I disagree with you.
For herd immunity to work you need to know about the immunity or have a vaccine and they don't.
I might be a few weeks behind on this and I might be wrong but I don't think either of the above apply.
They know how herd immunity works.
And you don't need a vaccine to create it either. Before my time, but in the 50s and into the 60s mummies would have Measles parties where one kid got the measles and they'd get all the other kids to play with them.
Measels when you're a kid is not a problem. Measles when you're older is.
And this created a herd immunity too. No vaccine required, but obviously a vaccine is a better answer.
Your granny knew about this stuff back in the 1950s.
And since we don't have a vaccine, what other options are there, other than herd immunity?