1/10 Let's unpack transmission of #COVID19 and whether airborne transmission is a real thing and if you really need those #n95mask @asidanz @acipc @healthgovau @normanswan @hnehealth @hneccphn @peripatetical

2/10 Breathing, talking, coughing create bioaerosols - droplets large and small. Smaller ones dessicate to droplet nuclei (1-5 µm) in low humidity. Large ones fall and contaminate surfaces. #COVID19

3/10 Droplet nuclei efficiently transmit disease at a distance- e.g. tuberculosis, primary varicella, measles- one reason for very high R0s (> 10) and high household transmission. #COVID19

4/10 Cf. respiratory viruses including #COVID19 exhibit much lower R0 values and much lower household transmission (10% or less).

5/10 Extensive studies of #COVID19 transmission chains and clusters finds no evidence of airborne spread at a distance (marker of transmission by droplet nuclei)

6/10 Randomized trials of fit tested #n95mask vs surgical masks show no difference in resp. virus (influenza mainly studied) HCW acquisition which argues against there being a significant droplet nuclei transmission (Long-Y 2020) #COVID19

7/10 Why does #COVID19 not transmit via droplet nuclei – ? insufficient viral inoculum ? lower resp tract deposition not an efficient infection route or ? viability ? relative receptor expression URT vs LRT (ACE2 receptors are found in lung)

8/10 If high risk AGPs efficiently spread #COVID19 (open question), likely due to excessive production of bioaerosols per se NOT droplet nuclei.

9/10 With #sarscov1, tracheal intubation was the most consistent association with HCW acquisition across 8 studies (Chen-K 2012 Systematic review)-i.e. Hazard from close, not distant exposure- i.e. rationale for droplet precautions #COVID19

10/10 NB. Relative efficacy of #n95mask vs surgical mask never studied with AGPs. Droplet precautions (surgical masks, eye protection) prevented transmission in reported large hospital exposure events, including recently @HNEHEALTH #COVID19.