Secular trend in influenza is due to -

Antigenic shift,is an brupt drastic discontinuation variation in the antigenic structure resulting in a novel virus strain unrelated antigenically to predecessor strains. Such chamges may involve hemagglutinin , neuraminidase or both. Antibodies to predecessor viruses do not neutralise the new variants and can therefore spreads widely in the population causing major epidemics or pandemics. The changes involved in antigenic shifts are too extensive to be accounted for by mutation REF:Ananthanarayan & Panicker's Textbook of Microbiology 8th edition pg no:496