Inclusion Criteria:  Aged 18 years or older at enrolment. Have received two doses of BNT162b2 OR two doses of CoronaVac, with the most recent dose at least six months 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 prior to enrolment. Have previously already received one or two doses of any COVID-19 vaccines except CoronaVac or BNT162b2, for example but not limited to BBIBP-CorV (inactivated vaccine, Sinopharm), AZD1222 (adenovirus vector-based vaccine, Oxford/AstraZeneca), Sputnik V (adenovirus vector-based vaccine, Gamaleya Research Institute) and Ad26.COV2.S (adenovirus vector-based vaccine, Johnson & Johnson). Individuals who report any medical condition, or as determined by a clinician, not suitable to receive mRNA or inactivated COVID-19 vaccines, including but not limited to allergies to the active substance or other ingredients of the vaccine. Currently 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). Administration of immunoglobulins and/or any blood products within 90 days preceding the planned administration of the study vaccines. Pregnancy, lactation or intention to become pregnant in the coming 3 months.