Thread by @AbraarKaran

**New CDC MMWR**

Those reporting consistent mask use in indoor public settings were significantly less likely to test positive for #covid19

The effect was greatest in those reporting consistent N95 mask use

cdc.gov/mmwr/volumes/7…

2/ The data in favor of using N95 respirators for shared air / public indoor spaces during surges will continue to grow. Those who are skeptics will eventually come around. But the time lag will result in many needless infections, hospitalizations and deaths. It already has.

3/ Limitations of studies are always important.

Authors mention: “First, this study did not account for other preventive behaviors that could influence risk for acquiring infection, including adherence to physical distancing recommendations”

So one confounder here…

4/ would be if those who were more likely to wear better masks were also less likely to be at higher risk indoor shared spaces like crowded bars versus those who reported wearing no mask.

Shared indoor spaces differ in terms of their risk of spread (ventilation, crowding etc)

5/ I think this explanation could account for some of the difference between mask/ no mask, but less so between the type of mask you were wearing (are people who wear N95 really that much more careful than surgical; are those wearing surgical much more than cloth?)

6/ And remember— many public health experts once argued that masks would increase risk taking (an argument they made for no masks), so my attempt to give one confounder here to explain away the effect is as a devils advocate