Summary of the Evidence For and Against the Routes of Transmission of SARS-CoV-2
https://tinyurl.com/aerosol-pros-cons
Jose L Jimenez, 21-Apr-2021, v 1.50
(w/ input from many scientists)
jose.jimenez@colorado.edu
Twitter: @jljcolorado
For references: click on numbered links like this example 1
Published 15-April 2021
https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(21)00869-2/fulltext
In Favor of Aerosols
Indoors >> outdoors 1, 2
Superspreading events (restaurant, choir, bus etc.)
Supersp. very similar to known aerosol diseases1
Very high R0 (~20) of superspreading 1
Poor ventilation helps transmission
Similar viruses transmitted via aerosols (SARS-CoV-1, influenza, MERS) 1
Virus stays infective in aerosols 1,2,3,4
Infectious virus detected 2-5 m from patients 1
Importance of close proximity (higher aerosol dose)
Consistency of close proximity & room 1
Importance of singing / shouting 1,2
Demonstrated w/ ferrets 1 & hamsters 2
Other pathogens most concentrated in fine aerosols 1
Aerosols from AGPs infective1, no reason why lower dose for longer time, no mask, would not infect as demonstrated for TB 2
Supermicron aerosols well filtered by well-fitted surgical masks 1
Source apportionment studies1,2
Nothing we have found, only misconceptions 1
Against Aerosols
In Favor of Fomites
Virus stays infective on surfaces 1,2
Similar viruses transmit this way 1
Intensive hand-washing program in UK reduced trans. 16% 1 (so plays role, but not dominant)
Demonstrated for hamsters (but inefficient) 1
Against importance
Lipid-enveloped viruses don’t survive long on human hands 1,2,3,4,5
CDC: possible, not major way 1
WHO: no specific evidence 1
Importance of close proximity encounters is not consistent w/ fomites dominant, pattern of transmission would be more random 1
Hamsters: “mainly by aerosols, rather than fomites” 1
Against Fomites
In Favor of Large Droplets
Large droplets have larger volume than aerosols 1,2
Against significant contribution
Never directly demonstrated for any disease 1
Droplets need to be ~300 ÎĽm to be efficient at reaching others, but very few of that size when talking 1,2
Very low efficiency of delivery to susceptible (eyes, nostrils, mouth are small targets) 1
Against dominance
Cannot explain outdoors << indoor
Cannot explain superspreading ev.
Cannot explain impact of ventilation
Importance of a/pre-symptomatic transmission (no cough, low droplets) 1
Against Large Droplets
Misconceptions about large droplets & aerosols I 1
For a thorough review see my Medscape perspective on this topic
Transmission in close proximity demonstrates large droplet transmission, and disproves aerosols
The opposite is true: large droplet transmission is unlikely when talking, and aerosol transmission in maximized in close proximity situations 1,2 This concerns the physics of motion of aerosols and droplets through the air, is published, and is understood best by aerosol scientists. And since aerosol have not been considered important for disease transmission, most ID and epi researchers are not experts on aerosols (e.g. persistent 5 um error).
A low R0 or lack of documented long-range transmission prove that this is not an airborne disease like measles
COVID-19 is indeed not like measles. Measles is a high-contagiousness aerosol-driven disease. COVID-19 is likely a lower-contagiousness aerosol driven disease. It infects best at close proximity, also at the room scale if we “help it along” (indoors, low ventilation, long time, no masks). And it has trouble infecting at long range 1
There is no basis for the R0 assumption. This is an artifact of history, which is being confused with a law of nature! Extreme resistance to aerosol transmission, due to adoption of the paradigm of Chapin (1910) deprived us of widely-accepted examples of less-contagious aerosol transmitted diseases, that do exist. 1 Only the most contagious ones (measles, and chickenpox) could be demonstrated, after being described as droplet/fomites for decades. If you “dial down” the contagiousness of measles, R0will decrease. There is no reason for nature to only generate diseases that are either supercontagious through aerosols, or not contagious at all. Intermediate cases are possible, and likely include SARS, MERS, and influenza.
