The virus has a head and tail. We make vaccines for the tail that evolves annually. We guess what it will be to make a vaccine for the following year.
The NIH is working on making a vaccine for the head of the virus.
It is sort of a one size fits all vaccine.
We have not yet succeeded.

Getting a sample of the 1918 flu was restricted for 5 years. It has opened up again. Samples anyone?

(simplified version)