From the Forbes Article:
"Peer-reviewed Evidence
The historical peer-reviewed evidence on universal mask-wearing to reduce community spread remains inconclusive. It is difficult—probably impossible—to conduct an ethical randomized controlled trial that could ever conclusively tell us whether an entire region wearing masks will substantially reduce transmission, especially since such an intervention should never be implemented without other evidence-based measures that we know work, such as testing, contact tracing, mask-wearing for the symptomatic, and social distancing.
Here’s a short summary of what the peer-reviewed evidence does show, all of which must be considered through the lens of other issues discussed further down and the fact that COVID-19 is a new disease caused by a new pathogen:
Homemade masks were not as effective as surgical masks in preventing wearers from expelling droplets, but they did reduce droplets and were better than no protection, according to a 2013 experiment.
The combination of wearing a mask and hand-washing—but not either one by itself—reduced household transmission of influenza by 35-51% in a 2010 study.
Flu-like illnesses occurred 13 times more often in healthcare workers wearing cloth masks compared to surgical masks, and 97% of particles penetrated cloth masks, compared to 44% penetrating surgical masks, according to a 2015 study.
Wearing a mask decreased infection risk by 60-80% when a parent was caring directly for a sick child in the same household, but mask adherence was well below 50%, leading the authors of a 2009 study to conclude that masks were “ineffective in controlling seasonal influenza-like illness” but might work better with better adherence. Further, the authors urged “caution in extrapolating our results to school, workplace, or community contexts, or where multiple, repeated exposures may occur, such as in healthcare settings.”
A 2015 systematic review of 9 randomized controlled trials consistently found that real-life use of medical masks overall did not reduce infections compared to no masks except in the 2009 study above when adherence was high. Five studies found small reductions in risk with a combination of mask-wearing and hand-hygiene, while the others found no benefits.
Five separate studies in the 2015 review above found that N95 respirators significantly reduced infections compared to surgical masks.
Masks blocked live influenza particles in a 2013 simulation experiment with a dummy, but studies showing that masks block droplets or even infectious particles cannot be assumed automatically to prevent infections.
Wearing masks appeared to reduce SARS transmission risk, with approximately one infection prevented for every 6 people wearing a mask, according to a 2008 systematic review.
Face masks were not helpful in reducing transmission of pandemic influenza, according to a 2017 systematic review and meta-analysis.