“The effectiveness of Pfizer's Covid-19 vaccine steadily declines over time, dropping to about 84% for vaccinated people about four to six months after getting their second dose, according to CEO Albert Bourla.”
Overall, I think’s a mix of theoretical concern and statistics that may have many factors involved (like the delta variant). Even as antibody titers fall, protection may still remain. The immune system has memory cells, ready to activate if re-exposed, often years later. But it may not be as good, and for the high risk people it’s probably best to not chance it. That said, 84% is still very good. And periodic booster for vaccines isn’t uncommon. Pneumonia and tetanus are two that leap to mind. 6 months is fast, but again I think we are erring on the side of safety here.