<prompt> Antigenic shift -Occurs every 2-3 yearsGradual change over timeResults from genetic recombinationOccurs in all influenza viruses  <response>: Antigenic shift is an abrupt, drastic, discontinuous variation in the antigenic structure resulting in a novel virus strain unrelated antigenically to predecessor strain. REF: Anantha Narayan AND PANIKERS TEXTBOOK OF MICROBIOLOGY 9TH EDITION PAGE NO:499 