[m12k]

There's several reason why even with highly effective vaccines, we might see worse infections when we do see them among the vaccinated. For example, if the vaccines work well, then there should be a bias where we see the most breakthrough infections among those for whom the vaccine doesn't do anything, i.e. people with compromised immune systems. So the people with normal immune systems will make up a much larger percentage of infections among the unvaccinated group, allowing them to drag down the average severity of infections - in the vaccinated group, they've been "removed" since most of them won't get infected, or have mild enough infections that they don't even notice, leaving the immunocompromised as a larger part of the infections that get counted in the statistics, and these infections are thus on average more severe.