Universal mask-wearing has been determined to be highly effective when combined with other measures such as lockdowns, social distancing, hand washing, and testing. There isn't much (or maybe any) data on how effective it is when only half the people wear masks - many assume that it's half as effective, but I haven't seen anything to support that. At this point, it is temporary and it can't hurt, so masks are recommended. With the way the virus is spreading and given how devastating it can be, doing anything will help. But don't think that wearing a mask without keeping 6 feet away from an infected person will necessarily protect you.