Min sample to confirm 0% success/fail from sub group

Hi All,

New to the forum, just hoping for some assistance on the following:

Problem: Data from the last year shows a 5% relapse rate from a new treatment tested on a population of 10000 patients with a certain medical condition. When we study a more specific sub group of 500 patients (the under 25s) we see 0% relapse rate.
Question: what is the minimum size of the sub group with 0% relapse rate to confidently conclude that future patients with the same attributes as the sub group do not relapse?
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