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kottke.org posts about Trisha Greenhalgh

Case Closed: SARS-CoV-2 Spreads Primarily by Aerosols

posted by Jason Kottke   Apr 16, 2021

In a letter published in The Lancet, a group of scholars argue, with an extensive review of the available evidence, that the primary mode of transmission from human to human of the virus responsible for Covid-19 is via aerosols, not through larger particles called droplets or through fomites (transfer from surfaces). Here are three of their ten reasons why:

Third, asymptomatic or presymptomatic transmission of SARS-CoV-2 from people who are not coughing or sneezing is likely to account for at least a third, and perhaps up to 59%, of all transmission globally and is a key way SARS-CoV-2 has spread around the world, supportive of a predominantly airborne mode of transmission. Direct measurements show that speaking produces thousands of aerosol particles and few large droplets, which supports the airborne route.

Fourth, transmission of SARS-CoV-2 is higher indoors than outdoors and is substantially reduced by indoor ventilation. Both observations support a predominantly airborne route of transmission.

Fifth, nosocomial infections have been documented in health-care organisations, where there have been strict contact-and-droplet precautions and use of personal protective equipment (PPE) designed to protect against droplet but not aerosol exposure.

The letter concludes with a plea by the authors for public health officials to finally embrace this reality: “The public health community should act accordingly and without further delay.”

I can’t believe we’re actually still arguing about this. One of the authors, Jose-Luis Jimenez, wrote this seminal Time magazine piece that provided the smoke analogy that is the mental model I’ve been using to think about potential risks during the pandemic.

When it comes to COVID-19, the evidence overwhelmingly supports aerosol transmission, and there are no strong arguments against it. For example, contact tracing has found that much COVID-19 transmission occurs in close proximity, but that many people who share the same home with an infected person do not get the disease. To understand why, it is useful to use cigarette or vaping smoke (which is also an aerosol) as an analog. Imagine sharing a home with a smoker: if you stood close to the smoker while talking, you would inhale a great deal of smoke. Replace the smoke with virus-containing aerosols, which behave very similarly, and the impact is similar: the closer you are to someone releasing virus-carrying aerosols, the more likely you are to breathe in larger amounts of virus. We know from detailed, rigorous studies that when individuals talk in close proximity, aerosols dominate transmission and droplets are nearly negligible.

Another of the authors, Zeynep Tufekci, has been arguing the case for aerosols (and masks & overdispersion) since early in the pandemic, and she succinctly explained in a Twitter thread how predominantly aerosol transmission fits with the mitigation methods that have really worked around the world:

Airborne transmission unites three things crucial to recognize for effective COVID-19 mitigation: transmission without symptoms (thus aerosols), clusters driving the epidemic (also aerosols) and masks/ventilation indoors being key (hey, also aerosols). This framework is coherent.

Her whole thread is worth a read — like this bit about how other respiratory pathogens are likely spread by aerosols and not droplets (as commonly believed):

Fascinatingly, you search the scientific record high and low, but there really is little to no direct evidence for gravity-sprayed droplets being predominant mode of transmission for respiratory illnesses outside of coughing/sneezing. It’s many… assumptions. Like a tradition.

If any good comes out of the pandemic at all, a better and more useful scientific understanding of how respiratory pathogens are transmitted would be a good start.

Update: One of the authors, Trisha Greenhalgh, responds succinctly to criticisms of the paper in this Twitter thread.

Criticism 1: “The paper is just opinion, and several authors aren’t even doctors.”

Response: No. It’s well-researched scholarly argument, produced by an interdisciplinary team of 6 professors including 3 docs, 2 aerosol scientists and 1 social scientist.