Objectives To assess the reactogenicity, measure the adaptive immune responses and track the long-term immune memory in healthy children and adults as well as pediatric patients receiving the COVID-19 vaccines-BNT162b2, CoronaVac-chosen by the Hong Kong Government; to compare the reactogenicity and immunogenicity across the vaccines used for these children and adults.

Hypothesis to be tested The safety profile and the magnitude and durability of immune responses to the COVID-19 vaccines in children are non-inferior to those in adults.

Design and subjects A single-site, comparative nonrandomised clinical trial for 450 healthy individuals or patients under 18 years old and one or both healthy parents and unrelated adults to receive one of COVID-19 vaccines by intramuscular injection (and intradermal injection)

Instruments Mobile app for subjects to record adverse effects, enzyme-linked immunosorbent assay, plaque reduction neutralization assay, luciferase immunoprecipitation system assay and flow cytometry.

Interventions BNT162b2 and CoronaVac, by intramuscular or intradermal route

Main outcome measures Types and frequencies of adverse effects within 7 days, and changes and peaks of antibody levels and antigen-specific memory T cell responses for 3 years.

Inclusion Criteria:

1. informed consent from the parents or a legally acceptable representative for an underage participant
2. biological parents of students enrolled in the trial or unrelated healthy adults
3. ability to adhere to the follow-up schedules
4. willingness to report reactogenicity daily for 7 days post dose 1, 2 and 3 (and 4) proactively
5. willingness to receive that vaccine available for that particular recruitment period (as student-parent pair, if applicable)
6. good past health, including pre-existing clinically stable disease, such as paediatric or immune disorders
7. prior COVID-19 (for COVID-19 survivor subgroup)

Exclusion Criteria:

1. reported pregnancy or breastfeeding