Naive question: If the null hypothesis is that there is no difference, wouldn't that imply 95/30,000, p = 0.0031, putting the probability of observing less than 5 cases at a much more reasonable 1*10^(-14).
I think Fisher's exact test [1] is most commonly used in these types of trials. But the P-value roland (parent comment) provided also make sense to me. For the Fisher's test R says...
Covid NoCovid
_____ _______
Vaccine 5 1495
NoVaccine 90 1410
9.0e-22 one tailed
4.5e-22 two tailed
No, I think you must estimate p only using the cohort where people were not treated, otherwise you will underestimate the population fraction. In order to test if the null hypothesis is true, we can't assume that's its true when constructing the test.