A study came out recently that should have gotten more attention. It looked at how athletes responded to getting "boosted" with the Pfizer vaccine: Effect of BNT162b2 mRNA booster vaccination on VO2max in recreational athletes: A prospective cohort study

What's nice about this study is that they measured various markers of health before and after booster injection, in the same individuals. A total of 42 athletes were included in the study.

They found a decrease in VO? max post booster dose

One of the things they measured was VO? max both before and after booster dosing. VO? max is a measure of aerobic capacity; it's the maximum rate at which the body can take up oxygen and utilize it during intense exercise. It's an indicator of overall cardiorespiratory fitness; having a higher VO? max generally means you're more fit.