Inclusion Criteria:  Aged 30 years or older at enrolment. Have received two doses of any inactivated COVID-19 vaccines (for example Sinovac- CoronaVac, Sinopharm-BIBP) with the most recent dose at least 90 days prior to enrolment. Currently resident and planning to remain resident in Hong Kong during the duration of the study, i.e. for 12 months after enrolment. Agreement to refrain from blood donation during the course of the study. Willing to provide blood samples for all the required time points. The individual or their caregiver have a home phone or cellular or mobile phone for communications purpose. Capable of providing informed consent.  Exclusion Criteria:  A history of laboratory-confirmed or clinically confirmed COVID-19 infection. Have received (at least one dose of) any COVID-19 vaccines other than inactivated vaccines. Individuals who report any medical condition, or as determined by a clinician, not suitable to receive mRNA vaccines, including but not limited to allergies to the active substance or other ingredients of the vaccine. With diagnosed medical conditions related to their immune system. Use of medication that impairs immune system in the last 6 months, except topical steroids or short-term oral steroids (course lasting ≤ 14 days). Have used immunoglobulins and/or any blood products within 90 days prior to enrolment (administration of the study vaccine). Pregnancy, lactation or intention to become pregnant in the coming 3 months.