Homologous vs Heterologous Third Vaccination in Kidney Transplant Recipients Kidney Transplant Recipients

Inclusion Criteria: * patient has received a kidney transplantation * full SARS-CoV-2 vaccination with mRNA vaccine (two doses) at least 4 weeks before screening * \> 18 years of age * no SARS-CoV-2 spike protein antibodies at least 4 weeks after the second dose of an mRNA vaccine Exclusion Criteria: * acute illness with fever * Prior documented infection with SARS-CoV-2 * triple anticoagulation therapy * Subject is currently enrolled in or has not yet completed at least 30 days since ending other investigational device or drug trial(s), or subject is receiving other investigational agent(s) * Subject has known sensitivity or intolerance to any of the products to be administered for the purpose of this study * Subject has any kind of disorder that compromises the ability of the subject to give written informed consent and/or to comply with the study procedures * Subject is pregnant or breast feeding