Here’s a short thread on some rather worrying misinterpretations of the meaning and implications of “asymptomatic” #SARS-CoV-2 or #COVID19. This is provoked by a @bmj_latest news item by https://www.bmj.com/content/bmj/369/bmj.m1375.full.pdf

Let’s start with two main claims: 1/9

1. “Four fifths of cases” of #COVID19 are not asymptomatic.

The fraction of people testing positive for #SARSCoV2 infection is not the same as the fraction with the disease #COVID19 2/9

2. The data Michael Day shows can't show that “the large majority of coronavirus infections do not result in symptoms.”

You can’t extrapolate a proportion, taken from data about people tested once, to make a general claim about the natural history of SARS-CoV-2. 3/9

Two groups might be asymptomatic at a particular point in time:
1. Truly asymptomatic infection: you can only establish this by following people with #SARSCoV2 until the end of the course of infection; probably a small proportion. And we don’t know how infectious they are. 4/9

2. Pre-symptomatic people: #SARSCoV2-infected people who are asymptomatic when tested, but go on to develop symptoms of #COVID19. You also have to follow these people up to find out. And they might be infectious 1 to 3 days before they develop symptoms. 5/9

Michael Day doesn’t have any “new evidence” and misinterprets the data he has. @yaneerbaryam explains what they actually show (ht @CT_Bergstrom). https://twitter.com/yaneerbaryam/status/1246158748047073286 6/9

China is conducting careful testing for #SARS-CoV-2 so that they can find infected people and isolate them to break transmission chains. It makes sense to test people who might not yet know they are infected, i.e. without symptoms. This is the point of contact tracing 7/9

The news item is even more misleading. Tom Jefferson misinterprets the data” as meaning “large swathes of the population” have already been infected and become immune. This is at odds with https://www.imperial.ac.uk/media/imperial-college/medicine/mrc-gida/2020-03-30-COVID19-Report-13.pdf which estimates <3% of the UK population has been infected. 8/9

We are doing a rapid #LivingSystematicReview to to give accurate information about #asymptomatic and #presymptomatic #SARSCoV2 and how they contribute to the epidemic ( https://osf.io/9ewys/ ). @bmj_latest should not have published this hyperbole https://www.bmj.com/content/bmj/369/bmj.m1375.full.pdf 9/9