<why the experts didn’t recommend masks 5 years ago.>
From what I can tell, masks work pretty well at preventing me from breathing directly into the air that you breathe in. Masks work considerably less well as a filter, unless you go for the N95 type.
Because of this, the person protected by the mask is not the person wearing the mask. It’s the person in front of the person wearing the mask.
This makes masks hard to study. It’s easy enough to put masks on half your nurses, and then measure the rate of respiratory disease amongst them. This measures the ability of the mask to protect the wearer. Years ago, they ran several of these studies and found out that masks are not great at protecting the wearer. We ran our studies and found out that masks are only slightly effective against flu. Case closed. Time to put up the hand washing posters, because the hand washing studies were much more impressive.
This kind of stalled research on masks. We thought we knew the answer, and we were wrong. (Or half wrong. We were right about hand washing.)
I’m not really a save one life kind of person. I’m more of a save 100,000 lives kind of person. My opinion on masks this fall depends on whether Delta is likely to cause 10,000 or 100,000 deaths. If it is 10,000, do what you like. If it is 100,000, then bring out the masks this fall. If it is a million, then close down high risk indoor businesses.