If you are vaccinated against COVID-19, you can still get infected with the virus, but you have far less chance of developing serious or even fatal disease.
When driving a car, the risk of serious injury increases with speed. For COVID-19, the risk of severe illness increases with age and is higher for people with underlying medical conditions.
If you don't get vaccinated, you won’t necessarily get infected, develop serious COVID-19 disease or die. Likewise, if you drive without a seatbelt, you won’t necessarily get in an accident or get injured.
But what if you do? A seatbelt cannot protect you 100%, but it greatly reduces the chances of serious harm from a crash.
This is also true of COVID-19 vaccines: they cannot protect you 100%, but they greatly reduce the chance that you will develop severe COVID-19 disease. The objective of both, the seatbelt and vaccination, is to reduce risk and keep you safe.