That’s the real factor isn’t it - that it is contagious before people even realize they could be contagious, then closing the borders is a MUST, not an IF!
FWIW I contacted and came down with chicken pox and never saw anyone with active chicken pox in the students I was in association with. So yes, I believe the doctor entirely. The virus has to replicate all over your body before your body succumbs to the disease symptoms.