Monday, April 27, 2020

Very Brief Blog: Cloth and Better Masks and Viral Transmission

There was a lot of debate about whether the public should wear masks or not during the COVID pandemic.   There's some somewhat ominous data about how far COVID nanodroplets (from talking, for example) can travel.  On the other hand, COVID isn't infinitely transmissable.  In a Chinese restaurant, about half the people in the room for an hour with a COVID positive person caught it, about half didn't (here).

Today, WSJ had an interesting article about a group of scientists who allied with some major politicos (or Goldman Sachs types) and have access to the White House staff (here).    WSJ also posted a 17-page report from the group (online here.)   Their white paper is a well-written document about key features and watch-outs in vaccine trials, drug development, etc.   It's not a lightning bolt from heaven; it is organized and assembled common sense on a wide range of COVID topics.

But their references 13 and 14 provide some original research reports on viral transmissions and different types of masks/.   These are Jefferson et al. 2008, and MacIntyre et al. 2015. 

As the white paper authors summarize,

  • Surgical style masks have an odds ratio of 0.32 of transmission.
  • N95 masks are better, but have an odds ratio close to 0.1 (10% of un-masked transmission).
    • N95 odds ratio as low as 0.03 (3%) when used perfectly.
  • Controlled studies of surgical masks and N95 masks in "real world settings" have found them similarly effective.
  • Cloth masks were "63% as effective in preventing symptoms" but "8% as effective in preventing influenza-like illness."
  • Coronavirus has a limited lifetime on paper and masks (not otherwise sterilizing) might be reused after 5-7 days of storage.

I've seem bits of these data in news articles but hadn't found the original citations.  Note that all these studies quoted are about protecting the mask user.  We've all seen reports that cloth masks are good at preventing transmission outward (e.g. coughing particles), so the mask protects people around the wearer.  So if everyone was wearing a cloth mask, everyone would be protected from everyone else.

As far as protecting the mask wearer himself from COVID, you can still wonder.  In the MacIntyre cloth mask study, it wasn't clear if the "cloth masks" used were very thick or had a very close fit.  You see people wearing surgical blue paper masks (surgical masks being more highly recommended) with a lot of open space on the sides, which does not seem that protective against nanoparticle viruses in free air.  Luthra makes this point at Kaiser Health News here.

Jefferson 2008 - Open access
Physical interventions to interrupt or reduce the spread of respiratory viruses: systematic review.

MacIntyre 2015 - Open access
A cluster randomised trial of cloth masks compared with medical masks in healthcare workers