The Department of Paediatrics and Adolescent Medicine and School of Public Health at HKUMed are now launching the COVID-19 Vaccination in Adolescents and Children (COVAC) study. Timely local data on the reactogenicity and immunogenicity of COVID-19 vaccines in adolescents and children will inform the public and the government whether the vaccines should be given to children, and allow complete school reopening as soon as possible. This study has been approved by the Institutional Review Board of HKU/Hospital Authority HK West Cluster. Current COVID vaccines are given by an injection in the muscle. Yet, experience with inactivated flu vaccine suggests intradermal route (in the skin) vaccination may result in superior immunogenicity. There is currently no evidence on effectiveness of intradermal administration of COVID vaccines. Hong Kong those below 12 are not allowed to receive CoronaVac yet. In the fully enrolled COVA study, adolescent participants were given CoronaVac via intramuscular route. In extension, we would like to recruit more healthy adolescents aged 3-17 for intradermal vaccination in the COVAC study.