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%.
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%.