The major flu season lasts up through April with the peak being Jan through mid-March. The flu shot takes about one month to work, so you do the math.
I find it funny that they can hardly give these flu shots away now. They've been offering at work now for free and no one wants them...
I'm not sure if the shot would help with whatever these "flu type symptoms" are that you are referencing. If she just feels off and gets minor illnesses this time of year, it's not going to help that.