Airborne vs Droplet: Turbulent Gas Clouds of Opinion!

There’s much about the COVID-19 pandemic that is unprecedented, at least in my lifetime. One aspect is very familiar, though: arguments about the primary mode(s) of transmission of a newly emerging respiratory virus.

Much of the problem stems from our need to divide transmission modes into simple categories in order to apply prevention measures effectively. When someone calls the infection prevention program to ask about precautions recommended for virus X on the respiratory viral panel, it's not helpful to begin the conversation by saying, 

“well, you know, droplet and airborne transmission is not really a dichotomy, it’s more like a continuum, and there are a lot of factors at play—can we talk in more detail about the patient’s condition, what procedures they might be undergoing, and whether they might break out in song during routine patient care activities?”

One recent review you may find useful was published in Current Opinions in Infectious Diseases in August of 2019 by Shiu, Leung and Cowling (talk about great timing…). A very important point made in this piece is that viral nucleic acids and (less often) viable virus can be found in air samples--including from healthcare environments--for influenza, RSV, adenovirus, rhinovirus, and other coronaviruses. So reports about airborne SARS-CoV-2 (which will keep coming out in both pre-print and peer-reviewed literature) are not surprising. Nor do they answer the most important practical question about SARS-CoV-2 transmission:

Is airborne transmission a major mode of COVID-19 spread in community and in routine (i.e. no aerosol-generating procedure (AGP)) clinical settings?

My view is that we should consider the epidemiology of COVID-19 thus far in the pandemic, to determine if transmission patterns are more consistent with that of other common respiratory viral pathogens, or more consistent with that of the agents we classically consider to be transmitted by the airborne route (measles, VZV and M. tuberculosis). We could compare, for example, attack rates in various settings (household, healthcare, public), and the infamous R0 (expected ‘average’ number of secondary cases from a single infected individual in a susceptible population).

For COVID I’ll point to two careful contact investigations—this one of the over 400 close contacts of the first 10 travel-related COVID-19 cases in the US, and this study from Guangzhou, China, which was ten-fold larger (4950 close contacts to confirmed cases). The US study examined symptomatic secondary attack rates, and the study in China did serial RT-PCR on all contacts in addition to monitoring for symptoms. The findings are remarkably similar: highest attack rates are among household contacts (10.5% in US, 10.2% in Guangzhou), with extremely low rates of transmission among healthcare or community contacts (zero in US study, 1% among healthcare contacts and 0.1% among public transport contacts in Guangzhou). As for the R0, which of course varies as a population begins prevention approaches, the best estimate in my opinion is the tragic natural experiment performed on the unfortunate passengers of the Diamond Princess: during the early stage of the outbreak the R0 was 2.3.

For measles, the R0 is 12-18 and the secondary household attack rates are >= 90%. 

For VZV, the R0 is ~10 and the secondary household attack rate is 85%.

For TB, the R0 for smear-positive untreated TB is up to 10 (per year) and the secondary household attack rate has been reported to be >50%.

Based upon the above, I’m confident that SARS-CoV-2 transmission is similar to that of other respiratory viruses we are used to encountering—for which experience suggests droplet + contact spread to be the primary route of transmission. The trick is determining under what conditions a higher-risk aerosol might be produced (i.e. what is our list of AGPs? See here and here, if you dare!).

Does this mean that every respiratory droplet falls to the ground immediately and within 6 feet of a coughing patient? No. Dr. Lydia Bourouiba has an excellent piece in JAMA about the role of “turbulent gas clouds” in allowing droplets to travel further, and to remain in the air longer, than our traditional “droplet-airborne” dichotomy considers. In my view, this kind of droplet + "gas cloud" production mostly contributes to the extensive surface contamination that results in the highest risk of transmission being among close household contacts.