11 April 2020

Nature Medicine: “Respiratory virus shedding in exhaled breath and efficacy of face masks”

Our findings indicate that surgical masks can efficaciously reduce the emission of influenza virus particles into the environment in respiratory droplets, but not in aerosols12. Both the previous and current study used a bioaerosol collecting device, the Gesundheit-II (G-II)12,15,19, to capture exhaled breath particles and differentiated them into two size fractions, where exhaled breath coarse particles >5 μm (respiratory droplets) were collected by impaction with a 5-μm slit inertial Teflon impactor and the remaining fine particles ≤5 μm (aerosols) were collected by condensation in buffer. We also demonstrated the efficacy of surgical masks to reduce coronavirus detection and viral copies in large respiratory droplets and in aerosols (Table 1b). This has important implications for control of COVID-19, suggesting that surgical face masks could be used by ill people to reduce onward transmission.

Among the samples collected without a face mask, we found that the majority of participants with influenza virus and coronavirus infection did not shed detectable virus in respiratory droplets or aerosols, whereas for rhinovirus we detected virus in aerosols in 19 of 34 (56%) participants (compared to 4 of 10 (40%) for influenza and 8 of 23 (35%) for coronavirus). For those who did shed virus in respiratory droplets and aerosols, viral load in both tended to be low (Fig. 1). Given the high collection efficiency of the G-II (ref. 19) and given that each exhaled breath collection was conducted for 30 min, this might imply that prolonged close contact would be required for transmission to occur, even if transmission was primarily via aerosols, as has been described for rhinovirus colds20. Our results also indicate that there could be considerable heterogeneity in contagiousness of individuals with coronavirus and influenza virus infections.

Donald K. Milton & Benjamin J. Cowling

There’s a growing movement online towards wearing face masks all the time, even home-made fabric masks in absence of proper surgical masks, and this article is being passed around as supposed evidence in favor. Having read the conclusion quoted above – see also my highlights – I am getting a strong sense that these people have either not read the study, or are deliberately framing the results to justify their personal opinions. And that’s a dangerous path towards misinformation and ‘fake news’, towards the kind of twisted reasoning that led to the anti-vax movement.

Let’s go through the reasons why the study doesn’t actually support universal wearing of face masks:

  • First and foremost, the study hasn’t included the current strain of coronavirus causing COVID‑19, so its conclusions may be extrapolated to SARS‑CoV‑2, but are by no means certain.
  • It studied shedding of viruses from infected individuals, not if and how many healthy people can be infected by wearing masks in an infected environment.
  • It studied the effects of surgical masks, not of home-made masks that some people are now recommending. I think we can all agree these will prove far less effective, in any scenario.
  • Moreover, results shows participants infected with other coronavirus strains shed few virus particles through breathing. As such, masks should be recommended for people with common colds, to stop infecting those around them, but would be of marginal importance for influenza and coronavirus infections. With this conclusion, the study is rather an endorsement for social distancing, preventing prolonged close contact between healthy and infected individuals.

The study doesn’t disprove mask wearing as a partially efficient method of reducing viral spreading, but it needs to be used together with social distancing, wide-spread testing, and quarantine for the infected. As a side-note, in my neighborhood I noticed increasing numbers of people wearing masks compared to past weeks – yesterday at the supermarket more that half of them were wearing one – at least people are taking this situation seriously.

I personally worry that convincing people to constantly weak masks in public will lead to a false sense of security, and a relaxation of other measures that are more important to containing the disease, such as social distancing and proper hygiene. Besides not being available in sufficient numbers everywhere, masks come with many complications, for example people may not be fitting them properly over their faces – I’ve seen many who don’t cover their noses – or they are pulling them down to smoke, drink coffee or eat. The safest bet is to limit interactions, as much as possible, not to test how much artisanal masks can protect us.

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