Amsterdam institute for Immunology and Infectious diseases

The SARS-CoV-2 pandemic has provided unprecedented challenges since 2020, but also unique opportunities for biomedical science. New and highlyeffective vaccines have been developed in record-time, and molecular approaches have enabled detailed single-cell evaluation of the adaptive immune response to a new pathogen. Using high-dimensional flowcytometry with fluorescent antigen tetramers, we and others have shown that memory B(mem) cells are durable for >8 months after infection, despite a rapid decline in serum antibodies. Currently, we apply this technology to evaluate Bmem formation, durability and capacity to recognise variants-of-concern (VoC). Sixtyseven healthy adults and 28 patients with primary immunodeficiency were sampled before and 3-4 weeks after each of their three vaccine doses (ChadOx1-S/nCoV-19; AstraZeneca or BNT162b2, Pfizer). Serum antibodies to Wuhan and VoC RBD were quantified, as well numbers and immunophenotype of Bmem cells. All 30 health donors that received BNT162b2 generated high levels of Spike RBD antibodies, which were neutralising, 4 weeks after their second dose. RBD-specific Bmem cells were formed after dose 1 and their numbers ncreased after dose 2, mainly due to an expansion of IgG1+ Bmem. After dose 1, about 50% of Wuhan-RBD specific Bmem cells recognised RBD from the Gamma and Delta variants. This increased to >65% after dose 2. Thus, BNT162b2 is highly efficient in generating antibody and Bmem responses to Spike RBD. The second dose is required for neutralising antibody levels and expansion of Bmem cells that bind to VoC. The increased efficiency of Bmem cells to bind VoC is potentially the result of additional affinity maturation that overcomes the epitope variant. Further studies are ongoing to evaluate the capacity of the vector-based ChadOx vaccine to induce Bmem cells and that of the third dose booster in healthy controls and patients with primary immunodeficiency. Outcomes of these evaluation could inform on the levels of protection against COVID-19 in vaccinated people with or without underlying conditions, and on the timing and formulation of future booster vaccinations to prevent severe disease.