Dear Interjection,
Someone who has symptoms would cancel his vacation. If he does not, there is not much you can do. He will party in Israel if he wants. The fact is Yidden get very touchy about cancelling their Rosh Hashana.
On a more serious point. Some Bresolvers avoid preventive medical safeguards. It is part of their life philosophy. We know that Tzfas Yidden went to Umann with the measels!