Combination of mRNA (Comiranty) and inactivated (CoronaVac) COVID-19 vaccination might improve the vaccine immunogenicity and safety when compare to vaccination with two doses of the same type of vaccine. This study by HKU Department of Medicine targets to recruit 150 adult subjects 18 years or above and randomise to receive a single dose of of Comirnaty followed by a single dose of CoronaVac, or two doses of Comirnaty or two doses of CoronaVac. Subjects will be follow-up and assessed for a total of 1 year for vaccine immunogenicity and safety.