“Even if they could find a commonality in flu which would allow development of a vaccine with long term protection, I doubt that they would.”
It has really nothing to do commonality between influenza viruses. Influenza viruses are ‘enveloped’ RNA viruses, and have a lipid envelope ‘coating’. There are proteins embedded in this lipid envelope, and these proteins are what the antibodies/immune system react against. Unfortunately, these proteins have a high rate of mutation and rearrangement, and so every year the proteins are a little bit different. This allows the virus to escape the immune system of the host (e.g. us). It makes it extremely difficult to develop a vaccine that will work year after year.
First of all, it wouldn’t be the Flu (whichever version) if there was no commonality. Second, it’s more than likely we just haven’t found the commonality or understand how to address it. In other words, what exists in all versions of flu which makes flu unique... even single RNA segments.