<prompt> False about antigenic drift -Occur under pressure for immunityResponsible for epidemics of influenzaOccurs only in influenza-AOccur every 10-12 years  <response>: Antigenic drift the gradual sequential change in antigenic structure occurring regularly at frequent intervals. Antigenic drift is due to mutation and selection. It accounts for the periodic epidemics of influenza. REF:ANANTHANARAYAN AND PANIKERS TEXTBOOK OF MICROBIOLOGY 9TH EDITION PAGE NO-499 