To determine the level of pre-existing human population immunity to bovine-derived A/H5N1 viruses, serum samples from two distinct cohorts—healthcare workers vaccinated with the 2024 seasonal influenza vaccine and individuals infected with seasonal A/H1N1 or A/H3N2 influenza during the 2023–2024 influenza season—were tested for the presence of antibodies recognizing the H5 HA or N1 NA proteins. The gene discussed is XK; the disease is influenza.