Well, Dan asked for a summary of the article that
@aygart posted, but it's disappeared. And while I wrote a summary, this (better) summary with pictures has been published, but I'll paste mine anyway. Is this too long?
https://www.forbes.com/sites/alicegwalton/2020/06/13/face-masks-may-be-the-key-determinant-of-the-covid-19-curve-study-suggests/#44ac7f116497Identifying airborne transmission as the dominant route for the spread of COVID-19, by Renyi Zhang and others, PNAS, published June 11, 2020
Short summary: Why did COVID-19 stop spreading so much faster in China than in Italy and the US? And why did new cases plateau so much earlier in NYC than in the rest of the United States? In both cases, the authors conclude that the onset of mandated face coverings made the difference, preventing tens of thousands of infections.
Longer summary: In China, the early response to the outbreak was SD, masks, extensive testing, and contact tracing, all put into effect simultaneously. In China, the number of new cases increased for about 3 weeks after the lockdown, and had reached a plateau about a week after that.
In NYC and Italy, the early response was hand washing, SD, stay-at-home, but masks were not required until a month had passed. In these places, the number of new cases increased rapidly for about 4 weeks after the early SD requirement, and began to decrease only later, after the face covering requirement was implemented.
In the rest of the US the early response was hand washing, SD, stay-at-home and masks were not required, and the number of new cases were continuing to rise steeply (at the time they looked, May 9).
The timing of these events suggests that mask-wearing was critical in decreasing the spread of coronavirus, quickly in China and after a delay in NYC.
A second piece of evidence they bring is from plotting the total cases in NYC, using a statistical technique called linear regression. Their graph shows that if the original policy (SD, stay at home) had continued, the number of cases would have been much higher than it actually was. The pivot point occurred on April 17, when masks were first required, and the caseload started to decline.
Similarly, their comparison of new infections in NYC compared to the US as a whole (minus the NYC cases) shows that corona cases declined steeply after the April 17 mandated face covering, while new cases remained high during that time throughout the United States.
Why would masks have such a dramatic effect?
Back in March, experts thought that the virus is communicated by droplets that we expel when we cough or sneeze. These droplets are pulled down by gravity, so are usually found within 3 feet of a sick person, so we were advised to stay away from an obviously sick person and to wash our hands and keep them away from our face to avoid transferring a virus-containing droplet to our mouth, nose or eyes. Masks were recommended only for the sick person themselves, to prevent their expelled droplets from contaminating others (WHO, April 6).
But later in April, scientists learned that there’s another way that virus can be expelled, in the form of much smaller particles, called aerosols, especially by people talking loudly or singing.
You can visualize the difference by spraying a window cleaner and an air freshener on a mirror. The window cleaner forms small droplets that soon start rolling down, attracted by gravity. The air freshener forms an aerosol of tiny drops that remain where they fell on the mirror. In fact, we spray air freshener into the air knowing that these miniscule particles – and their pleasant odor – will remain suspended in the air for quite some time.
The realization that viral particles were found in aerosols convinced the experts that the virus could be inhaled by someone who was more than 3 feet from an infected patient, or even in a room where an infected person had previously exhaled, and this led to expansion of the advice to wear masks:
• The knowingly infected should wear masks to avoid spreading their droplets and aerosols, which can contain virus.
• The unknowingly infected (that is, presymptomatic, when viral expulsion seems highest) should similarly wear masks to avoid spreading the virus.
• The healthy uninfected should wear masks to avoid inhaling the aerosols of the two previous groups.
They conclude that “wearing of face masks in public corresponds to the most effective means to prevent interhuman transmission, and this inexpensive practice, in conjunction with extensive testing, quarantine, and contact tracking, poses the most probably fighting opportunity to stop the COVID-19 pandemic, prior to the development of a vaccine.”
My comment:
Finding that the number of cases in NYC began to decrease around the same time as mask wearing was required is not sufficient to conclude that mask wearing was responsible for the decrease. Did people actually obey the mask-wearing regulation? Were there other events that occurred around that time that contributed to the decline in new infections? This paper provides evidence that mask-wearing is important, but it's weak evidence, and the argument could be strengthened by other research.
