One dose of an MF59-adjuvanted pandemic A/H1N1 vaccine recruits pre-existing immune memory and induces the rapid rise of neutralizing antibodies

► Antibody CD4 T and B cells response to 2009 A/H1N1 pandemic vaccination in adults. ► Comparable day 0 frequency of CD4 T and memory B cells reactive to pandemic/seasonal H1N1. ► Vaccination expand pandemic- and seasonal-specific CD4+ T cells and memory B cells. ► Antibodies and plasmablasts accumulating by day 8 are IgG-switched. ► By day 8 in antibodies inhibiting hemagglutination and infection rose up to 8-fold.