Sample collection, preparation and storage

Peripheral venous blood samples were collected in Vacutainer CPT (362761, BD, San Diego, CA, US) tubes and coagulation promoting tubes at four time points, including the day before vaccination (BV), 7 days after the first dose (1V7), 7 days after the second dose (2V7) and 14 days after the second dose (2V14). The blood samples were processed in 2 h after collection. The PBMCs were isolated using Vacutainer CPT tubes according to the manufacturer’s protocol [23]. The viability of PBMCs in each sample was confirmed to be >90% by Trypan Blue staining. PBMCs were immediately used for scRNA library construction and sequencing, or were cryopreserved at −80°C in 10% dimethylsulfoxide in fetal bovine serum. Blood samples in the coagulation promoting tubes were incubated upright for 20 min at 4°C. Then, the coagulation promoting tubes were centrifuged at 1000 × g for 10 min at 4°C. The supernatant serum in each tube was extracted and stored at −80°C for future assays.