Influenza causes 250,000-500,000 deaths each year worldwide. There is an effective vaccine to prevent or reduce the symptoms of influenza infection. However, because the influenza virus mutates rapidly, the major circulating strains are constantly changing. This means that the vaccine must also constantly change in order to remain effective against the most prevalent strains.
The WHO has the 411 on the flu strains currently causing trouble in the world. Every year, they meet with the Global Influenza Surveillance and Response System (GISRS) to discuss the influenza strains currently making their rounds around the round world. These agencies then use their crystal balls (and important scientific data) to make predictions about which strains are likely to continue circulating. They use this info to make a recommendation about which strains should be included in the upcoming influenza vaccine.
The vaccine prediction is usually accurate, but there have been several years where the circulating strains were different from the strains in the vaccine (making it useless). In 2009, when the H1N1 pandemic hit and spread around the world, the vaccine was ineffective because 2009 H1N1 was a completely new strain that was formed by a mix of several other viruses. In fact, they had to rush a new vaccine through production which was special for the 2009 H1N1 strain.