PF4 and Hypokalemia: Five serious adverse events occurred in the 4-week follow-up period after vaccination: 1 patient died of a myocardial infarction (vector group); 2 patients were hospitalized for urinary tract infections (1 in each group); 1 patient developed thrombophlebitis 3 weeks after vaccination (vector group) and required systemic anticoagulation (laboratory work-up results showed no signs of thrombocytopenia and platelet factor 4 antibodies were negative)23; and 1 patient was hospitalized for treatment of symptomatic hypokalemia (mRNA group).