Say there are 1000 unvaxxed people and we are counting deaths over a time interval of 1 year. Say 50 of them die during that year.

The person-time at risk = 1000 people times 1 year = 1000 person-years

The number of deaths is 50

So the death rate is 50 per 1000 person-years or 0.5 per 100000 person-years

That one's on me - if I got that wrong we both have to go back to AI school!