The primary endpoint in clinical trials for COVID-19 vaccines is "vaccine efficacy" which is essentially what percentage of COVID cases didn't happen because people were vaccinated. This is obtained, basically, by comparing similar groups of people, one vaccinated and one not, and seeing how many cases develop in each. The Pfizer and Moderna numbers are both around 95% effective.
However, from a "personal risk" perspective, that's not the whole story because there are two extremes that could cause those numbers:
Every vaccinated person is 95% resistant to COVID. It would take 20x the exposure to the virus to get sick (on average) than before you were vaccinated.
95% of individuals are 100% immune to COVID, 5% are just like they were before.
And of course it could lie somewhere on the spectrum in between those two. Basically what I want to know is "If a sick person comes and coughs repeatedly directly in my mouth (as a fully vaccinated person), what's the probability I get COVID?" A 95% vaccine efficacy rate is compatible with both 5% and 100% as answers to that question. I don't think the media has done a good job recognizing that distinction. Based on the clinical studies, I'm not sure we even know the answer to that. Has anyone seen any additional research into that question?
This thread seems relevant to the question: https://mobile.twitter.com/biosbenk/status/1425164080805986312
To me it looks like the differences between the breakthrough cases and the non-breakthroughs in terms of immune response is pretty small, which I would interpret as evidence for a stronger #1 component (everyone has equal risk) than a #2 component (some individuals have all the risk)