Evaluation of age-related differences in the immunogenicity of a G9 H9N2 influenza vaccine

► Seroprevalence to a G9 influenza A/H9N2 strain was higher than against a G1 strain. ► Persons born after 1970 responded as well to a G9 vaccine as older persons. ► Immune responses following vaccination with G1 and G9 H9N2 strains differ.