I had a similar reaction. 10 days after the Moderna booster I started with itchiness around my neck and scalp. Then hives all over my body (very itchy ). That night I felt tightness in my throat and even had reflux symptoms. Esophagus tightness and some coughing that would come and go but very intense at times. Got tested for Covid , it came negative. Drs sent me prednisone for a week and an antihistamine. The prednisone helped with the esophagus and reflux issue but the hives kep coming and going. got tested for allergies and no allergies were found. It has been amore than a month and I can not pass 2 days without taking the antihistamine because I start getting hives all over. I never had any hives before and I am starting to read that many people have been getting them after the moderna shots.