inclusion criteria: participants aged 6-17 years at the time of enrollment. obtain written informed assent from participants and consent from parents, guardians or legal representatives. able and willing to complete all the scheduled study procedures during the whole study follow-up period of 12 months. have not received any covid-19 vaccines (for primary groups only).

inclusion criteria: participants aged 6-17 years at the time of enrollment. obtain written informed assent from participants and consent from parents, guardians or legal representatives. able and willing to complete all the scheduled study procedures during the whole study follow-up period of 12 months. have not received any covid-19 vaccines (for primary groups only).