Yes, the flu shot is the best basic level of insurance against the flue---the premium for which is the cost of the shot ($0.00 under my BC/BS policy).
Insurance allows you to spread the typical risks associated with living among entities that allow you to protect yourself against "externalities" which you might experience---and which are by definition, out of your control.
What companies you choose to pay a small fee to for assuming a designated percentage of the risk is up to you---and frankly, is your responsibility.
Choose wisely.