History of palpitations,2 episodes of syncope last 20 yrs.had my yearly physical 4 hrs after moderna booster,1/2 dose. ecg showed bradycardia,53 beats per minute.next two days resting heart rate was fluctuating as low as 48 bpm.fine now,from vaccine?

One cannot tell in an individual if it is the vaccine or not. You did not mention if you are taking meds such as beta-blockers which can slow heart rate and are used commonly in men your age. The EKG reading was it by the machine or by the Dr reading the EKG. Can be normal if you are an exerciser. Discuss with your med team. They know you we do not.

Bradycardia with recorded heart rates of 53 and 48 bpm are unlikely to be related to the vaccine. In view of the history of syncopal episodes, would have your physician evaluate you fully and possibly refer you to a cardiologist. You will need a thorough examination including heart rate response to exercise, vagal stimulation such as carotid massage. Holter monitor may also be appropriate.