While immunity in itself is a wide concept, and there are different ways in which our body actively protects us, immunity with respect to COVID-19 has been seen to wane after a while, and re-expose a person to risks. From what is known to us right now, immunity and antibody response generated via natural exposure and antibodies tend to lower down after a while and do not remain as effective as before. However, the degree to which the efficacy comes down is questionable at the moment.
As per previous findings, immunity gained after fighting COVID-19 (natural immunity) remains at its peak for 3-5 months post infection, and then starts to fall down. Waning antibody counts, and low levels of immunity posit risks. With vaccines, while the immune protection was considered to be more maximising has now been found to lower down with time, be less effective. As per some research available on the matter, vaccine-driven immunity, as its best, protects an individual for 6-9 months time, before it starts to lower down.
Vaccine-driven protection has also been seen to become less effective with the virus mutations in circulation. Not only does the virus become smarter as it mutates, it also gains the ability to surpass vaccine-driven antibodies and spread to organs quicker.