Some travel medicine specialists consider flu vaccination to be the most important of all travel vaccines.
Protection against seasonal flu is recommended regardless of where you are travelling.
Respiratory infections account for over 25% of health problems while travelling, and you are three times more likely to contract a cold or flu when you are away from home. There have been well-documented outbreaks of flu on planes and especially on cruise ships, so flu vaccination is a precaution that is well worth taking.
Keep in mind that in tropical countries, flu is not seasonal, it occurs all-year round. Travellers to tropical countries are vulnerable to flu at any time of the year. And of course, in the Southern Hemisphere, flu seasons are 6 months out of phase with seasons in the North.
(If you find yourself in the Southern Hemisphere during their winter months, consider having a flu jab locally, since the recommended strains may be different.)