Increased humoral responses after pandemic influenza A/H1N1 vaccination. Specific antibodies against the pandemic influenza virus A/H1N1 were measured via (a) hemagglutination inhibition and (b) microneutralization. The box plots show quartiles and medians. The dotted lines indicate a cut-off value of 1:40, with p ≤ 0.05 (pre-vaccination vs. post-vaccination), as determined using the Wilcoxon singed rank test (n = 60). (c) The presence of IgG antibodies against hemagglutinin A/H1N1 (HA1) was assessed in sera obtained prior to H1N1 vaccination using ELISA, with the dotted line indicating the cut-off value (n = 22). The antibody titers against the pandemic influenza virus A/H1N1 were assessed post-vaccination in sera from individuals with (n = 13) or without (n = 9) anti-HA1 antibodies prior to vaccination using the hemagglutination inhibition assay. The depicted box plots show the quartiles and medians. The dotted line indicates the protective cut-off value.