Nobody authoritative ever said they prevented transmission. Ever. They reduce transmission. Even if you choose to ignore that, consider: Vaccinated people have

Nobody authoritative ever said they prevented transmission. Ever. They reduce transmission. Even if you choose to ignore that, consider: Vaccinated people have shorter recovery times and are less likely to require hospitalization. That results in a shorter time window for spreading the virus. Furthermore, if you can recover at home you are less likely to spread it to health care workers or other patients.