Subject recruitment and clinical characteristics
Between 1st December 2020 to 31stMarch 2021, 31 patients who had recovered from COVID-19 were recruited from Princess Margret Hospital, Hong Kong SAR. Fourteen (45.2%) were boys and 17 (54.8%) were girls with a median age of 12 years (range 2.7-18 years). Twenty age-matched uninfected controls were also recruited from Queen Mary Hospital, Hong Kong SAR, China and from the community. Subject demographics and clinical characteristics are shown in Table 1. The majority of subjects were Chinese (80.6%). Among the COVID-19 cases, 83.9% were domestic cases, 32.3% were asymptomatic, and the remaining cases (67.7%) had mild disease. Blood samples were collected at 29 to 219 days after recovery.