Assessing Immune Response of Different COVID-19 Vaccines in Older Adults

Inclusion Criteria (Part A): * Subject is ≥75 years old. * Prior to study entry the subject was vaccinated with BNT162b2, mRNA-1273 or ChAdOx-1-S (same vaccine product for 1st + 2nd dose) BNT162b2 + BNT162b2 mRNA-1273 + mRNA-1273 ChAdOx-1-S + ChAdOx-1-S * 9 ± 3 months since the second vaccine dose at time of enrolment for the planned 3rd vaccine dose in the trial. Vaccination status should be documented in the source data and captured in the eCRF. * No contra-indication against any of the vaccine products in the trial. * Written informed consent from subject has been obtained. Exclusion Criteria (Part A): * Primary vaccination performed with different vaccine products as sole (e.g., COVID-19 Vaccine Janssen) or, 1st and 2nd vaccination doses (heterologous vaccination scheme). * Subjects with any significant or uncontrolled disease posing a risk due to vaccination as judged by the investigator. * Current immunosuppressive therapy, for example continuous glucocorticosteroid treatment equivalent to \>10 mg/day prednisolone. * Participation in other interventional trials. * Subjects unable to report solicited adverse events. * Subject with any contraindications to the vaccines in the trial at randomisation. A list of contraindications as listed in the Summary of medicinal Product Characteristics (SmPC, the Fachinformation in Germany), if appropriate. * Use of drugs with significant interaction with the investigational product according to the SmPC or similar documents. * D…