That might be quite difficult to standardize. You would need to have their histories of prior capabilities on the treadmill test before vaccination/infection. That data simply is unlikely to exist and the numbers of individuals that even have that data (probably highly trained athleted or people needing stress tests) would be both limited and unusual cases. But on the surface why would people sort themselves into fitter/less fit groups by vaccination status? Do people more prone to avoid vaccination tend to avoid exercise? Presumably the sample excluded those who were seriously ill...if not then that should have been done. And presumably gender and age cohorts were standardized.

Mind you that if the above wasn't done you might end up by chance with the previously infected unvaccinated group showing better fitness


I'd view this as a highly suggestive study that requires larger sample sizes or bootstrap analysis.

There is a statistically significant tendency, though.