Thread by @jljcolorado

1/ Thread on the relative importance of transmission modes

A very difficult question, everyone reluctant to guess. As evidence accumulates, I decided to venture a guess, hoping to start some discussion. Pls comment w/ flaws in my arguments, other arguments, give your guess.

2/ Three modes: fomites, ballistic drops & aerosols

a) Fomites (touching objects, then touching eyes, nose, mouth): increasing consensus that they are minor. WHO: “no specific evidence. CDC: “it may be possible - not thought to be the main way”.

3/ Lancet letter argues against it:
- thelancet.com/journals/lanin…
- Good news piece on topic with interviews: cbc.ca/news/health/co…

People should still wash hands etc. But appears unlikely to be MAJOR contributor to spread. If anyone disagrees, pls reply and explain why

4/ b) Ballistic drops (HT @Don_Milton, analogy to rain drops) fall to ground quickly. These are WHO’s “droplets” fall in 1-2 m, 1-2 sec (graph). Can infect by impacting on eyes, nostrils, mouth. No direct evidence, but definitely possible. What fraction? Keep reading.

5/ c) Aerosols: don’t fall to ground quickly unlike drops, stay in the air tens of sec to hrs (depend on size). Infect by inhalation (or deposition onto eyes). Substantial evidence it happens (academic.oup.com/cid/article/do…), not 100% proof, @WHO resists.

6/ So 2 main players: drops & aerosols.
2 pieces of evidence suggest relative importance:
- Pandemic driven by superspreading events (eg nytimes.com/2020/06/30/sci…). Easy to explain w/ aerosols. Harder w/ drops. Transmission requires “prolongued close contact” (PCC).

7/ Need PCC with lots of ppl in a location. E.g. Skagit Choir (we investigated), fixed singing positions w/ 0-2 people within 6 ft in landing area of drops. A couple of 10 min breaks. Could index case “spit” drops on 53 ppl in such short breaks? No time for PCC!

8/ Guangzhou restaurant, buses, other cases. “Contorsionist thinking” (B. Nazaroff) required to explain w/o aerosols
- Note that when face-to-face, majority of infection likely through aerosols, much more concentrated right in front of person: sciencedirect.com/science/articl…

9/ -There is PCC both indoors & outdoors. Drops are ballistic, no time for dilution or UV to remove virus, don’t care indoors or outdoors, should be similar. Aerosols are carried by the wind, incredible dilution, more time for UV to destroy virus (very quick).

10/ Run Skagit choir case in aerosol transmission estimator (tinyurl.com/covid-estimator “choir” sheet), reproduces infection rate. Now move exact choir outdoors (“outdoors” sheet), infection drops from 83% to 0.4%.

11/ Real-world: "The vast majority of transmission seems to be through close contact with an infected individual, primarily in an indoor setting." (CBC article above). Only aerosols can explain this.

12/ Conclusion: for the first time, I am ready to say publicly that my *guess* is that the majority (>50%) of the spread is through aerosols. Pls comment w/ your take.

@linseymarr @ShellyMBoulder @CathNoakes @Don_Milton @SaskiaPopescu @angie_rasmussen @eliowa @GidMK

13/ @CorsIAQ @kprather88 @jeremyphoward @nataliexdean @j_g_allen @EpiEllie @WargockiPawel @apoorva_nyc @RichReadReports @allegranzib @mvankerkhove @WHO

14/ PS: thanks to @mauricedehond , his article (maurice.nl/2020/07/11/a-f…) inspired me to think about this. His guess is 98% aerosols. Seems high to me now… but so did >50% earlier in the pandemic.

15/ PSS: a day later, this has gotten more attention than expected. Some clarifications:
- As @linseymarr said, nobody knows the real fractions. This was clearly labeled as a guess. Obviously couldn't get a paper published asserting this.

16/ I did this on my own, and it does NOT represent the views of the group of 36.
- I did estimate as a thought experiment, to motivate discussion, especially with ppl skeptic that aerosols matter
- After much discussion, not major arguments or flaws that I have seen

17/ I think my main conclusion so far is that this estimate is PLAUSIBLE. There is not a lot of evidence that confirms aerosols unimportant. A bit that suggests they are. Precautionary principle: adapt guidance to protect community settings.

18/ Thanks to many scientists who have engaged in the discussion, kept open mind
- If members of media are listening, please don't report on the estimate. Not a scientific result, just a guess. But hopefully the discussion is useful to see how scientists debate disagreements.

19/ - We've said this multiple times, but again we don't want to undermine WHO or make their life difficult. They have a very difficult job under very difficult circumstances. We want to support WHO.

20/ Finally, there are so many replies that I can't simply reply to all. If there is something important and I don't respond, I probably haven't seen it, shoot me an email.