Overly simplified:
- If you've had COVID or get Vaccinated, your body develops antibodies.
- The amount/strength of those antibodies varies based on LOTS of factors.
- An mRNA vaccination seems to generate a MUCH stronger immune response, so theoretically will last longer.
- That response deteriorates at an unknown rate, that seems to depend on factors we don't 100% understand yet.
- As your antibodies deteriorate, your resistance to infection declines.
- If you still have sufficient antibodies remaining, you are resistant to infection. If not, you are not.
ANALOGY: View antibodies like fuel in a gas tank. As long as you have some gas, you're still moving (somewhat immune). Generally speaking, those who are vaccinated start with a more full tank than those who have post-infection immunity.
If you're low on gas (previously infected a while back, or had a low response), you can still drive... it's just a bit more risky. You don't NEED more gas, but if you top off (get vaccinated), you reduce risk and buy more time.
If you've got a lot of gas already, topping off has limited benefit.
Now imagine you don't really have a gas gauge. If you can't accurately tell when your tank will run dry, it's generally a good idea to top off when you get low... especially if you're getting a ####load of fuel (stronger mRNA vax immune response).
Now, antibody tests can give a general idea of how full your tank is, and immunity isn't a on/off full/empty scenario... so the analogy isn't perfect... but you get the idea.
Your decision to get vaccinated after having a natural infection involves a LOT of variables... your initial immune response, time since recovery, your personal risk tolerance, environmental risk (how prevalent is COVID, your risk of exposure).
Asking for concrete proof of something that we're still learning is basically just saying "I don't want to", which is fine. Just be honest about it instead of trying to hide behind convoluted "science".