Distinct antibody and memory B cell responses in SARS-CoV-2 naive and recovered individuals after mRNA vaccination

mRNA vaccines for SARS-CoV-2 have been authorized for emergency use. Despite their efficacy in clinical trials, data on mRNA vaccine-induced immune responses are mostly limited to serological analyses. Here, we interrogated antibody and antigen-specific memory B cells over time in 33 SARS-CoV-2 naive and 11 SARS-CoV-2 recovered subjects. SARS-CoV-2 naive individuals required both vaccine doses for optimal increases in antibodies, particularly for neutralizing titers against the B.1.351 variant. Memory B cells specific for full-length spike protein and the spike receptor binding domain (RBD) were also efficiently primed by mRNA vaccination and detectable in all SARS-CoV-2 naive subjects after the second vaccine dose, although the memory B cell response declined slightly with age. In SARS-CoV-2 recovered individuals, antibody and memory B cell responses were significantly boosted after the first vaccine dose; however, there was no increase in circulating antibodies, neutralizing titers, or antigen-specific memory B cells after the second dose. This robust boosting after the first vaccine dose strongly correlated with levels of preexisting memory B cells in recovered individuals, identifying a key role for memory B cells in mounting recall responses to SARS-CoV- 2 antigens. Together, our data demonstrated robust serological and cellular priming by mRNA vaccines and revealed distinct responses based on prior SARS-CoV-2 exposure, whereby COVID-19 recovered subjects may only require a single vaccine dose to achieve peak antibody and memory B cell responses. These findings also highlight the utility of defining cellular responses in addition to serologies and may inform SARS-CoV-2 vaccine distribution in a resource-limited setting.

Automated summary

mRNA vaccines exhibit robust serological and cellular priming, with naïve individuals requiring two doses for optimal antibody responses, especially against the B.1.351 variant. Memory B cells specific for spike protein and RBD were efficiently primed by vaccination, while recovered individuals showed significant boosting after the first dose, correlating with preexisting memory B cell levels. Identifying distinct responses based on prior SARS-CoV-2 exposure suggests that recovered subjects may only need one vaccine dose for peak responses, which can inform vaccine distribution strategies in resource-limited settings.