Study measuring viral particle shedding with an without surgical masks, with symptomatic people who are actually infected with influenza, rhinovirus (colds), and coronaviruses (also common colds).
Results: the masks didn't do very much for either influeza or rhinoviruses, and not enough people were infected with coronavirus to know if the results are valid.
@pete as far as I did read, results are more nuanced that what you are stating.
e.g. "Our findings indicate that surgical masks can efficaciously reduce the emission of influenza virus particles into the environment in respiratory droplets, but not in aerosols."
Mask are no panacea of course but they surely are helpful in some situations.
@roshii That's their interpretation. If you look at their actual data, they're claiming that on what's essentially a sample size of two (for Influenza B).