Antigenic drift and antigenic shift is seen in

ANTIGENIC DRIFT It is a minor change occuring due to point mutations in the HA/NA gene ,resulting in alteration of amino acid sequence of the antigenic sites on HA/NA , such that virus can escape recognition by the host's inmune system. Seen in both influenza type A and B More frequent- every 2-3 years ANTIGENIC SHIFT It is an abrupt , major drastic , discontinuous variation in the sequence of a viral surface protein , that occurs due to genetic reassoment between genomes of 2 or more influenza viruses infecting the same host cells , resulting in a new virus strain, unrelated anti genetically to the predecessor strains Occurs only in influenza type A Less frequent - every 10 -20 yrs Ref : essentials of Medical microbiology apurba sastry 1st edition page 459