Of course it is clear that if someone has covid and wear a mask, the probability to transmit covid will decrease (I was actually encourage people to wear a mask 6 months ago), but will it stop ? no. people can still transmit it by touching you. So yeah this is decreasing the risk, but is it decreasing the risk high enough that it will prevent the virus to spread ? Nobody has answered the question yet.
In France the mask is mandatory almost everywhere and still the number of cases keep increasing