Flu (Influenza) Can One Vaccine Protect Against All Strains?

If researchers could develop a long-lasting and very protective vaccine, we would not have to worry about getting a flu shot every year.
Vaccine against flu helps to activate our immune system by inserting proteins that found on the coat of the flu virus. But, unfortunately, flu virus doesn’t have the same coat every year; it changes each season. And, when the coat changes, it can no longer recognize the proteins that were taken from the coat of previous virus. That’s why we need to reestablish our immunity to the flu by getting new shot every year.
Scientists are working on finding proteins from the virus that are more permanent and don’t change so fast. This would help to develop vaccines that last longer. Some scientists already found stable proteins. Dr. Gerard found M2, and Dr. Van found NP. They are trying to make more of the version of such proteins by bioengineering. Also, they’re examining for how long the immunity provided by these vaccine would last, and what is the possibility for these proteins to mutate.

http://www3.niaid.nih.gov/topics/Flu/Research/vaccineResearch/oneVaccine.htm