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Summary of the Evidence For and Against the Routes of Transmission of SARS-CoV-2

A collection of scientific evidence for and against the following methods of transmission of Covid-19:

- Aerosols

- Fomites

- Large Droplets

coronavirus, science, aerosol, masks

Summary of the Evidence For and Against the Routes of Transmission of SARS-CoV-2

Jose L Jimenez, 18-Oct-2020, v 1.48

(w/ input from many scientists)

In prep. for publication - Suggestions & discussion welcome: [email protected]

Twitter: @jljcolorado

For references: click on numbered links like this example 1

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 μm 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




Outdoors << Indoors



Similar viruses demonstrated


Animal models


Superspreading events




Supersp. Patterns similar to known aerosol diseases



Importance of close proximity



Consistency of close prox. & room-level



Physical plausibility (talking)


Physical plausibility (cough, sneeze)

Impact of reduced ventilation



SARS-CoV-2 infectivity demonstrated in real world



SARS-CoV-2 infectivity demonstrated in lab


“Droplet” PPE works reasonably well

Transmission by a/pre-symptomatics (no cough)


Infection through eyes

Transmission risk models

More details and references: Only including the items that could bear on multiple pathways. See other slides for details and references


✔: evidence

✔✔: very strong ev.

X: no evidence

X: evidence against

n/a: not applicable

(v1.48, 18-Oct-2020)

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

But only one study, difficult to study. WMO says not strong evidence. And humidity also affects the host 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:

From Milton, 2020:

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)

Summary of the Evidence For and Against the Routes of Transmission of SARS-CoV-2
Tags Coronavirus, Science, Aerosol, Masks
Type Google Slide
Published 20/01/2021, 17:53:31


COVID-19 transmission patterns only seem explainable by aerosols