2.1 Sample collection
A total of 112 participants were recruited in Macheng and Beijing, China. Participants comprised 51 COVID-19 vaccinated convalescents (2Y-Vac), 35 healthy vaccinated controls (HC-Vac), and 26 healthy unvaccinated controls (HC-Unvac). 23 out of the 51 2-year convalescents were also kept prior to vaccination during the followed up at 6 months (6m) and 1 year (1Y) after symptoms onset (Table 1). Sera and PBMCs of whole blood were separated and frozen immediately for subsequent analysis (Supplementary Methods).