Usually they used randomized controlled trials for this sort of thing, which in a well designed study will account for this sort of bias

Whether the population in the trial generalized to the population at large is a bit different question and I think is where the bias you’re highlighting could come into part

It cannot be randomized for a vaccine trial since the participants are all people who want to be vaccinated.

'... people who care enough to get a booster are more likely to care proactively about their health'

Are there studies to show people who care about health get boosters? My neighbors are hyper healthy and undosed. I am undosed and I exercise regularly.

I generally see many people who sat on couch for two years put on considerable weight. Getting dosed while eating ourselves into 2x pants sizes doesn't seem to be good for health, as it increases the co-morbidity counter.