Artist’s depiction of a coronavirus such as SARS-Cov-2
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Previously, I wrote about the evidence for airborne transmission of Covid-19. Here I want to consider how to protect against airborne transmission and what airborne transmission might mean for the epidemiology of Covid-19.
Wear a mask
Homemade face masks protect against infection with SARS-CoV-2
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The good news is that face masks reduce airborne transmission and they do this two ways: (1) by protecting the susceptible wearer, and (2) by protecting everyone else when the wearer is the one who is infectious.
Even though aerosol filtration by common fabrics is not perfect, it may be comparable to N95 masks. Even homemade face masks made of tea cloth have been shown to reduce the inhalation of aerosols by about 50%. This isn’t terribly bad compared to surgical masks, which reduced exposure by about 75% in the same experiment. (Both homemade masks and surgical masks were inferior compared with FFP2 masks, a European mask similar to the N95 masks available in North America, which reduced exposure by about 99%.)
Highly protective N95 face mask
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Another study, using surgical masks, looked at aerosols generated during a 30 minute period of normal breathing by individuals infected with one of two human coronaviruses, influenza virus, or rhinovirus. The coronaviruses used in this study — known as OC43 and HKU1 — are relatives of the virus that causes Covid-19, but are not the same species. Coronavirus was detected during thirty minutes in the exhaled breath from four out of ten participants without masks but zero out of eleven participants with masks. These sample sizes are small, but statistically significant.
Perhaps most interestingly, the data collected in the second study also included how many times participants coughed. Despite infection, some participants didn’t cough at all. Of these “non-coughers” there were only four with coronavirus and none of these generated detectable virus in aerosols or droplets. But one out of nine non-coughers with flu and five out of seventeen non-coughers with rhinovirus generated virus-laden aerosols. These results strongly support the value of masks worn even by people who are not symptomatic, if it’s possible that they may be infected.
Why being outdoors helps: Sunlight and ventilation
Regardless of transmission route, one thing is certain: increased contact with others, both in duration and proximity, increases risk of infection. The risk is compounded in enclosed spaces, where social distancing is more difficult. One study found that the odds of transmitting Covid-19 in a closed environment is 18.7 times greater than in an open-air environment. Being outside provides an astonishing reduction in risk.
This is partly due to ventilation. A key element in reducing risk indoors is ensuring proper ventilation. When ventilation is poor, aerosols remain in the air longer. For individuals in the same enclosed space as an infected person, poor ventilation increases risk of airborne transmission by allowing aerosolized virus particles to accumulate.
In looking further at how risks of indoor spaces may be mitigated, studies have demonstrated that it is the sun itself that reduces the infectiousness of SARS-CoV-2. As little as eight minutes of exposure to sunlight reduced the viability of an aerosolized sample by 90%. A control sample left in the dark required more than four and half hours for the same reduction in viability.
Time (in minutes) to 90% loss of viability of SARS-CoV-2 under simulated sunlight.
Image: John M. Drake. Based on data from Schuit. et al. 2020.
Why is there still skepticism about airborne transmission?
Despite the many studies suggesting the possibility of airborne transmission, there is still skepticism that it plays anything more than a negligible role in total infections. One argument focuses on the reproduction number (i.e. the number of secondary cases rising from each infected person). The reproduction number for Covid-19 — even before strict prevention measures — is probably somewhere between 2 and 4 (with some of that variation depending on the transmission setting). This is far lower than viruses known to be airborne, such as measles, which has a reproduction number of 12 to 18.
This is a compelling argument. But there are alternative explanations. For instance, it could be that transmission is generally rare (whether by aerosols, droplets, or fomites), but sometimes, perhaps only very occasionally, results in a lot of airborne exposures. Rarity and magnitude would balance each other out to result in the modest reproduction number estimated by epidemiologists. A prediction of this hypothesis is that SARS-CoV-2 transmission may occur in bursts. There is anecdotal evidence that Covid-19 transmission occurs in bursts quite often (think about the numerous reports of large simultaneous exposures like the Skagit County choir practice). A feature of the original SARS virus, which caused a series of epidemics in 2003 and is related to the virus that causes Covid-19, was transmission in bursts
To sum up, aerosols have the chance of exposing a much larger number of people than droplets, due to the longer time they remain aloft and longer distances traveled. But there are ways to reduce risk — by wearing masks, socializing outside, and ensuring adequate indoor ventilation. Without taking these precautions, even if the number of exposure events may be small, it is quite possible that a relatively large number of individual cases of Covid-19 have been caused by airborne transmission. The important thing is that this is neither surprising nor insurmountable.
