Randomized Trial of COVID-19 Booster Vaccinations (Cobovax Study)

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 syste…