Dr. Sadrzadeh said he brought his EpiPen to his vaccine appointment because of his serious allergies. He said that within minutes of the vaccine injection at 3:30 p.m., his heart rate had spiked to 150 beats per minute, about twice its normal cadence; his tongue prickled and went numb. Before long, he was drenched in a cold sweat and found himself feeling dizzy and faint. His blood pressure also plummeted, he said. His immune system, he realized, was in revolt.