This was a pretty contentious part of the debate on the CDC’s recommendations and has come up elsewhere as well: how effective are masks at preventing infection when worn by an uninfected individual? How about preventing spread when worn by an infected individual? Are there things you can do that make it more or less useful?
This is a thread for evidence to answer those questions and related ones. Obviously the more rigorous the better, but case studies like this one about one dude on a bus would be useful had they not been retracted (thanks to gwillen for pointing this out).
The majority of studies either compare two different kinds of masks to each other, in a hospital setting, which is interesting but not the main thing we want, which is a comparison between mask and no mask. There are also a few studies looking at households where the housemates of a person with influenza-like illness are randomly assigned to use masks (or not); these studies find masks are not effective, but also show low compliance. In a household setting, most of the variance in whether a household member gets infected is likely explained by vaccination, other sources of prior immunity, and kitchen hygiene; whereas when considering infections of coronavirus occurring in public, none of these are factors. All studies had problems with noncompliance, and confounding between compliance and other precautions (ie people who comply with masks wash their hands at rates.)
This study is interesting in that it effectively has a “placebo mask” arm. There were three arms: a no-intervention arm in which health care workers continued wearing whatever masks they did before (less often than in either intervention arm, but still a significant amount), an intervention arm where hospital workers are given reusable cloth masks which do not work, and an intervention arm in which they’re given disposable medical masks. The disposable medical mask arm did best, the cloth-mask arm did worst (worse than the no-intervention arm, due to a combination of not using other masks and taking fewer non-mask precautions). Presumably, this was able to get past an IRB because the study authors didn’t know how bad cloth masks are.
Compliance (defined as “mask wearing more than 70% of working hours”) was 57% in both the cloth mask and medical groups, and 24% in the no-intervention group.
The majority of infections in the study were from rhinovirus, which is transmitted via aerosol and contact droplets. These are the same modes of transmission as SARS-CoV2, but in different proportions; rhinovirus causes sneezing, so it generates a lot of aerosol, whereas SARS-CoV2 doesn’t and aerosol is believed to be responsible for only a small portion of its transmissions. Other diagnosed infections in the study were from hMPV and influenza B. Rhinovirus has a smaller diameter than SARS-CoV2 (30nm), so it’s unlikely that mask aerosol penetration of rhinovirus is higher than that of SARS-CoV2. In particle penetration tests, the cloth masks were almost completely ineffective, and the medical masks had some effectiveness but much less than N95.
Compared to cloth masks, medical masks reduced clinical respiratory illness from 7.6 to 4.8%, laboratory-confirmed viral infection from 5.4 to 3.3%, and influenza-like illness from 2.3 to 0.2%.
Simple Respiratory Protection—Evaluation of the Filtration Performance of Cloth Masks and Common Fabric Materials Against 20–1000 nm Size Particles
Five major categories of fabric materials including sweatshirts, T-shirts, towels, scarves, and cloth masks were tested for polydisperse and monodisperse aerosols (20–1000 nm) at two different face velocities (5.5 and 16.5 cm s−1) and compared with the penetration levels for N95 respirator filter media. Average penetration levels for the three different cloth masks were between 74 and 90%, while N95 filter media controls showed 0.12%
Scott Alexander has done a literature review called Face Masks: Much More Than You Wanted To Know.
(If someone wrote an answer briefly summarising his conclusions and main evidences in 2-3 paragraphs, that would be a much better answer than this one.)
Surgical Mask vs N95 Respirator for Preventing Influenza Among Health Care Workers
Two groups of nurses were assigned to wear either medical masks or N95 masks when near patients with febrile respiratory illness (n=446). The study found no significant difference in the occurrence of flu.
Effectiveness of facemasks to reduce exposure hazards for airborne infections among general populations
Two mannequins were set up, one to simulate realistic coughing and the other realistic breathing. Transmission was tested with a variety of mask fits. Perfect fit did very well, normal fit did worst but was 40% effective.
Wrong link, should be https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3306645/.
I’m curious whether “normal” is meant to simulate a proper OSHA-approved fit, or “how normal people wear masks”. My impression was that the OSHA fit test standard for N95 masks was meant to achieve an extremely high-quality seal—they test with volatile substances, and you fail if you can smell them. (https://www.osha.gov/video/respiratory_protection/fittesting_transcript.html) It seems like the “normal” setting here is probably a lower standard than what an OSHA fit is supposed to achieve, whereas “fully sealed” (with tape) is possibly higher.
Fixed, thank you.
Face mask use and control of respiratory virus transmission in households
Among 286 exposed adults from 143 households with children sick with influenza-like illness, self-reported mask use correlated negatively with infection. Adherence was <50%.
I don’t have a lot of faith in this one- parents who wear masks reliably are likely more conscientious at other preventative measures (e.g. handwashing) .
Effectiveness of surgical masks against influenza bioaerosols.
A dummy test head attached to a breathing simulator was used to test the performance of a variety of surgical masks against a viral challenge. Live influenza virus was measurable from the air behind all surgical masks tested. The data indicate that a surgical mask will reduce exposure to aerosolised infectious influenza virus; reductions ranged from 1.1- to 55-fold (average 6-fold), depending on the design of the mask
N95 Respirators vs Medical Masks for Preventing Influenza Among Health Care Personnel
Two groups of nurses were assigned to wear either medical masks or N95 masks when near patients with respiratory illness (n=2371). The study found no significant difference in the occurrence of flu or respiratory illness.
Some comments from Tara Mac Aulay on masks, in the context of donating to hospitals:
She also mentioned that she expects standards in hospitals to decline over time, and that they will eventually accept other mask types for donations, if they don’t already.
Protecting healthcare workers from pandemic influenza: N95 or surgical masks?
A metareview of 21 studies and 25 lab-based publications. The majority of laboratory studies identified both mask types as having a range of filtration efficiency, yet N95 masks afford superior protection against particles of a similar size to influenza.