Is that many don't wear them when they are most vulnerable to getting the virus. Example, you wear yours everywhere, but take it off in your house, obviously. If you are like me, and have a14 year old who we let hang out with his buddies....if he had it, suddenly it's spread inside our house.

My belief for them to be effective, you'd have to wear it, literally, all the time. Much like they do in hospitals. The virus doesn't care that you've worn in 90% of the time, but then taken it off when you are around someone that might have it. There is no credit for good behavior, unfortunately.