Misconceptions about large droplets & aerosols II 1
That “droplet PPE” seems to work in hospitals disproves aerosols
Droplet PPE should be renamed as “droplet and lower-contagiousness / larger particle size aerosol PPE.” The particle sizes where SARS-CoV-2 most likely is (> 1 um)1 are reasonably well-filtered by surgical masks.2 And very sick people in ICUs etc. are not very contagious, outside of aerosol-generating procedures.1 Plus typically there is good ventilation in hospitals 1
Lack of long range transmission disproves aerosols
Lack of long range transmission is actually expected for a less contagious virus such as SARS-CoV-2. There is no reason in Nature why every virus that can infect through aerosols needs to be extremely contagious.1
Summary of Evidence vs. Modes of Transmission
Droplets
Fomites
Aerosols
Outdoors << Indoors
X
âś”
✔✔
Similar viruses demonstrated
X
âś”
âś”
Animal models
?
âś”
âś”
Superspreading events
X
X
✔✔
Supersp. Patterns similar to known aerosol diseases
n/a
n/a
âś”
Importance of close proximity
âś”
X
✔✔
Consistency of close prox. & room-level
X
X
âś”
Physical plausibility (talking)
X
âś”
âś”
Physical plausibility (cough, sneeze)
âś”
âś”
âś”
Impact of reduced ventilation
X
X
âś”
SARS-CoV-2 infectivity demonstrated in real world
X
X
âś”
SARS-CoV-2 infectivity demonstrated in lab
X
âś”
âś”
“Droplet” PPE works reasonably well
âś”
âś”
âś”
Transmission by a/pre-symptomatics (no cough)
X
âś”
âś”
Infection through eyes
âś”
âś”
âś”
Transmission risk models
âś”
âś”
âś”
More details and references: http://tinyrul.com/aerosol-pros-cons Only including the items that could bear on multiple pathways. See other slides for details and references
Key:
âś”: evidence
✔✔: very strong ev.
X: no evidence
X: evidence against
n/a: not applicable
(v1.48, 18-Oct-2020)
From https://twitter.com/JenniferKShea/status/1373840694176010240
Additional items for discussion, but not considered good evidence for now
Additional Items, not considered solid evidence so far
Someone added against droplets: “Bypasses immune system for asymptomatic people--meaning it travels deep into lungs (only tiny aerosols do this)”
But many ACE2 receptors in upper resp track, no need to reach deep lung. (Also not sure what is meant by “bypasses immune system”
Suggested in favor of aerosols (possibly fomites?): more transmission when air is dry
https://www.eurekalert.org/pub_releases/2020-08/lift-css082020.php
But only one study, difficult to study. WMO says not strong evidence. And humidity also affects the host https://www.annualreviews.org/doi/10.1146/annurev-virology-012420-022445 So it seems too uncertain to add it as a strong piece of evidence for any modes at present
Suggested in favor of aerosols: more transmission to tall people
Logic is sound because exhaled breath rises due to buoyancy
But study that reported this was very limited, not considered solid, see this thread and discussion in comments
Background on the transmission issues
From CDC web page, “Aerosols 101” presentation: https://t.co/HXnHGnf2up?amp=1
From Milton, 2020: https://academic.oup.com/jpids/article/doi/10.1093/jpids/piaa079/5875939?guestAccessKey=72cf62f5-ae56-4073-a264-7b3818371f07
Droplets vs. Aerosols
Older Slides - Backup
Not being updated, kept here just for convenience
Reasons why the mitigation measures work
-No close proximity: much reduced aerosols (talking), also less droplets (cough)
- Avoid indoors: no room-level aerosols, less close proximity aerosols (wind, UV), less fomites (UV)
- Avoid crowded + long duration: reduced close proximity + room-level aerosols, reduced fomites and droplets
- Avoid low ventilation: reduced room-level and long range aerosols
- No masks: reduced close proximity, room-level, long-range, fomites, and droplets
- Talking / shouting / singing: reduced close proximity, room-level, long range, droplets (also less generation of fomites)