On September 18, the US Centers for Disease Control and Prevention (CDC) updated its guidelines to recognize that SARS-CoV-2 can be transmitted by aerosols. This change was withdrawn on the 23rd, five days later, and today this type of transmission is no longer mentioned.
The controversy over aerosol transmission is not new. In July, 239 scientists from 32 countries, led by Australian Lidia Morawska, signed an open letter urging recognition of the importance of aerosol transmission. Since then, both the World Health Organization (WHO) and governments have accepted it, even minimizing its potential impact.
Droplet size matters, but it’s not the only thing
Let us put aside the exaggerated risk of transmission by fomites (objects and surfaces). Also other routes that are uncertain or with less potential for diffusion (ocular, fecal-oral, vertical and through breast milk, sexual and blood).
The predominant mode of transmission for COVID-19 is respiratory. The problem is that the label “respiratory” has a notable controversy surrounding it.
Some scientists defend the almost exclusive role of transmission by droplets. These would be exhaled by the infected people and would fall to the ground in minutes. Therefore, they would only reach short distances (hence the recommendation of the 2 meters distance).
Others postulate that much smaller droplets (micro-droplets) in the form of aerosols (airborne transmission) would be determining factors in the transmission of SARS-CoV-2. These would remain for hours in the air and would reach distances much greater than two meters.
Part of this debate is due to the fact that, for historical reasons and conceptual simplification, epidemiologists work with an outdated dichotomous framework for respiratory transmission routes that opposes droplets (5-10 µm) to aerosols (
However, droplets exist in a continuous range of sizes, and their ability to travel distances and their infective capacity depend on many other variables apart from their size. For example, the force of emission, ventilation, exposure time, temperature and humidity of the environment. It is even pointed out that the phonetic characteristics of some sounds could have a greater transmission capacity than others.
Evidence supporting droplet transmission
Transmission by droplets and over short distances, probably with less experimental evidence than transmission by aerosols, is mainly supported by empirical data.
On the one hand, numerous studies show that a good part of COVID-19 cases are linked to direct infections (person-person) with someone close to them; either with symptoms or in the incubation period. That is why close contact tracking is the foundation of all lead tracing strategies.
On the other hand, the dynamics of the transmission of SARS-CoV-2 is, in general, much more similar to that of diseases transmitted by droplets (such as influenza, with reproduction rates R₀ around 2-3), than those transmitted by aerosols (such as measles and chickenpox, which reach R₀ greater than 15).
Similarly, the secondary attack rate (the proportion of contacts that develop the disease) is too small in occasional contacts to suggest aerosol transmission. The spread of SARS-CoV-2 seems to require closeness, exposure time and lack of protection. Therefore, the definition of close contact for tracing requires less than two meters of distance for more than 15 minutes.
Likewise, the similar protection offered by masks with very different aerosol filtering capacity points to the greater importance of direct respiratory transmission.
Evidence supporting aerosol transmission
But there are also numerous (very large) experimental evidences and empirical data that suggest the importance of aerosol transmission, at least under certain conditions.
Dozens of studies have shown that –beyond aerosol-generating procedures such as intubation–, talking –especially out loud–, coughing and sneezing produce aerosols in which SARS-CoV-2 can travel distances greater than two meters and stay viable in the air for hours.
Although this does not demonstrate the infectivity of aerosols, which depends on many other factors, it does leave the possibility of transmission by this route very open.
In the same way, the study of some outbreaks in choirs, restaurants, buses, residences and some other places, suggests the importance of the transmission by aerosols in interior spaces, especially when they are not well ventilated.
Likewise, the explosive behavior of SARS-CoV-2 in some closed spaces, such as companies (meat), collective centers (residences, hospitals, prisons and others), nightclubs, long-term transport, even without ruling out the possibility of transmission by droplets, also strongly suggests aerosol transmission.
Currently no scientific group denies the transmission by aerosols, which is otherwise widely recognized in poorly ventilated indoor spaces. The differences lie in how much importance is given to it in the overall transmission.
Aerosol transmission can have complex implications
Admitting that aerosols play a widespread role in the transmission of SARS-CoV-2 would have a huge social and economic impact.
First, we would have to make the leap to the much more expensive and less available masks with high aerosol filtration capacity (FFP2 or FFP3 in EU standards, N95 in US standards, KN95 in Chinese standards) . This would be difficult in developed countries and almost impossible in the rest of the world. Perhaps it would spread panic without good protection alternatives.
More difficult still: we should also check the air conditioning systems. Not only those of the transport of people (buses, trains, subways, planes), but also those of shopping centers and companies. These systems would not always be adaptable to the use of high-efficiency filters, germicidal devices, or other environmental control technologies. We should probably also undertake structural renovations in many public buildings to ensure sufficient ventilation.
Under these circumstances, and with the current level of uncertainty about the global importance of each route of transmission, it seems reasonable that the WHO and government health protection agencies require more evidence for aerosol transmission than for droplet transmission – approachable with simple measures: distance and conventional masks – or by fomites.
In the latter, beyond hand washing, nobody is very clear about the usefulness of the “hygiene theater”, while worrying about the environmental impact of so much disinfectant, the expense that could be more useful in other measures and the false sense of security .
Ventilate and you will conquer
Requiring sufficient evidence for some specific aspects of aerosol transmission should not be at odds with addressing others, simple and potentially very beneficial. These require less evidence for their development and, above all, would hinder both droplet and aerosol transmission.
An example would be to minimize the number of people who share interior spaces and the widespread use of masks and distance in these spaces.
Indoor spaces, with more or less crowds of people (bars, restaurants, subways, buses, trains), increase the risk. Up to 20 times more. Sufficient ventilation significantly reduces it. In collective centers, keeping the most closed spaces, such as bedrooms and bathrooms, ventilated also helps. Reducing air recirculation as much as possible is also a good idea.
While many administrations retain the curious mania of closing parks and imposing harsh restrictions on beaches, mountains and lonely streets, they keep open interior spaces, sometimes poorly ventilated, with significant gaps and in which in many cases the use of masks is obviated.
Perhaps it is time to limit outdoor activities less – avoiding crowds and very partitioned spaces – which have much less risk of transmission than closed spaces and, above all, incorporate ventilation as an essential strategy in the latter.
As in other times those who watched the young people at dances said: “Let the air run!”
Salvador Peiró (researcher, Health Services Research Area, FISABIO SALUD PÚBLICA, Fisabio)
This article was originally published on The Conversation. Read the original.
