The flu shot every year also doesn't prevent one from getting the flu. Supposedly, if you do, you will get a less severe case, because you body is ready to start fighting sooner.
As for natural immunity, meaning immunity after you had and recovered from covid, supposedly that immunity only lasts 6 months or so. (I have seen claims of 8 months). I emphasized supposedly, because I feel like there may be some protection longer than that. Protection isn't just measured by antibody levels. There are cells and even nymph nodes that retain the information and can aid in ramping up protection more quickly. Despite that I feel that way, based on what I read, I went with the 6 month rule. In my position, I am not willing to take on too much risk with this F-ing virus.
Now how long protection from a vaccine lasts, I think is anybody's guess still. I know the UK is already planning a booster shot for those over 65 beginning in September, so they seem to be thinking the 6 month rule as well.