Serum hemagglutination-inhibition antibody titers. Peripheral blood was obtained prior to (baseline), and one and six months following vaccination with trivalent influenza vaccine (TIV) throughout multiple years. Influenza-specific serum antibody was measured by hemagglutination inhibition assay and geometric mean titer (GMT) determined for the indicated influenza subtype. GMT (upper 95% confidence interval (CI)/lower 95% CI) indicated. Relative difference in GMT among rheumatoid arthritis (RA) groups as compared with the healthy control (HC) group at same timepoint indicated by color. Cells with bold lines indicate significant difference (P < 0.05) compared with HC group at same timepoint. N, number of subjects measured per year. MTX, methotrexate.