False about antigenic drift -: (A) Occur under pressure for immunity, (B) Responsible for epidemics of influenza, (C) Occurs only in influenza-A, (D) Occur every 10-12 years
Answer is C. 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 PANIKER'S TEXTBOOK OF MICROBIOLOGY 9TH EDITION PAGE NO-499