The vaccine decreases the severity of covid, rather than preventing it. Using the vaccine kept more people out of the hospital and morgues. Also, people that are immunocompromised or older generally have weaker immune systems (one reason why covid hit older folks the hardest).
I'm 69 and got the covid vaccine 6 months before I caught covid after sitting next to an asymptomatic infected person in a van for an hour (high viral load, plus vaccine effectiveness in oldsters decreases through time). I would have been much sicker if I hadn't been vaxxed. Statistically, the hardest hit by covid were older, immunocompromised, and unvaccinated.