inclusion criteria: 1. willing and able to provide informed consent for participation in this study; 2. aged ≥21years at the time of study enrolment; 3. received the second dose of bnt162b2 or mrna-1273 coronavirus disease 2019 vaccines at least 6 months prior to enrolment; 4. willing and able to comply with all scheduled visits, vaccination plan, laboratory tests and other study procedures. for phase d inclusion criteria 3 will be amended to 'received the first booster dose of bnt162b2 or mrna-1273 coronavirus disease 2019 vaccine at least 5 months prior to enrolment'. only individuals recommended to receive a second booster dose under moh recommendations will be enrolled.

inclusion criteria: 1. willing and able to provide informed consent for participation in this study; 2. aged ≥21years at the time of study enrolment; 3. received the second dose of bnt162b2 or mrna-1273 coronavirus disease 2019 vaccines at least 6 months prior to enrolment; 4. willing and able to comply with all scheduled visits, vaccination plan, laboratory tests and other study procedures. for phase d inclusion criteria 3 will be amended to 'received the first booster dose of bnt162b2 or mrna-1273 coronavirus disease 2019 vaccine at least 5 months prior to enrolment'. only individuals recommended to receive a second booster dose under moh recommendations will be enrolled.

inclusion criteria: 1. willing and able to provide informed consent for participation in this study; 2. aged ≥21years at the time of study enrolment; 3. received the second dose of bnt162b2 or mrna-1273 coronavirus disease 2019 vaccines at least 6 months prior to enrolment; 4. willing and able to comply with all scheduled visits, vaccination plan, laboratory tests and other study procedures.

inclusion criteria: 1. willing and able to provide informed consent for participation in this study; 2. aged ≥21years at the time of study enrolment; 3. received the second dose of bnt162b2 or mrna-1273 coronavirus disease 2019 vaccines at least 6 months prior to enrolment; 4. willing and able to comply with all scheduled visits, vaccination plan, laboratory tests and other study procedures.

inclusion criteria: willing and able to provide informed consent for participation in this study; aged ≥21years at the time of study enrolment; received the second dose of bnt162b2 or mrna-1273 coronavirus disease 2019 vaccines at least 6 months prior to enrolment; willing and able to comply with all scheduled visits, vaccination plan, laboratory tests and other study procedures.

inclusion criteria: willing and able to provide informed consent for participation in this study; aged ≥21years at the time of study enrolment; received the second dose of bnt162b2 or mrna-1273 coronavirus disease 2019 vaccines at least 6 months prior to enrolment; willing and able to comply with all scheduled visits, vaccination plan, laboratory tests and other study procedures.