Updated: Jun 5
A recent article was posted on line (not peer reviewed yet) from late last month performing an epidemiological model based analysis studying the potential benefit of mask wearing on viral spread. They do theoretical calculations (simulated computational models) and find it fits well to the observed data for various countries that practiced varying degrees of mask wearing, finding even that homemade masks can make a difference.
This study hits on an important aspect of targeted mitigation strategies necessary to lift lockdowns and still slow the spread of the virus.
Together with the recent Nature medicine paper demonstrating how masks can restrict respiratory spread of virus (surgical masks in that study), this provides some empirical support for the wide use of masks in public.
Some people have doubts whether masks, especially homemade ones, can really restrict the spread of the virus. Some people misunderstand the purpose of wearing a mask -- it is not so much to protect the wearer, but to protect others FROM the wearer should they be an asymptomatic infected. Given a vast majority of cases seem to be from asymptotic spread, broad societal level mask-wearing, at least when in settings where closeness to others is unavoidable, could make a real difference. Second, medical and biological professionals duly note that viral particles are small enough to pass through homemade masks, or even medical masks that don't have an N95 or higher rating. While this is strictly true, it also seems to be the case that infection may result from exposure of a certain volume of virus, and it seems that even if some virus gets through the mask, most would be blocked by the mask, so even a simple, homemade mask could still make a huge difference on respiratory and aerosol spread from asymptomatic infected.
Update: An earlier paper published in the Annals of Internal Medicine, a top journal, that suggested (from a study with 4 subjects!!!) that surgical and cloth masks are not effective in stopping SARS-CoV-2 spread, has been retracted. This paper, along with early rhetoric intended to prevent people from hoarding N95 masks that were in shortage and desperately needed by healthcare workers as personal protective equipment (PPE), has caused some of the confusion that still prevents people from going along with the mask wearing requirements that are a key part of a targeted mitigation strategy that could control the viral spread without lockdowns.