This is an open-label clinical trial on COVID-19 vaccine booster in Hong Kong organised by the School of Public Health, the University of Hong Kong. We will offer a booster dose of a mRNA COVID-19 vaccine (Fosun/Pfizer/BioNTech-Comirnaty) to adults aged 30 years and older who have received two doses of inactivated COVID-19 vaccine (for example Sinovac or Sinopharm). Then, we will track these participants for a period of 1 year (2021 – 2022) to monitor the post-vaccination reactions and immune protection after receiving COVID-19 booster vaccination.