My son is an avid runner and monitors his heart rate closely. He said after each vaccine his heart rate went up for a period of days over the normal rate. The mRNA vaccines are known to cause myocarditis and pericarditis occasionally. I'm thinking that maybe it is not all that surprising that they may be a trigger for A-fib, especially for those already prone towards it. I haven't had any more bouts since November 24, the day I started taking Metoprolol. I'm weaned down to almost none at this point and no recurrences. I'm off caffeine and alcohol though, and plan to staff off those substances. In spite of it all, I will still take the next booster when it is available. I will feel better knowing I now have the tools (meds) to address A-fib if need be.