Some data on mask effectiveness
We’re still in the early days of understanding exact effectiveness. Few randomized control trials have been done.
However, there’s some good meta-analysis done on the geographical level, in the US, Germany, and within hospital systems.
You probably already believe this but please wear a mask. They aren’t a cure-all but they help.
I wanted to take a look at masks and just how “how effective” they are. I think there’s a general consensus around my cohort and the scientific community that masks are pretty effective at reducing the spread of covid, help reduce your viral load intake, and are generally good. But I think the details are a little hazy (what amount of filtration helps how much?). This is partly motivated by personal curiosity (what do we know, given the policy advice?), and partly motivated because I saw a former classmate commenting about sheeple. What follows is a brief overview of some studies and surveys from the last few months.
Some caveats:
For some simplicity, I’m going to avoid studies about droplet size and filtration ability (N95 vs cloth). Most people aren’t wearing N95’s.
There is a lack of randomized control trials in this space. But there are some ethical concerns around doing so, and if one’s argument is that masks help prevent community spread, then it seems like the measurement should be done on entire communities that have adopted the mask standard. So there are a few observational studies pre- and post- mask intervention.
In some order:
CMU stats on Mask-wearing [link]
Delphi is a research group that tracked diseases like flu and dengue. When the pandemic began, they pivoted to Covid. They worked with Facebook to do mass surveys of people on whether they wore masks or not. This led to their being able to visualize how mask adherence and infection rates relate. In particular, I think this graph is really cool:
This is done for 50 states - each state is its own datapoint (I’m personally proud to see New York and my native Maryland on the top left). This is probably as direct as data gets. Places where a lot of people wear masks, tend to have fewer per-capita new cases of covid.
Massachusetts Hospital Workers [link]
The Massachusetts Brigham network has 12 hospitals and more than 75k employees. Early on in the pandemic, they were experiencing a rising rate of infection. By March, they were seeing a positivity rate of 21% and case doubling every 3.6 days. They then created a universal mask policy and instituted a comprehensive testing strategy. After doing so, positivity rates declined from 14.7% to 11.5%. Which isn’t super-drastic, but what I think is notable here is that it presumably measures mask effectiveness in an environment that’s already presumably dense in Covid.
Poor Hamsters [link]
In one of the more controlled studies, a researcher placed hamsters in cage systems with exposed, one-directional airflow (from a covid-compromised hamster). In the test case, they placed a surgical mask over the partition. The result? In the control group (no mask separation), 67% of hamsters got covid. There were two follow-up tests, simulating different types of covid exposure (with mask), but in those the infection rates dropped to 25% and 16%. The numbers are pretty small (~50 hamsters for the whole experiment), but the differences are pretty large.
By the way, you might have noticed that our posting frequency is down right now. Some of it is due to the holidays, but some of it due to us having a different main project, and being pretty busy with that. But with that being said, we’re still going to do research and write posts, it just won’t be super frequent :).
And some more links, if you’re interested:
(https://gh.bmj.com/content/bmjgh/5/5/e002794.full.pdf) - Researchers looked at covid transmission within households where one member had gotten covid. Their conclusion was that wearing masks is good, and spending significant time together (eg. eating dinner) was bad. But there are numbers behind this!
(https://www-ncbi-nlm-nih-gov.libproxy.lib.unc.edu/pmc/articles/PMC7253999/) - This an overview and survey over previous studies involving face masks and respiratory viruses. It included 21 studies. The reason I didn’t include this above was because it was heavily weighted by studies on the flu and SARS.
(https://www.healthaffairs.org/doi/10.1377/hlthaff.2020.00818) - A look at changes in case rates in US states after mask mandates. It shows a decrease in daily case rates, but I found their descriptions of their stats a bit confusing.
(https://www.pnas.org/content/117/51/32293) A study that looked at covid growth rates in various German districts, because different places instituted mask mandates at different times. Aggregated over all districts they looked at, they found that the mask mandate reduced infection rate by 47%.
(https://www.acpjournals.org/doi/10.7326/M20-6817)
I do want to mention the Dutch study, which got some headlines. That was a randomized experiment, but had a dearth of data. For example - it only could have measured the effect of masks on the wearer not getting covid, but because they only asked a small number of people (3000 people throughout Denmark) to wear masks, there was no way to measure the decrease in transmission from already infected people. Their end statistics hinted that mask wearing reduced to a reduction in transmission. But it was far from statistically significant.