The vaccine doesn't last forever, and so it would need advised to get boosters. COVID is a fairly fast mutating virus. For example, the virus that causes HIV is very fast with its mutations, hence why there isn't a vaccine against AIDS. COVID doesn't mutate as fast, but fast enough to where a vaccine will be almost useless after a multitude of months.
There are many other variables that dictate how poorly a person will become with COVID. Things such as how much of the virus they take into their body and are exposed to, vitamin D levels, sleep, diet, genetics, age, smoker or not, liver health and so on.