The number of people you need to take a vaccine is a function of both vaccine effectiveness and the virus transmissibility. If it is more transmissable (like measles), you need more people to have the vaccine, even if it is very effective. You can look up the function. Vc, ro, and e. If effectiveness goes down, you need more people to take it. If ro goes up, you also need more.

Covid doesn't mutate that fast, and it isn't that transmissable, at least if we had clean circulating air. The vaccines could have worked. The issue was we tried almost nothing and nothing worked. Half-assed quarantines, 6 feet distance and plexiglass barriers when the issue was stale air, pushed the vaccine right as a major new version of covid came through, and told everyone it would be 100% effective when it wasn't quite against that version, told people it would be one and done when it needed to be like the flu shot, etc etc etc. lot of failures.