Face masks couldn’t be doing that because they don’t work to slow transmission whatsoever. The whole theory of face masks - even before accounting for real world usage - rests on the assumption that the primary transmission mode is respiratory droplets and not aerosols which is not the case. They’re just medical security theater, except worse because they have obvious negatives. I advocate instead for performing rain dances; they’re no less effective but provide a bit of exercise to boot.

Anyway, on a related vein, non-infection-protecting vaccines can do exactly what you’re saying and select for more virulent/pathogenic strains. Take a look: https://journals.plos.org/plosbiology/article?id=10.1371/jou.... Because all the COVID/19 vaccines do not prevent infection nor transmission (they reduce it but don’t fully block it), our incredibly shortsighted mass vaccination campaign is likely to select for more pathogenic strains. Go science!

> Could some vaccines drive the evolution of more virulent pathogens? Conventional wisdom is that natural selection will remove highly lethal pathogens if host death greatly reduces transmission. Vaccines that keep hosts alive but still allow transmission could thus allow very virulent strains to circulate in a population. Here we show experimentally that immunization of chickens against Marek's disease virus enhances the fitness of more virulent strains, making it possible for hyperpathogenic strains to transmit. Immunity elicited by direct vaccination or by maternal vaccination prolongs host survival but does not prevent infection, viral replication or transmission, thus extending the infectious periods of strains otherwise too lethal to persist. Our data show that anti-disease vaccines that do not prevent transmission can create conditions that promote the emergence of pathogen strains that cause more severe disease in unvaccinated hosts.