It is pretty clear to me. Early immunity sets in as antibodies, for which few can be infected while those are in one's body. Then those fade away, leaving people vulnerable, at its worst, 52% have enough residual immunity to stop a COVID-19 infection. Then, with the second dose, it drives the T-cells to finish creating the long term immunity that leaves nearly everyone unable to get COVID-19.
We need to assume that to get those percentages, efforts were made to infect people or to check the adequacy of their immunity levels throughout the time reviewed.
Maybe your problem is that the human immunological response is not clicking in your mind.
So... If one was to graph the immunity for a large group of people, it would peak after the 1st shot, gradually decline to about half effectiveness*, then the 2nd shot creates another rise which is a plateau (basically) with a very slow decline following the 2nd rise.
Therefor there is a period of relative vulnerability between the two peaks, and if one wants to avoid being infected, some precautions would be in order in that brief period.
*Maybe for most people their “low point” vaccinated (”partially vaccinated”?) immunity is strong enough to ward off a moderate viral load, but not enough to defeat a heavy one?
If this peak—dip—plateau function is the case, every vaccine recipient needs to get a little graph showing it — visual aids are often much more effective than “complicated” verbal descriptions. I’d also suggest lots of 30 second PSA’s on TV and the Web, so the rest of the population better grasps this as well.