..from experience this seems to not be very effective at all. My mother in law gets it every year and is ill for the next week after geting it, so she is basically making herself ill to stop getting ill. They still convince her shes too high a risk not to get it though
I got it when pregnant, and a couple of months later was struck down with flu for the first time in my life.
So yeah, I won't be inflicting this on my kids tbh.
Its possible we were just unlucky, but there seems many many stories the same too.