Nobody knows for sure but I'm addicted to news and remember this well. A bigshot at CDC was working out the guidelines at home and his 14 yo daughter doing homework they started talking about science. He asked her what might be a safe distance in school...3 feet.....5 feet.....10 feet? And she said 6 feet not too close and not too far. He liked the answer and went with it.

I remember telling this story back then and of course nobody believed me....but then too nobody believed me about lockdowns either.