Yes: "athletic heart syndrome" is a thing--but not recognised as a formal diagnostic category. And what a coincidence! I too developed A-Fib following administration of the bivalent mRNA C19 vaccine. I've posted elsewhere on this site about this incident (& the AFib was preceded by months of gastro-intestinal disorders that started with the first monovalent booster; I'm still being treated...13 months & counting, for dysbiosis, IBS, & SIBO-C). But it's infuriating: I'm now 5 months beyond the bivalent mRNA booster (October 2022) & my first incidence of A-fib...& I'm still experiencing paroxysmal A-fib. As previously noted: I've got 8-plus years of FitBit fitness/cardiac fitness data to document my exceptional fitness/cardiac fitness...prior to the bivalent mRNA insult & unprecedented onset of A-Fib (Fitbit consistently estimated my VO2 max at 58-62--& I am 65 years old!). Notwithstanding the paroxysmal A-fib (just had an overnight bout on Tuesday), I have resumed my (sometimes extreme) fitness routines. And my cardiac fitness metrics--measured both by my Fitbit & now Apple Watch 8, have returned to pre-bivalent-booster/pre-AFib levels: RHR = 44; walking heart rate = 52; VO2 max = 58-62 (up from 51-55 following the bivalent mRNA booster). But that I am having paroxysmal A-fib is infuriating & worrisome. I live alone (with cats) & not infrequently wonder when I go to bed if I will wake up the following morning. Worry mostly about dying in my sleep from a heart attack & not being discovered for days...while the cats are uncared for/unloved. Angry that I might not be able to get the Novavax booster, given the termination of the C19 Pandemic Emergency Policy (& funding for additional vaccine & therapeutics development, not to mention policies permitting Telemedcine that also will be terminated). Will NEVER accept another mRNA vaccine--for any indication, using the lipid nanoparticle delivery technology. Never!