The half life of the antibodies averages six months but varies. It's not as severe at diseases like smallpox where a single exposure results in antibody product

The half life of the antibodies averages six months but varies. It's not as severe at diseases like smallpox where a single exposure results in antibody production for life. So our body will make energy trade offs by ramping down antibody production, including to zero. Antibodies aren't the only immune response, you can also "train" your T cells. It's the difference between having a team of soldiers train for a particular enemy, vs. training for "enemies at large". It is likely that the first exposure would lead to the most serious infection, because of the delay in the short term immune response due to novelty. This is why people have a really bad week two with covid (delay followed by panic). It could end up being like the flu, where due to mutation and waning immune response getting the vaccine annually makes sense. It's unlikely we would need the vaccine every 6 months for healthy people.