Only two of 11 neutralizing antibodies were highly specific for the pandemic H1N1 strain alone (Fig. 3 B and Fig. S2 A) , including a low avidity antibody, 1000-2G06, which only showed slight neutralization capacity in vitro, and EM-4C04, which was very effective at neutralizing the pandemic H1N1 influenza. We conclude from these experiments that a surprising 82% (9/11) of the neutralizing plasmablasts that we isolated during pandemic H1N1 influenza infections were broadly cross-reactive to multiple influenza strains.