Just because antibodies disappear, that doesn't mean you have no protection anymore. If memory serves me correctly, the body produces antibodies after getting the vaccine, however, after time has passed the body stops making the antibodies because the immune system realizes that there is no need to be making them. T-Cells should remember what COVID "looks like" so if the immune system ever detects COVID, the T-Cells should resume making antibodies.
I could be wrong, but I'm pretty sure this behaviour is the same for all vaccines (it's not like we have measles antibodies, but our T-Cells should be able to make the antibodies as needed)
The Moderna vaccine has exhibited the same behavior as well:
https://globalnews.ca/news/8233115/canada-covid-19-boosters-christmas-moderna