Thread by @Jabaluck

An intuitive way to grasp the effectiveness of masks: extrapolating from our results, every 600 people who wear masks for a year in public areas prevents 1 person from dying of COVID given status quo death rates in the US.

Note that this is *taking into account current vaccination rates in the US*. Despite the availability of vaccines in the US, the weekly death rate is higher than at any point prior to November 2020.

Here is how I arrived at this number. Our study shows that inducing a 30 pp increase in mask-use prevented 35% of COVID cases among the elderly.

First, let's assume that we see a proportionate reduction in mortality. In the US, death rates for the past year and a half (lower than in the past week!) have been 1,200 people per day.

Of these, 80% are among those 60+. Our 35% number would imply about 120,000 preventable deaths.

A 30 pp increase in mask-use among adults is ~75 million people wearing masks regularly. Dividing 75 million by 120,000 gives ~600 people that need to wear masks for a year to prevent each death.

Let me first contextualize this number and then run through some caveats. The average American knows about 600 people by name that they associate with (not celebrities). nytimes.com/2013/02/19/sci…

That means that if you and every person you know by name wear masks in public areas for a year, on average, it will prevent one of you from dying of COVID.

Caveat #1: of course, there is substantial heterogeneity across places and times. If you live in a place where few people are dying of COVID (e.g. due to higher vax rates), the benefits are smaller. If more people are dying, they are larger.

Caveat #2: this doesn't mean you need to wear a mask 24/7. Our study doesn't assess this. Based on other work, I suspect that mask-wearing in crowded, indoor environments with poor ventilation is especially valuable.

My best guess is that mask-wearing outdoors has few benefits outside of very crowded areas, like rock concerts, rallies, sports events, or the kind of crowded markets we see in Bangladesh where people are breathing directly on one another.

Caveat #3: there are numerous external validity issues depending on the operative mechanism in our original study. For example, if part of the effect comes from the fact that people with masks physically distance more, would this be equally true in the US?

This kind of calculation is very far from definitive. But it's also not "fixed" one way or the other. I didn't choose the numbers and assumptions to get a desired result. You can get something that looks even better for masks if you use death rates in the last week.

I plugged in the numbers from the study plus reasonable assumptions in order to produce our best guess estimate of the impact of masks on mortality, and that impact appears to be very large given status quo vaccination rates in the United States.

(here is a link to the study that is the basis for these calculations: poverty-action.org/sites/default/…)