Data are only presented for proportions of patients with H3N2 serotype of influenza A, which is typically associated with higher risk of admission to hospital and death than the H1N1 serotype or influenza B.1,2 Protective haemagglutination inhibition (HI) titre for studies was 1:40 or greater unless otherwise noted. Studies without healthy control groups are not presented. No studies provided data on haemopoietic stem-cell transplant recipients relevant to these figures. Also excluded are data for corticosteroid users, because of inability to extract relevant data for figures. Results in patients with HIV infection (A), stratified by CD4+ T cell counts, and healthy controls. Fowke and colleagues32 not shown, because CD4 strata were not readily integrated with more conventional CD4 strata above. Brydak and colleagues44 did not include healthy control group, but data were provided for CD4 strata, and therefore included. Results in solid-organ transplant recipients and healthy controls (B): Hayney and colleagues46 data extracted from published figures. Dengler and colleagues47 reported percentage with either HI titer 1:40 or greater or with four-fold or greater rise in HI titre postvaccination. Birdwell and colleagues48 reported HI titer of 1:32 or greater. Results in patients treated with chemotherapy for malignancies and healthy controls (C): Feery and colleagues49 and Gribabis and colleagues50 data extracted from published figures. Stiver and colleagues51 reported HI titre of 1:32 or greater. Results in patients on hemodialysis and healthy controls (D): Beyer and colleagues52 reported HI titre of 1:100 or greater. Rautenburg and colleagues53 data extracted from published figures and reported IgG titre of 1:2 or greater. Antonen and colleagues54 data extracted from published figures.