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. * 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. inclusion criteria (part b): * subject is ≥75 years old. * prior to study entry the subject was vaccinated with one of the following vaccination regimens (1st + 2nd + 3rd dose): bnt162b2 + bnt162b2 + bnt162b2 bnt162b2 + bnt162b2 + mrna-1273 mrna-1273 + mrna-1273 + mrna-1273 mrna-1273 + mrna-1273 + bnt162b2 chadox-1-s + chadox-1-s + bnt162b2 chadox-1-s + chadox-1-s + mrna-1273 the last dose of the above listed vaccinations must have been administered at least 1 month prior to study entry. vaccination status should be documented in the source data and will be captured in the ecrf. - written informed consent from subject has been obtained. exclusion criteria (part b): * prior to study entry the subject got vaccinated with a regimen not included in the list given above. * last anti-sars-cov-2 vaccine dose administered less than one month prior to study entry. * vaccination against a disease other than covid-19 within 2 weeks prior to study entry. only exception: influenza vaccination which is allowed at any time. * 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. * subject simultaneously participates in another clinical trials or has participated in the past 30 days. * subjects unable to report solicited adverse events. * subject with any contraindications to the vaccines in the trial. a list of contraindications as listed in the summary of medicinal product characteristics (smpc, the fachinformation in germany), if appropriate.

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. * 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. inclusion criteria (part b): * subject is ≥75 years old. * prior to study entry the subject was vaccinated with one of the following vaccination regimens (1st + 2nd + 3rd dose): bnt162b2 + bnt162b2 + bnt162b2 bnt162b2 + bnt162b2 + mrna-1273 mrna-1273 + mrna-1273 + mrna-1273 mrna-1273 + mrna-1273 + bnt162b2 chadox-1-s + chadox-1-s + bnt162b2 chadox-1-s + chadox-1-s + mrna-1273 the last dose of the above listed vaccinations must have been administered at least 1 month prior to study entry. vaccination status should be documented in the source data and will be captured in the ecrf. - written informed consent from subject has been obtained. exclusion criteria (part b): * prior to study entry the subject got vaccinated with a regimen not included in the list given above. * last anti-sars-cov-2 vaccine dose administered less than one month prior to study entry. * vaccination against a disease other than covid-19 within 2 weeks prior to study entry. only exception: influenza vaccination which is allowed at any time. * 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. * subject simultaneously participates in another clinical trials or has participated in the past 30 days. * subjects unable to report solicited adverse events. * subject with any contraindications to the vaccines in the trial. a list of contraindications as listed in the summary of medicinal product characteristics (smpc, the fachinformation in germany), if appropriate.

inclusion criteria: * subject is ≥75 years old). * prior to study entry the subject was vaccinated with one of the following vaccination regimens (1st + 2nd + 3rd dose): bnt162b2 + bnt162b2 + bnt162b2 bnt162b2 + bnt162b2 + mrna-1273 mrna-1273 + mrna-1273 + mrna-1273 mrna-1273 + mrna-1273 + bnt162b2 chadox-1-s + chadox-1-s + bnt162b2 chadox-1-s + chadox-1-s + mrna-1273 the last dose of the above listed vaccinations must have been administered at least 1 month prior to study entry. vaccination status should be documented in the source data and will be captured in the ecrf. - written informed consent from subject has been obtained.

inclusion criteria: * subject is ≥75 years old). * prior to study entry the subject was vaccinated with one of the following vaccination regimens (1st + 2nd + 3rd dose): bnt162b2 + bnt162b2 + bnt162b2 bnt162b2 + bnt162b2 + mrna-1273 mrna-1273 + mrna-1273 + mrna-1273 mrna-1273 + mrna-1273 + bnt162b2 chadox-1-s + chadox-1-s + bnt162b2 chadox-1-s + chadox-1-s + mrna-1273 the last dose of the above listed vaccinations must have been administered at least 1 month prior to study entry. vaccination status should be documented in the source data and will be captured in the ecrf. - written informed consent from subject has been obtained.

inclusion criteria: subject is ≥75 years old). prior to study entry the subject was vaccinated with one of the following vaccination regimens (1st + 2nd + 3rd dose): bnt162b2 + bnt162b2 + bnt162b2 bnt162b2 + bnt162b2 + mrna-1273 mrna-1273 + mrna-1273 + mrna-1273 mrna-1273 + mrna-1273 + bnt162b2 chadox-1-s + chadox-1-s + bnt162b2 chadox-1-s + chadox-1-s + mrna-1273 the last dose of the above listed vaccinations must have been administered at least 1 month prior to study entry. vaccination status should be documented in the source data and will be captured in the ecrf. - written informed consent from subject has been obtained.

inclusion criteria: subject is ≥75 years old). prior to study entry the subject was vaccinated with one of the following vaccination regimens (1st + 2nd + 3rd dose): bnt162b2 + bnt162b2 + bnt162b2 bnt162b2 + bnt162b2 + mrna-1273 mrna-1273 + mrna-1273 + mrna-1273 mrna-1273 + mrna-1273 + bnt162b2 chadox-1-s + chadox-1-s + bnt162b2 chadox-1-s + chadox-1-s + mrna-1273 the last dose of the above listed vaccinations must have been administered at least 1 month prior to study entry. vaccination status should be documented in the source data and will be captured in the ecrf. - written informed consent from subject has been obtained.

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

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