sure @mattyglesias had the scoop in January...): https://jamanetwork.com/journals/jama/fullarticle/2762028 It documents a single case of asymptomatic transmission (confirmed by RT-PCR).

Then, this paper, by Gao and Ji (published 06 March) discussed asymptomatic and presymptomatic transmission in a bit more depth: http://cas.medline.org.cn/?service=http%3A%2F%2Frs.yiigle.com%3A80%2Fyufabiao%2F1183795.htm&gateway=true&loginUrl=%2F The two are actually different – genuine asymptomatic transmission is limited to patients who don't get sick.

The @CDCMMWR from Friday has an article on asymptomatic and presymptomatic transmission in a nursing facility in King County, WA: https://www.ncbi.nlm.nih.gov/pubmed/32240128  – proving on one hand that asymptomatic and presymptomatic transmission is *possible*, but not its relative likelihood.

So, to wrap this up: extrapolating from other coronaviruses, we've had a good hunch since December that asymptomatic transmission is theoretically possible. The evidence that it happens with SARS-CoV-2 is recent & mostly limited to few cases & circumstances (e.g. nursing homes).

Should an assumption that asymptomatic spread is possible have governed the public health response? There isn't really a public health response to asymptomatic spread, especially for asymptomatic or presymptomatic spread of indefinite duration, so... nope?

There's also extremely little hard evidence of when presymptomatic shedding begins and there are relatively few case clusters that were not exposed to someone symptomatic. If asymptomatic transmission were that significant, we'd see random clusters all the time over the place.

In addition, most people presenting for testing do NOT have SARS-CoV-2, so symptomatic or not, they cannot transmit the disease. Nowhere is the test positive rate at or above 50%.

There's a difference between 'assuming' and 'knowing'. Science is about what we know, not what we assume. While assumptions are cheap, knowledge takes time and effort.

And now, if you'll excuse me, I've got to do some yoga to calm the hell down. Jeez.