TCFL5 and influenza: Since individuals with a previous history of influenza vaccination and/or infection already carry rare sbnAbs, partly due to the less frequent existence of sbnAb-specific memory B cells or plasmablasts, it was hypothesized that immunization with cHA once or twice might suffice for the clonal expansion of sbnAb-specific cell populations to induce high titers of sbnAbs [72].