Earlier quoted context omitted.
That doesn't make any sense. If the immune response didn't wane on a personal level we wouldn't be needing any boosters.
You can't get 15% of a case of severe COVID. At a personal level, the vaccine either worked at preventing it, or it didn't. We can compute a population-level probability of those two scenarios for you, but you can't really say "15% effective, after 2 jabs, for me personally".
Getting Covid or not is binary, yes, but the severity of the disease is on a spectrum. I can surely get a 15% less severe disease thanks to a 4-5 month old vaccination. Just like the severity of the disease depends on the viral load you're exposed to.
Of course there's no way of knowing what the exact percentage is for me personally, so in that sense you're right - it's just an average. But it's surely indicative, which is why you should be getting boosters after a specific amount of time, if you're in a risk group.