COVID AND THE VACCINE - TRUTH, LIES, AND MISCONCEPTIONS REVEALED, page-5460

Things are more complex than simply antibody levels = protection.

There's a very robust T cell response to the second dose of the vaccine. T cells are being theorised to play a major role in the immune response against covid.

We still aren't sure exactly what role each plays a part in, but I wouldn't be too hyper focused on just the antibodies.

Look more at the data of breakthrough cases. That will be a good indication of vaccine efficacy.