How long the vaccine will last, is unknown at the moment.
You think they would know, but they don’t know. So for example, if you look at people who’ve had, COVID 19 infection, their antibodies do disappear over a period of weeks or months. But that doesn’t tell you much about long lasting immunity, because there are other cells inside the blood called T-cells, which hold the memory of the infection. And the question is how long they’ll hold the memory. Most people in the vaccine area think that you won’t have to have annual injections, but that’s yet to be proven.
The scary thing is to how they’ll find that out is, there’s no magic about this, if the disease starts to come back in vaccinated populations, then they’ll know that you need boosters. But at the most they don’t.