Figure 1. Older subjects have lower baseline titers and a reduced vaccine response. The influenza antibody response to all three vaccine strains in the seasonal vaccine was measured pre-vaccination and 28 days post-vaccination by HAI. (A) The percentage of young (white bars) and older (grey bars) subjects showing ≥4-fold increase in HAI assay on day 28 to the indicated number of strains. (B) Baseline titers for each of the three strains among young (white bars) and older (grey bars) subjects. Medians are indicated by the bold line, while the box boundaries indicate the 25th (lower line) and 75th (upper line) percentiles, with outliers indicated by (o). (C) and (D) Comparison of baseline (pre-vaccination) HAI titers with fold-change (day 28 vs. pre-vaccination). The size of the circles (small to large) depicts the number of subjects (1 to 7 for young and 4 to 11 for older). The set of vaccine responders and non-responders selected for microarray experiments are indicated by the plus and cross within the circle symbols, respectively. For simplicity, the response against the strain that induced the maximum fold-increase in HAI titer on day 28 in vaccine responders is plotted. For non-responders, the highest baseline titer among the three vaccine strains is depicted.