Doesn't have to be all eligible people.

There's also natural immunity for the ones who'd rather have that than the scary scientist-designed shot. The most irresponsible ones have the best chance of getting nature's vaccine. Depending on how long immunity lasts, that might drive it very low.

And as far as I know, the vaccines are highly effective and very few people are actually medically unable to get this vaccine. Honestly, if only people who chose not to be protected from it die... too bad for them.