> 78% effective how could they possibly have any kind of respectable sample size when it is such a rare disease.

The UK has had more than 3500 confirmed cases, and almost exclusively among MSM. That is more than enough.

> Are they having vaccinated people expose themselves to the virus to see because if they don't get exposed to it then there's no way to know if it is effective or not.

Like with any vaccine study, they are comparing infection rates between vaccinated and unvaccinated people.