Thread by @DrEricDing

CDC experts on masks:

Community mask wearing substantially reduces transmission of #SARSCoV2 in 2 ways:

📍Source control of exhalation

📍Protecting the wearer

➡️ But they are *not equal*. CDC just advised double masking.

Important explainer 🧵.
jamanetwork.com/journals/jama/…

2) First, masks prevent infected persons from exposing others by blocking exhalation of virus-containing droplets into the air (termed source control)—important because at least 50% or more of transmissions are from presymptomatic & asymptomatic #COVID19 cases.

3) In recent laboratory experiments, multilayer cloth masks were more effective than single-layer masks, blocking as much as 50% to 70% of exhaled small droplets and particles.2,3 In some cases, cloth masks have performed similar to surgical or procedure masks for source control.

4) Second, masks protect uninfected wearers. Masks form a barrier to large respiratory droplets that could land on exposed mucous membranes of the eye, nose, and mouth. Masks can also partially filter out small droplets and particles from inhaled air.

5) Multiple layers of fabric & higher thread counts improve filtration. However, **effectiveness of cloth masks to protect the wearer is lower than their effectiveness for source control**, and filtration capacity of cloth masks can be highly dependent on design, fit, & materials

6) CDC just released new recommendations to either double mask with at least cloth + surgical, or tighten the fit of surgical masks with special technique.

https://twitter.com/drericding/status/1359536244464766980

7) In the new CDC study, having both the source and receiver masks is best. But if only one is masked, then double masks seems better (comparing purple circles below).

https://twitter.com/drericding/status/1359540044739059716

8) Also notice that if only one sloppily fitted surgical mask is worn by one person, it’s more critical that the SOURCE person exhaling the virus wears it. (Red circles)

9) Also, there is a glaring finding that one sloppy mask worn by the receiver actually doesn’t give enough protection much at all (yellow circle). But if double masked or carefully fitted & sealed, receiver only masking does work!!!

https://twitter.com/drericding/status/1359549413652389890

10) This is why we need to tell people that a sloppy mask is very poor protection. If only one mask, then use a medical grade surgical one—and make sure it is tied this way...

11) “Epidemiology has helped quantify the benefit of mask wearing. At a hair salon in which all staff and clients were required to wear a mask under local ordinance, 2 symptomatic, infected stylists attended to 139 clients and no infections were observed in the 67 clients”

12) During a COVID-19 outbreak on the USS Theodore Roosevelt, persons who wore masks experienced a 70% lower risk of testing positive for SARS-CoV-2 infection. Similar reductions found when contacts were masked5 and in household clusters in which household members were masked.6

14) “increasing number of studies have also provided persuasive evidence that universal mandatory mask wearing policies have been associated with reductions in the number or rate of infections and deaths (Table).

15) “These studies did not distinguish the types of masks used in the community. The association is strengthened because, in many cases, other mitigation strategies had already been deployed before enactment of mask wearing policies, after which reductions were (still!) observed.

16) “A study that examined changes in growth of infections in 15 states & DC before and after mask mandates showed that growth before the mandates were enacted, but slowed significantly after, with greater benefit the longer the mandates had been in place. pubmed.ncbi.nlm.nih.gov/32543923/

17) “children aged 7 to 13 years have been shown to be able to make accurate inferences about the emotions of others with partially covered faces, and FDA recently approved a transparent surgical mask that may be useful in such circumstances. pubmed.ncbi.nlm.nih.gov/33362251/

18) Concerns about reduced oxygen saturation and carbon dioxide retention when wearing a mask have not been supported by available data. pubmed.ncbi.nlm.nih.gov/33003954/

19) “Similar to principle of herd immunity for vaccination, the greater the extent to which mask wearing in this case—is adopted by the community, the larger the benefit to each individual. (Greater) mask use in community may be of greater importance than the type of mask worn”

20) “It merits noting that a recent study has been improperly characterized as showing that cloth or surgical masks offer no benefit. This randomized trial in 🇩🇰 was (**only**) designed to detect >50% reduction in risk for persons wearing surgical masks.

21) “...Findings were inconclusive, most likely because the actual reduction in exposure these masks provided for the wearer was lower.

22) “More importantly, the study was far too small (ie, enrolled about 0.1% of the population) to assess the community benefit achieved when wearer protection is combined with reduced source transmission from mask wearers to others”.

23) Basically, that Danish study was not informative or useful for interpretation, as deemed by the CDC authors. jamanetwork.com/journals/jama/…

24) I want to reemphasize this figure from the recent CDC study (see links in posts above)— everyone masking clearly more effective than 1 person masking. And if only 1 person, double masking better than poor unfitted surgical mask.

25) The other thing to consider is, where available, if possible, switch to premium masks from just cloth. Assuming tight fit, premium offers better filtration than cloth or surgical masks alone. The new #B117 is just too infectious. Take more precautions.

https://twitter.com/drericding/status/1353186978519539712