Subject is ≥75 years old. For study entry in Part B the subject was vaccinated with one of the following vaccination regimens (1st + 2nd + 3rd dose): o BNT162b2 + BNT162b2 + BNT162b2 o BNT162b2 + BNT162b2 + mRNA-1273 o mRNA-1273 + mRNA-1273 + mRNA-1273 o mRNA-1273 + mRNA-1273 + BNT162b2 o ChAdOx-1-S + ChAdOx-1-S + BNT162b2 o ChAdOx-1-S + ChAdOx-1-S + mRNA-1273 The last dose of the above 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. No contra-indication against any of the vaccine products in the trial. Written informed consent from subject has been obtained.

Subject is ≥75 years old. For study entry in Part B the subject was vaccinated with one of the following vaccination regimens (1st + 2nd + 3rd dose): o BNT162b2 + BNT162b2 + BNT162b2 o BNT162b2 + BNT162b2 + mRNA-1273 o mRNA-1273 + mRNA-1273 + mRNA-1273 o mRNA-1273 + mRNA-1273 + BNT162b2 o ChAdOx-1-S + ChAdOx-1-S + BNT162b2 o ChAdOx-1-S + ChAdOx-1-S + mRNA-1273 The last dose of the above 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. No contra-indication against any of the vaccine products in the trial. Written informed consent from subject has been obtained.

Elderly (≥75 years old). Already fully vaccinated adults. Primary vaccination (1st and 2nd dose) using BNT162b2, mRNA-1273 or ChAdOx-1-S). No contra-indication against any of the vaccine products in the trial at time of enrolment.

Elderly (≥75 years old). Already fully vaccinated adults. Primary vaccination (1st and 2nd dose) using BNT162b2, mRNA-1273 or ChAdOx-1-S). No contra-indication against any of the vaccine products in the trial at time of enrolment.