Wear a Mask! Here’s the science behind mask use.

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Photo: Juan Rojas. Toronto

Note: This article has been updated. Scroll to the bottom to see the update.

The science around wearing a mask has been, lets say, evolving since the start of the Covid-19 pandemic. Right wing conspiracy theorists and anti-science folks have been claiming, among other things, that masks simply don’t work.

It is true that randomized, controlled trials (RCT’s) are the best type of evidence, scientifically speaking. The problem is that it is difficult to properly assess how effective mask wearing really is. You can’t exactly go around spraying masked and unmasked volunteers with Covid-19 and compare the infection results. So we must look at previous studies based on other diseases such as influenza.

Scientists have done studies over previous decades, and systematic reviews have assessed the results.

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In a 2020 meta-analysis study, two different types of masks were studied to determine which was more effective at preventing influenza epidemics. Surgical masks are “primarily designed to protect the environment from the wearer.” N95 respirators, on the other hand “are supposed to protect the wearer from the environment.” These were compared, and the study concluded that both masks provided similar amounts of protection in practice. It also determined that these masks provide a “protective effect” against influenza.

The N95 respirator has been shown in lab testing to be more effective than surgical masks, but they are also used improperly and can cause discomfort. In practice they offer about the same amount of transmission prevention and are not recommended for the public.

The N95 respirator has been shown in lab testing to be more effective than surgical masks, but they are also used improperly and can cause discomfort. In practice they offer about the same amount of transmission prevention and are not recommended for the public.

One of the trials within this study was done in Australia in 2009. The study, which was considered to be robust, came to this probable conclusion:

We estimated that, irrespective of the assumed value for the incubation period (1 or 2 days), the relative reduction in the daily risk of acquiring a respiratory infection associated with adherent mask use (P2 or surgical) was in the range of 60%–80%. 

Previous systematic reviews have also been done, with less definitive results, and these are typically the studies that ‘anti-maskers’ tend to cite. Systematic reviews in 2011, 2010, and 2007 came to less definitive conclusions; there is “limited evidence base supporting the efficacy or effectiveness of face masks to reduce influenza virus transmission,” but this is due to the “substantial gap in the scientific literature.”

These older studies were of poorer quality with problems such as small sample sizes (one study only had 32 participants in Japan), and possible publication biases. Citing these studies as proof that ‘masks don’t work’ is the the fallacy that “absence of evidence is not evidence of absence.” Newer science such as the 2020 study earlier referenced is emerging.

Simulations show that the smaller the amount of initial cases in an outbreak, the higher the chances that outbreak will be contained. If masks can prevent even a fraction of cases in an outbreak, on a city, provincial, or national level this will make containing waves of the coronavirus much easier on our medical system.

Source: Feasibility of controlling COVID-19 outbreaks by isolation of cases and contacts, The Lancet

Though not as robust as RCT’s, case reports can provide some insight and should be taken into account. In May 2020, two hair stylists at salon in Springfield, Missouri became infected with Covid-19. They continued to work, with symptoms, with 139 clients who had an average age of 52. Everyone wore masks, and no clients were infected out of the 67 tested, and all clients were monitored afterwards. This is astounding, considering how infectious this disease is and how long it can stay airborne. It is reasonable to conclude from this case that masks have a significant role to play in containing this potential outbreak.

This particular outbreak also suggests that not only do masks protect the environment from the wearer, but also the wearer as well, to an extent.

Testing with hamsters in compartmentalized cages, separated by barriers made of surgical masks has shown a dramatic decrease in exposure. After 15 days, about 67% of hamsters in a single cage had become infected. Hamsters that were separated by a mask barrier, were only infected at 16%. The result stated that the “rate of non-contact transmission – in which the virus was transmitted via respiratory droplets or airborne particles – dropped by as much as 75 per cent when masks were present.”

Absence of evidence is not evidence of absence

Visual evidence of speech-generated droplets shows that face coverings of any kind can significantly reduce the amount of droplets ejected from ones mouth during normal speech.

It is easy to see why national policy on mask wearing has flip flopped during the initial months of the pandemic. Evidence was sparse, mixed, or inconclusive and public policy reflected that. But as new evidence is emerging that does support the use of face masks by the general population, so to, does public policy. The precautionary principle states that we should sometimes act without definitive RCT evidence, and at this point it is reasonable to believe that masks do provide some level of protection.

It is important to note that wearing masks will not be the be-all-end-all. Physical distancing and hand hygiene will be required, but masks will be a useful tool in the fight against Covid-19 until a vaccine is available.

Update Sept. 15, 2020:

Mats of CovidDash.org has provided a twitter thread of 70 different studies proving the effectiveness of masks. This is a very valuable resource.

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