It could still happen. There are going to be more waves of coronavirus that are better adjusted to work around the current crop of vaccines. Nobody has a crystal ball for this stuff. That is why the scientific bodies keep talking about "evidence" and "no evidence" instead of making blanket claims.

Plus if I recall correctly there is some evidence that people who get COVID after being vaccinated (ie, breakthrough cases) get worse cases. We haven't had time for the Phase IV trials yet, so it is entirely possible that there are some interactions that haven't come to light yet.

> Should people make "their own" decisions on how much diesel exhaust their cars produce, or whether they can carry around airborne nerve gas?

Yes to the diesel. And no to the nerve gas.

But I'll point out that coronavirus is quite different to nerve gas - you're going to be exposed to it anyway regardless of vaccination rate. And statistically speaking, the person who exposes you is likely to be vaccinated.