Does this mean that evolution is useless? Not at all. Does this mean that evolution is invalid? Not at all.
I would disagree with the first one...it is essentially a restatement of the obvious.
Being unable to make specific predictions does not invalidate the theory... but is still does not make it any more useful than the paper it is printed on...
We do know that it will be substantially the same and that certain proteins will be similar. This is because evolution predicts that the new variety will be a daughter of the previous one. Thus you know where to start with making a vaccine for a new variety of a virus rather than starting from scratch.
We also know that our children will be substantially similar to ourselves too. Does that make that prediction anymore or any less creditable than ToE... no. We knew that 6000 years ago...
It is a useless theory... that has essentially contributed virtually nothing to empirical science. It can not predict what, where, or when a mutation will occur. That is how your allusion to the flu vaccine falls apart as it would be predicting what the mutation will be, where in the gene it will occur, and what chemical changes will occur. It does none of the above... so it therefore is useless as it pertains to vaccines.