A tiresome SPAT

I’m surprised that we can’t stop arguing about the modes of SARS-CoV-2 transmission, despite the fact that most experts (including our friends at WHO) agree on the important issues. Our colleague Jorge Salinas very nicely summarized these issues (and their implications) in this post.

The latest kerfuffle: media coverage of 239 experts who are upset that the WHO is not acting as decisively as they’d like on an evidence base that the experts themselves admit is far from definitive.

As we’ve outlined here and here, a major problem plaguing this discussion is the false dichotomy between “droplet” and “airborne” transmission that we use in healthcare settings (for simplicity of messaging, and because it has served us well for several decades—for reasons I’ll get back to later). This dichotomy divides application of transmission-based precautions between those pathogens spread via respiratory droplets, all of which must absolutely fall to the ground within 6 feet of the source, and those pathogens which become airborne, meaning they travel long distances on air currents, remain in the air for very long periods of time, and most importantly, can cause infection after their airborne sojourns if they find the right mucosal surface.

But we know (and WHO experts know) that there is no such dichotomy—it’s more of a continuum. At the very least there is a middle category, let’s call it Small Particle Aerosol Transmission (or SPAT). Many respiratory viruses (not just SARS-CoV-2) can remain suspended in aerosols and travel distances > 6 feet. As Jorge outlined, it’s probable that transmission events occur when these aerosols are concentrated in closed, poorly ventilated spaces or in very large amounts (e.g. a 2+ hour choir practice, a 3 hour indoor birthday party, a crowded bar). This may explain the superspreading events that drive a lot of SARS-CoV-2 transmission.

It’s important to distinguish SPAT from “classic airborne transmission” (let’s call it CAT). The CAT pathogens (TB, measles, VZV) have very different transmission dynamics than SPAT pathogens, as I outlined here (R0s of >10, household transmission rates of 50-90%). The distinction is important because for most healthcare epidemiologists, using the term “airborne” implies a common set of “one-size fits all” interventions to prevent transmission, interventions that require resource-intensive engineering controls and PPE requirements. It is not at all clear that such interventions are required to prevent transmission of SPAT pathogens. In fact, most evidence (and real world experience) suggests that they are not. This is why the droplet-airborne dichotomy has served us fairly well over the years—either because droplet precautions appear to be pretty effective at preventing SPAT, or because SPAT is rare even among those viruses capable of it.

I could say more about my feelings about aerosol-scientists criticizing epidemiologists and clinicians for having an “overly medicalized view” of the evidence, but I don’t want to be CAT-ty. I just want to end the SPAT.

So let’s redirect the discussion instead to: with the limited information we have, what additional interventions should WHO and/or CDC recommend for transmission prevention during the pandemic? Masks in crowded indoor spaces? Sure, but avoiding such spaces is preferred. Improved ventilation in all indoor environments? Absolutely, let’s get to work on that. N95s in the community? Don’t make me laugh, it might generate aerosols.* N95s for all patient care? Fair to consider, but by now we’ve gathered quite a lot of experience safely delivering care using existing WHO recommendations. And as Jorge aptly pointed out, “a debate only centered on whether respirators or medical masks are needed can distract us from the bigger challenges.” Indeed.

*Clarification as this comment, made in jest, has been misinterpreted.  N95 masks do not generate aerosols. They are unrealistic for community use, as they must be fit-tested and worn properly (even if we had an unlimited supply, which we do not). Nor are they, in my opinion, necessary for community protection. Face shields or medical/cloth masks are preferred for community use.