Decreased induction of IgG, IgG1 and IgG3 by the influenza vaccine in the elderly. A. Sera isolated before (t0) and after vaccination (t7 or t28) were collected and evaluated in H1N1-specific IgG, IgG1 and IgG3 ELISA. Data are expressed as OD at t7 or t28 minus OD at t0. All values here demonstrate a positive response. Data would look similar if only OD at t7 or t28 are shown. Mean comparisons between groups were performed by Mann-Whitney test (two-tailed), using GraphPad Prism 5 software. P values (two-tailed) are indicated in each graph. The numbers of individuals are given in Table 1. B. Individual IgG1 and IgG3 responses at t7 and t28 are shown. The numbers of individuals are given in Table 1.