Thank you for commenting. I think this systematic review study on healthy vaccinee bias may interest you: https://bmcinfectdis.biomedcentral.com/articles/10.1186/s12879-015-1154-y.

Specifically, the systematic review authors said healthy vaccinee bias exists "if vaccinated participants had significantly fewer comorbidities (or respective indicators such as medical visits) than unvaccinated participants, as indicated by baseline characteristics."

That's one way to determine it but there're other ways. The natural experiment study, for instance, ruled out the healthy vaccinee bias by showing that the vaccine-eligible and -ineligible group had similar rates of other vaccinations and healthcare utilizations.

But I agree the healthy vaccinee bias can be contradictory since those who are not vaccinated may be healthy and not at risk of the disease. Precisely because of such confusion and confounding, conventional observational studies are highly flawed by design.