I’m Not a Poisek, but when Kanarek asked the Shaila, he left out vital info. I don’t know for sure he left out this info, but this is the way it seems. Acc. to R’ Dunner, the reason of the P’sak was because it’s a remote a possibility. Did Kanarek mention these cases & still R’ Chaim Paskened the way he did? Here are the cases:
“Update On Measles Outbreak In Frum Community In NYC – See more at: http://www.theyeshivaworld.com/news/general/171304/update-on-measles-outbreak-in-frum-community-in-nyc.html”
“Study: Mumps Outbreak In Orthodox Community Traced To Face-to-face Schooling”
“Israel: Health Ministry Bracing to Contain Measles Breakout”
I don’t think he did. I think the P’sak would be – letting these kids into schools, would be a Sofek of Ritzchecha!