So if you understand that 95% efficacy means that 95% of a cohort will be protected, surely you understand that this means 5%, or 1 in 20 will not be protected?
If it wasn't so, and that 5% had some level of protection, then the efficacy would be higher.
It doesn't mean that. A 95% efficacy means that the chance of a vaccinated person getting Covid is 95% less than someone who is not vaccinated. This does not mean that the other 5% of the population is 100% guaranteed to get Covid or a severe case of covid. So to simply say that 5% of 500k people or 25,000 vaccinated people are at risk of Covid which Philip Nolan claimed and seemed to suggest is in his model is simply incorrect. There are other public health measures and other reasons that will reduce the risk further so only a % of the 5% will actually catch Covid and even then might not be severe.
Going by what Nolan said, they have modelled 25,000 (5% of population) vaccinated elderly people catching covid because of the 95% efficacy and calculated deaths and hospitalisations on this number.