Let's start with the high level conclusion, the one they use to sell the results.
Mandating masks was associated with a decrease in daily COVID-19 case and death growth rates within 20 days of implementation. Allowing on-premises restaurant dining was associated with an increase in daily COVID-19 case growth rates 41–100 days after implementation and an increase in daily death growth rates 61–100 days after implementation.Wow! That means masks matter! To heck with the years of previous studies from the same CDC, as we talked about last November that said 14 Randomized Controlled Studies did not show a substantial effect from masks or hand washing on Influenza transmission (and presumably other respiratory viruses). This is new and improved science!
What are the implications for public health practice?
Mask mandates and restricting any on-premises dining at restaurants can help limit community transmission of COVID-19 and reduce case and death growth rates. These findings can inform public policies to reduce community spread of COVID-19.
So let's jump ahead to the math because that's the important part. Bold and underline added.
During March 1–December 31, 2020, state-issued mask mandates applied in 2,313 (73.6%) of the 3,142 U.S. counties. Mask mandates were associated with a 0.5 percentage point decrease (p = 0.02) in daily COVID-19 case growth rates 1–20 days after implementation and decreases of 1.1, 1.5, 1.7, and 1.8 percentage points 21–40, 41–60, 61–80, and 81–100 days, respectively, after implementation (p<0.01 for all) (Table 1) (Figure). Mask mandates were associated with a 0.7 percentage point decrease (p = 0.03) in daily COVID-19 death growth rates 1–20 days after implementation and decreases of 1.0, 1.4, 1.6, and 1.9 percentage points 21–40, 41–60, 61–80, and 81–100 days, respectively, after implementation (p<0.01 for all). Daily case and death growth rates before implementation of mask mandates were not statistically different from the reference period.Note the underlined last statement in the first paragraph. Daily case growth and daily death growth rates were not statistically different than the reference period, 1–20 days before implementation of the mask mandate.
During the study period, states allowed restaurants to reopen for on-premises dining in 3,076 (97.9%) U.S. counties. Changes in daily COVID-19 case and death growth rates were not statistically significant 1–20 and 21–40 days after restrictions were lifted. Allowing on-premises dining at restaurants was associated with 0.9 (p = 0.02), 1.2 (p<0.01), and 1.1 (p = 0.04) percentage point increases in the case growth rate 41–60, 61–80, and 81–100 days, respectively, after restrictions were lifted (Table 2) (Figure). Allowing on-premises dining at restaurants was associated with 2.2 and 3.0 percentage point increases in the death growth rate 61–80 and 81–100 days, respectively, after restrictions were lifted (p<0.01 for both). Daily death growth rates before restrictions were lifted were not statistically different from those during the reference period, whereas significant differences in daily case growth rates were observed 41–60 days before restrictions were lifted.
Allow me to put the numbers from the first paragraph in a format I find more understandable: Mask mandates were associated with 0.5, 1.1, 1.5, 1.7, and 1.8 percent decreases in daily COVID-19 case growth rates at 1–20 days, 21–40, 41–60, 61–80, and 81–100 days after implementation. Ordinarily, in this sort of study, when they present numbers like the first paragraph, those are relative numbers. We don't know the daily growth rate from this study, but for illustration, let's assume it was growing at 10% per day. That decrease of 0.5% at 1-20 days is not 10% - 0.5% or 9.5%. It's 10-(0.5% of 10) or 9.95% growth rate. You would need a lot of samples to detect that small a difference and they did: they had 2,313 datasets for the first data set and 3,076 for the second.
In plain English, if your chances of getting the infection was 10% without a mask, wearing the mask improved it to 9.95%. Take the worst daily case growth rate increase of 1.8%. Converted from relative risk to absolute risk, that's 9.82%. Just for fun, let's bump the odds of getting infected from 10% to 20%. Those two numbers turn to 19.9% and 19.64%.
A very reasonable question is whether a relative risk of 0.5 or 1.8% really matters IRL. Again, that's 1.018 chance vs a 1.000 chance. The relative risk of things we absolutely know are bad, like smoking, aren't 2% worse, they're 2 times worse or more. Relative risks like this could be from anything and not the thing they think they're measuring. For example, I find no mention of the type of mask; whether an actual N95 mask, a surgical mask, or a cloth gaiter. Are all masks the same? I doubt it. Read my article with the table of spurious correlations that make up much of the junk science we're subjected to daily.
Their figure from the study.
The exact number on what your chances are of getting infected are hard to find. There are tons of factors that must be accounted for: how much virus you're exposed to, the health of the tissues in your respiratory tract, and a broad swath of other metabolic health questions. A reasonable question might be, "do masks matter?" Based on this I would have to say yes they do. They just don't matter much at all.