I have never heard of a population wide mandate for a flu vaccine however.
And since 2004 the average effectiveness for the flu vaccine is about 40% so I guess if you are in a high risk group it might increase your survival a bit, but not enough effectiveness for a national mandate. The death rate of the flu is about 0.01% for all populations.
I mean, would you get a surgery that had a 40% success rate if your chance of dying without the surgery was 0.01%?