Probably something like “Masks aren’t proven to help yet, but unless covid is unlike any other respiratory disease they probably won’t hurt.” Yes, this could’ve caused a rush on them (though maybe that would’ve alleviated some of the need in hospitals?) and we would’ve had to deal with that, but lying to people in the name “for their own good” is not empathizing with them.
Getting too deep into technical reasons in the middle of such an event is not understanding the definition of teachable moment. Most people aren’t open to learning deep knowledge when their fear or other emotions are up, but it can take real skill to avoid diving into details as an expert. And I have some sympathy for people getting metaphorical microphones shoved in their face, but that just goes back to my point about public health (and medical people in general) needing to know that their job is as much soft skill as hard.