For the vast majority of people, there is no downside to getting the flu vaccine. A gamble implies there's something to lose.

By getting the shot you need to spend time and money.

Insurance doesn't really change this, because if everyone did it, the premiums would be increased. (No chance of the former, so I guess that's just a tiny wager)

So yeah, you always stand to lose something. It might be worth it to you though

With gamble I meant that the vaccine being developed for the correct strain active in the flu season.