The difficulties of maintaining a long-term immune memory to influenza viruses are due to

I am certain that the difficulties of maintaining a long-term immune memory to influenza viruses are due to the Antigenic shift from zoonotic sources. 
The adaptive immune response is intiated when the DCs present viral antigens to the naive and memory T-lymphoctes. During an influenza infection, the antigens are acquired by the DCs through two different mechanism.