Heterologous Primary and Booster Vaccination

At 5.5 months after the heterologous primary vaccination, and 2 weeks (antibody titers peaked around 14–19 days post initial heterologous vaccination (25 (link))) after the BNT162b2 ‘booster’ (7 months post primary vaccination), blood was drawn into S-Monovette^® Serum Gel (Sarstedt) or S-Monovette^® K3 EDTA tubes. Serum gel collection tubes were centrifuged at 1,500 × g at 20°C for 15 min, aliquoted, and stored at -20°C until further use. PBMCs were obtained from EDTA tubes using density gradient centrifugation by Pancoll human (Pan Biotech, Germany), and erythrocytes were removed by ACK lysis buffer (Lonza, Walkersville, MD, USA). Mononuclear cells were counted for viability using a Countess II Automated Cell Counter (Thermo Fisher) with trypan blue stain and were cryopreserved in aliquots of up to 1 × 10⁷ cells in 10% DMSO in heat-inactivated FCS.