The vaccine platform used for COVID-19 primary immunization shapes the quality of the human B cell response to a vaccine boost.

Improving long-term protective immunity elicited by prime-boost vaccinations requires a deeper understanding of the immunologic outcomes of different vaccine platforms. Given the variety of platforms used to develop vaccines against SARS-CoV-2, we reasoned that SARS-CoV-2 offered an opportunity to compare vaccine platforms in humans. We used flow cytometry and single-cell transcriptomics to explore the B cell response to different homologous and heterologous vaccine regimens. We found that an adenovirus vector prime followed by a messenger RNA (mRNA) vaccine boost showed the greatest short-term B cell expansion and preferentially elicited an activated atypical B cell subset that was associated with antibody binding titers against spike protein. In contrast, an mRNA primary series followed by homologous boost induced a different activated B cell subset with more proliferative potential and high frequencies of a long-lived resting memory subset. Moreover, immunoglobulin A (IgA)-expressing memory B cells had more somatic hypermutations than the predominant IgG-expressing B cell population. This heterogeneity in vaccine-elicited B cell responses underscores the potential of tailoring vaccine regimens that combine different platforms to achieve potent and durable protection against infectious diseases.