Part 2 – Prove to me it’s airborne – well it’s the aerosol scientists that have done that…

This week’s blog is by Evonne T Curran (@EvonneTCurran) and is the second in a series exploring the evidence that covid is airborne.

It’s not droplets (Part 1) showed that neither the WHO nor the CDC provided evidence for the ‘droplet only transmission route’. Before we explore airborne, it’s worth questioning why people were reporting ‘droplets’ in papers and guidelines. If you believe ‘close = droplets’, you will see droplets, you will report droplets and you will take comfort from everyone else seeing and reporting them also. Textbooks will confirm droplets (without presenting direct evidence for ‘droplets’).

So, was the reporting of droplets in the infection prevention and control literature, a case of belief in an unproven maxim? Well this report of influenza transmission shows that it can happen. It includes a re-evaluation of outbreak reports looking for whether the original authors had evidence to exclude any Mode of Transmission (MoT). The re-examination identified that in several reports, airborne transmission had been eliminated without evidence (Table 2 p40). Thus, a Sherlock Holmes MoT exclusion approach must be applied when outbreak reports state transmission is by a particular route, when they have failed to exclude other routes.

Four types of evidence to consider here to find the dominant MoT – physics, Occam’s razor, the epidemic curve, and once again the Sherlock Holmes transmission MoT exclusion approach. Let’s apply these to person-to-person MoTs of SARS-CoV-2 via the three routes: airborne, droplets and/or contact. To begin my non-specialist subject, physics.

1. Physics

Most of the bullets below are taken from the Wang et al 2021 review (and the et al here are impressive aerosol scientists). This document of 206 citations can be considered as us deferring to experts and their expertise – a high-reliability characteristic. (Remember in comparison, the WHO had no citations in support of their statements regarding the physics of aerosols or droplets).

• Respiratory aerosols are produced during all forms of respiratory activity by both infectious and non-infectious people.
• Normal breathing has been shown to release up to 7200 aerosol particles per litre of exhaled air.
• ≤100μm represents the largest particles that remain suspended in still air for >5 s (from a height of 1.5 m), that travel beyond 1 m from the infectious person, and can be inhaled.
• Aerosols are small enough to linger in air, accumulate in poorly ventilated spaces, and be inhaled at both short and long ranges.
• Most exhaled aerosols are <5 μm, and a large fraction are <1 μm for most respiratory activities, including those produced during breathing, talking, and coughing.
• Droplets (>100 μm) are primarily produced from saliva in the oral cavity.
• Although coughing can produce more aerosols in a short period of time, it is much more sporadic than continuous breathing and speaking, especially for infected individuals who display no clinical symptoms.
• Pathogens predominate in the aerosols that are <5 μm.
• In an animal model, the aerosol exposed route resulted in more severe disease despite receiving a dose lower than that used for the intra-nasal route.

In summary, the evidence from physics is that: we all produce aerosols, most of what we exhale is aerosols, any aerosols ≤100μm can be inhaled, most virus is in aerosols and in an animal model the airborne route requires a lower dose to cause infection. Thus, evidence from physics supports airborne transmission being the dominant route.

2. Occam’s razor

Occam’s razor is a ‘rule of thumb’ that if there are several ways a thing could happen, the one with the least assumptions is usually correct. If it looks like a duck, flies like a duck, quacks like a duck, its likely to be a duck. Now look at what needs to happen for each MoT below (Table 1), acknowledging the physics that the virus is mostly in exhaled aerosols.

Table 1: What needs to happen for each MoT

MoT	What needs to happen
Airborne	Infectious person exhales aerosols containing virus.Aerosols containing virus, in a sufficient load, are inhaled by a susceptible person.
Droplet:	Infectious and symptomatic person exhales droplets containing virus.Droplets and virus land in sufficient numbers on the oral mucosa, nasal mucosa, or conjunctivae of a susceptible person within 2m of the infectious person.Requires the susceptible person to have glasses off, or mouth open or head tilted to get droplets into the nose (depending on height).
Contact direct	Infectious person whilst exhaling aerosols and droplets containing virus, contaminates their hands with a sufficient viable load of virus. The infectious person’s hands are then put directly in / on a susceptible person’s conjunctivae, mouth or in their nose], orThe infectious person transfers virus in a sufficient viable load to the susceptible person’s hands which then transfers (still in a sufficient viable load) to their own mouth, nose, or conjunctivae.
Contact indirect	Infectious person exhales aerosols droplets containing virus, which in a sufficient viable load land on a surface or are deposited onto a surface via contaminated hands.The surface must retain a viable virus load until a susceptible person touches the contaminated surface and transfers a viable load of the virus to their hands.The susceptible person then transfers the virus, still in a sufficient viable load, to their mouth, nasal mucosa, or conjunctivae.

Occam’s razor, applied to the possible MoTs shows that the airborne route has the fewest events that need to happen for transmission to arise. [Note again, the physics shows most virus to be found in exhaled aerosols].

3. The epidemic curve

The pandemic’s epidemiological curve is of multiple millions of people affecting every continent of the planet. The epidemiological curve cannot be explained by droplets, and or a lot of dirty hands and/or a lot of dirty surfaces. The only MoT that can explain the epidemic curve of SARS-CoV-2 is airborne via inhalation of aerosols containing the virus.

This argument is best summarised by Wang et al 2021:  “If large droplets dominated transmission, distancing alone would have effectively suppressed the transmission of SARS-CoV-2.”

4. Transmission exclusion evidence (Sherlock Holmes)

If people are within droplet range when transmission occurs, and they either do or do not touch items or each other, no MoT can be excluded – so the important thing is to omit these scenarios when examining possible MoTs.  

One of the excellent papers written during the pandemic is this by Prof Greenhalgh et al 10 reasons why it’s airborne. The first reason, they note is that super-spreading events are consistent with airborne transmission and cannot be explained by fomites or contact (people were not playing pass-the-parcel). This is illustrated in the Figure 1 which has two lanes that show when transmission must include airborne and one where airborne cannot be excluded. 

Figure 1: When transmission must include airborne and where airborne cannot be excluded (Greenhalgh et al. 2021)

If people were outwith the same room, and there was no common touch item – transmission must include airborne. Likewise, if people were outwith droplet range (>2m) and there was no common touch item – transmission must include airborne. This is the 2^nd point of Prof Greenhalgh et al. The 3^rd is that asymptomatic people are not producing droplets with which to transmit infection – but like everyone they will always produce aerosols. 4^th is that most transmission happens indoors and is reduced by improving ventilation. 5^th nosocomial transmission has happened when contact and droplet precautions were well-applied (and it has been reduced when airborne precautions were used). 6^th viable virus has been found in the air. 7^th virus has been found in places (filters etc.,) outwith droplet range. 8^th its spread around corners and outwith droplet range via caged animals. 9^th there is no evidence to exclude airborne and 10^th there is no alternative hypothesis. And a final reason – it spread 5 metres backwards on a bus (not achievable by droplets or contact MoTs).

Thus, physics, Occam’s razor, the epidemic curve and transmission exclusion evidence has eliminated everything apart from airborne as the dominant route. Ergo, the cumulative evidence shows that SARS-CoV-2 is spread mainly via the airborne route.

Finally it is worth noting that there were 206 citations in the Wang et al paper, 25 in the Greenhalgh paper and 145 in Fennelly’s paper. In contrast in the WHO pandemic preparedness related to physics there are NO citations.

Still think it’s droplets? Present your evidence, not your beliefs.