I've been meaning to post a thread talking about COVID-19 transmission that breaks down my current thinking on the topic. This is that thread. (1/25)
By far most of the cases seem to be from direct person to person transmission from individuals who are symptomatic (i.e. sneezing and coughing)... (2/25)
with a smaller number of documented cases of transmission from people who are pre-symptomatic (i.e. not sneezing and coughing, but who will manifest symptoms later because they’re already infected). (3/25)
This particular article from MMWR is a good example of *symptomatic* transmission. cdc.gov/mmwr/volumes/6… I find Figure 1 to be particularly helpful in understanding how epidemiologists investigate and diagram this type of disease transmission. (4/25)
Solid blue boxes are symptomatic individuals and vertical solid lines represent opportunities for transmission to other individuals that appear at other vertical strata. Onset times shown by the distance spanned by the solid horizontal lines. (5/25)
I have found two papers on the topic of pre-symptomatic transmission that I think are interesting (to be discussed below). Please note that there are also other examples, but they are not in the form of peer reviewed papers (discussed first). (6/25)
One widely read (non-peer reviewed) example is from the LA Times: latimes.com/world-nation/s…. The singers gathered for a 2.5 hour choir practice, (with social distancing). Three weeks later 45 out of 60 individuals had COVID-19 or it's symptoms. (7/25)
Another example appears in this news article from the South China Morning Post: scmp.com/news/china/sci…. (8/25)
I could not find the scientific article implied. In the comments on the article one commenter notes the paper on which the article is based has been retracted. (9/25)
Now, two articles that have been peer reviewed, and point out some interesting evidence regarding pre-symptomatic transmission. The first comes again from MMWR cdc.gov/mmwr/volumes/6… and details seven clusters with pre-symptomatic transmission. (10/25)
Turning again to the figure... Stars indicate the source patient, lightest blue color when likely infected, medium blue when that pre-symptomatic individual spread COVID-19 to others, and darkest blue when patients develop symptoms. (11/25)
The most interesting transmission in the whole article (to me anyway) is to A5 who occupied the same seat in the church that A1 and A2 had occupied earlier that day. More details on that transmission found here: ncbi.nlm.nih.gov/pubmed/32192580. (12/25)
Note that in the Lancet article the patient numbers have changed. The Lancet article says: C1 and C2 attended the 2-h morning church service, left immediately after. Based on closed-circuit camera findings... (13/25)
C5 occupied the same seat as did cases C1 and C2 at the prayer meeting after the morning service. So we have a possible transmission from the church seat or surrounding area, separated by a short but unquantified amount of time. (14/25)
The next article I want to discuss this location here: wwwnc.cdc.gov/eid/article/26…. And yes, you guessed it I'm going to tell you to look at the figure which is here: wwwnc.cdc.gov/eid/article/26…. (15/25)
The index patient is A1, and this circle is filled in (yellow) on the figure.
This individual was not sick at the time of the meal, but developed symptoms later that day. Other dates indicate symptom onset of the others at the restaurant. (16/25)
This individual was not sick at the time of the meal, but developed symptoms later that day. Other dates indicate symptom onset of the others at the restaurant. (16/25)
Bonus points if you can find the typographical error in the figure. But what this shows is that virus transmission is possible over a moderate distance, but the airflow patterns have to be just right. (17/25)
Paraphrasing the paper: Virus transmission cannot be explained by droplet transmission alone. Larger respiratory droplets (>5 μm) remain in the air for only a short time and travel only short distances, generally <1 m. (18/25)
Distances between A1 and others, especially those at table C, were all >1 m. Strong airflow from the air conditioner could have propagated droplets from table C to table A, then to table B, and then back to table C. (19/25)
So what are my takeaways from all of this? It's really good that we are all practicing social distancing. It's also really good (most?) people did not go to church or sing on Easter. (20/25)
We have evidence of pre-symptomatic spread via inanimate objects (church seat), which was contaminated by two infected people contacting it for two hours. The third individual was in that same seat for some unspecified period of time. (21/25)
I know there's some number of individuals who are really angry I'm telling people i don't think they need to sanitize their groceries. There's a larger number of people who are relieved by the same advice. (22/25)
As I've said in other threads, If it makes you feel better to sanitize your groceries (or for that matter wash your produce in soap), go for it, but I don't want to know. (23/25)
Thanks as always for reading. If you have questions or comments, I'll do my best to read and respond. Trolls will be blocked. People that just want to argue will eventually be muted. (24/25)
Stay home if you can, wash your hands and use hand sanitizer, and take care of those who need it most.
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