First off flu does not simply mutate. It actually mixes with other strains which then propagate within the host breed ( flu of concern is found mostly in birds, swine, and equine ).
Secondly, we have not been able to create a vaccine against all, even though it is being worked on. So, the best we can do, is simply vaccinate against the type that we think will be here. IOW, scientist look at patterns around world, esp in China, and then guess what we will see.

Do note that if we ever develop a vaccine against all flu, I'm not sure I would take it. The 1918 epidemic was believed to be avian. However, it was different in that most ppl that died we're 20-40 y.o. in fact, relatively few old/young died in that. It hints that it does not operate the same as other flu and attacks those with good/great immune responses.