Inclusion Criteria:  Elderly (≥75 years old) Already fully vaccinated adults Primary vaccination (1st and 2nd dose) using BNT162b2, mRNA-1273 or ChAdOx-1-S 9 ± 3 months since the second vaccine dose at time of enrolment for the planned 3rd vaccine dose in the trial No contra-indication against any of the vaccine products in the trial at time of enrolment Genders (female, male) Written informed consent from participant has been obtained  Exclusion Criteria:  Primary vaccination performed with different vaccine products as sole (e.g., COVID-19 Vaccine Janssen) or, 1st and 2nd vaccination doses (heterologous vaccination scheme) Participants 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 Participants unable to report solicited adverse events Participants 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), if appropriate Use of drugs with significant interaction with the investigational product according to the SmPC or similar documents Diseases or findings that may have a significant effect on the target variables and which may therefore mask or inhibit the therapeutic effect under investigation Any current SARS-CoV-2 infection or proven in the preceding 3 months Persons with any kind of dependency on the principal investigator or employed by the sponsor or principal investigator Legally incapacitated persons Persons held in an institution by legal or official order