Inclusion Criteria:  Informed consent signed and dated by study patient and investigator/authorized physician Minimum age of 18 years Ability to understand the nature and requirements of the study ESKD patients on maintenance hemodialysis with a minimum of 4hrs three times a week at the time of recruitment and a dialysis vintage ≥3 months in NC clinics Patients fully vaccinated (2 doses) against COVID-19 with either Comirnaty (Portugal) or Sputnik V (Russia) as detailed in the manufacturer's instructions  Patients that are scheduled to receive a third vaccine dose  Portugal: Comirnaty Russia: Sputnik light (Sputnik V first dose vaccine)  Exclusion Criteria:  Any conditions which could interfere with the patient's ability to comply with the study Participation in an interventional clinical study during the preceding 30 days Previous participation in the same study Patients with proven current COVID-19 infection as identified by routine clinical practice Patient in therapy with immunosuppressive medications / immunomodulators Patients who already received a third COVID-19 vaccination dose