* Formation of two separate cohorts of approximately 1000 study subjects each from the three categories (dialysis patients, solid organ transplant recipients and medical staff) of participants vaccinated using either Biontech or Moderna vaccine (Biontech cohort and Moderna cohort, approximately 2000 subjects total).
* Study time points: Before administration of the 1st SARS-CoV-2 vaccine dose, before the 2nd vaccine dose, and 8 weeks and 6, 9, 12, and 18 months after administration of the 1st vaccine dose and after suspected or proven SARS-CoV-2 infection.
* At all time points, a questionnaire or eCRF will be completed with questions about the dialysis center and the clinical course of the patients.
* Humoral immune response will be determined in all study participants.
* Formation of a study subgroup of up to 300 subjects in each cohort (Biontech or Moderna cohort) for detailed evaluation of the cellular immune response (max. 600 subjects in total).
* In case of non-sustained immunity \> 6 months as well as the occurrence of a 3rd SARS-CoV-2 infection wave in autumn 2021, this will be investigated analogously to the current wave.
