How effective are face masks? [closed]

Question

Actually, all the countries worldwide are discussing policies about face masks. Masks were, up to now, mainly used by surgeons, and now, all medical workers are working with masks. In Switzerland, my country, face masks are now mandatory in public transportation. Some countries are right now discussing making them mandatory in public spaces.

At the same time, I'm discovering the whole "Unmasking the surgeons" discussion, which is a movement in medicine which try to evaluate the effectiveness of wearing face masks in the theatre. From what I understand, the evidence about reducing nosocomial infections is old and not that conclusive.

At the same time, a lot of people are spreading the idea that masks can actually be detrimental (bactery can multiply on the fabric, you have less oxygen, you are less careful with a mask, etc.), which seems kinda clumsy to me.

Coming from these, I hear a lot of people arguing about face masks as a response to COVID-19, and I don't know what to think about it and how to argument it, so here are the questions :

• Is this "Unmasking the surgeons" a serious thing really discuted ?
• How is the scientific consensus about the fact wearing masks can reduce the spread of viral respiratory infections in some case ? In which case ? Are those evidences good quality ?
• I believe that the people at WHO are issuing this policies, based on the fact that it is very probable that the outcome will be better. How are they arguing this ?

Answer 1

There was a good article on face masks on SSC way back in March, which reviewed pre-existing evidence on face masks.

I think a large reason the issue became contentious is the way that officials treated face masks as unnecessary in the beginning and then changed their minds later. I'm not sure why this happened, but one important issue may be, as described here, that "there is little evidence for X" means two very different things and a lot of professionals fail to distinguish between those two things:

1. "There is no evidence that parachutes prevent deaths from falling" - No published studies have addressed the question of whether people are more likely to die if they fall from 10,000 feet without a parachute. Certainly a gold-standard "randomized controlled trial" on parachutes seems to be missing.
2. "There is no evidence that vaccines cause autism" - Published studies that were specifically designed to study the question showed that vaccines don't cause autism.

Before the pandemic there were few studies on the effectiveness of low-grade (cloth/paper) face masks, leading to a type-1 lack of evidence. Plus, early on, the concept of presymptomatic and asymptomatic carriers also lacked positive evidence. This could lead officials that are overly beholden to studies to deny a need for face masks.

My understanding is that it is still thought that wearing a face mask will not have much effect on your chance (as the mask-wearer) of catching COVID by breathing it. However, there are two common-sense arguments that face masks work:

1. Even if a face mask blocks 0% of microscopic/aerosol virus particles, it can still block the much larger droplets coming out of your mouth, protecting others from you if you are a presymptomatic carrier.
2. The mask discourages you from touching your nose and mouth, reducing the chance you will be infected after touching a surface that has the virus on it.

Googling around, the Guardian suggested WHO was about to change their face mask stance based on "new evidence". The common sense existed before, but WHO might well be the sort of organization that needs clear peer-reviewed evidence before it recommends something, while simultaneously being unwilling to say the correct response in a type-1 situation which is "we don't know if you should wear masks or not", thus they said "don't wear masks" until a study showed they were wrong.