they don't prevent it, they make it less likely. which is still good obviously, but long covid for vaccinated people is a massive issue. vaccinated individuals are more likely to believe the vaccine makes them immune and therefore don't need to mask.

in case you're not trolling:

https://www.sciencedirect.com/science/article/pii/S1074761322005581

https://biologyinsights.com/what-is-a-sterilizing-covid-vaccine-and-is-one-possible/