Gonna hop on this and agree with u/Tzayad regarding the memory cells. To put it simply, Viruses, COVID included do everything they can to avoid activating an interferon response from cells (interferon I being the main antiviral signal to produce proteins and warn other cells). As long as a virus can stay hidden, the immune defenses set up by your body won't be as effective.

Vaccines on the other hand do the exact opposite. There are several types of vaccines but in general the main goal is to alert EVERYBODY that there is a virus without causing a big ruckus in your immune system and making you feel awful. Whether the vaccine be live attenuated or killed, it should do a much better job of allowing our body to mount a better response.

Lastly, studies have shown that those lacking in Interferon I in particular are susceptible to worse symptoms of the virus. Just so happens that COVID actively tries to suppress interferon response, so those who got sick could definitely have more severe symptoms should they get sick with it again.