Thread by @PShaniuk

Well, it looks like 😷are in the news again. With Dr. Atlas posting doubt that masks are effective (and @Twitter subsequently squashing the tweet), it’s probably time for a quick refresher (just in case anyone is still wondering). Let's answer the misinformation with data 1/11

First off. Here’s a quick reminder that, yes, COVID can spread before symptoms develop. This contact tracing study from Taiwan found a small, but statistically significant risk of acquiring COVID-19 even when exposed to pre-symptomatic patients. jamanetwork.com/journals/jamai… 2/11

So, do masks work? A lot of different types of studies evaluate this question. One of the more interesting ways to look at this is to look at local infection rates before & after mask mandates. It’s sort of an observational study. 3/

Here in the great state of #Ohio, we saw a large (34.9%) reduction in COVID-19 incidence in the month after
@GovMikeDeWine instituted mask mandates msn.com/en-us/news/us/… 4/

Next, a retrospective cohort study from Beijing. 335 people in 124 families with at least 1 lab conf. case. Mask use by the case and family contacts before symptom onset was 79% effective in reducing transmission (OR=0.21, 95% CI 0.06 to 0.79). gh.bmj.com/content/5/5/e0…

Next, the holy grail; the meta-analysis. This one in
@TheLancet
looked at over 170 studies evaluating MERS, SARS & COVID-19. Masks (both in health care & non-HC settings) led to reduction in risk of infection (n=2647; aOR 0·15, 95% CI 0·07 to 0·34, RD −14·3%, −15·9 to −10·7)

A picture is worth a thousand words. From
@TheLancet study

thelancet.com/journals/lance…

Honestly, though, this may be my favorite.
@DukeHealth experience. 70% of HCWs acquired COVID from co-workers not patients. They flattened this curve with a universal mask mandate. The elegance of this small study was the use of community-acquired incidence in HCWs as a control

Masks basically drove the rate of new cases acquired at work to near zero.

cambridge.org/core/services/…

Finally, these articles evaluate if a mask protects the wearer, but remember, my mask protects you and yours protects me. This article argues that absence of evidence in this case means we SHOULD wear a mask until we have evidence of harm. the “precautionary principle.” 10/

Basically, even limited protection could prevent some COVID-19 transmission and save lives and because this is such a serious threat, wearing masks in public should be advised. bmj.com/content/369/bm… 11/15

Also, as a post-script, I left out the purely anecdotal case reports, but these two are pretty powerful examples. The first being the absence of COVID transmission in nearly 140 patrons of 2 symptomatic hair stylists with confirmed COVID-19 12/15
cdc.gov/mmwr/volumes/6…

The second powerful anecdote is the man on a 15 hr flight who developed symptoms of COVID-19 en route and later tested positive. He was wearing a mask, but packed into coach. ZERO of the contacts tested positive. 13/15 cmaj.ca/content/192/15…

Compare to a recent CDC study from September of a case in March on an airplane with a 62% attack rate. Mask usage is unknown for this study, but this occurred before mask usage was recommended in vogue, so presumably can work as a control 14/15 wwwnc.cdc.gov/eid/article/26…

So, in summary.
1) Presymptomatic transmission happens
2) Yes, 😷= ⬇️risk, esp if prolonged close contact
3) Precautionary principle states we should #maskup until we know we shouldn't.
4) Yes, these studies are >3 months old. Why are we still talking about this??? 15/15

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