Various studies have shown that neutralizing antibodies (nAbs) drop off considerably over the 6 months after the second dose.
It is normal and not at all unexpected for nAbs to drop.
Cellular immunity, which is primarily responsible for reducing the severity of disease, remains strong, with very little decline in effectiveness.
A third dose or booster restores nAbs to levels even higher than the second dose.
There is ongoing research about 1) how we can achieve more durable antibody levels after the second dose by optimizing the interval between first and second dose (ie: adding additional weeks between doses), and 2) how much waning we will see after a third dose, and whether we will need additional doses