Why the CDC still wants you to wear a mask after being vaccinated
The simple answer is that we haven't had enough time or data to prove whether vaccines stop you from catching the virus at all, or whether it just keeps you from feeling virus effects.
Meaning you could still carry and transmit the Coronavirus despite being fully vaccinated. CDC research confirmed the risk of infection dropped by 90% for first responders who were fully vaccinated, but that's not 100% and although it's a small amount of people, the vaccine might not work for everyone. It's also not clear how long immunity from the vaccine will last, so until we know more, the CDC is asking that we err on the side of caution and continue to mask up.