Flu vaccinations are approximately 70-80% effective, provided that there is a good match between the strains used to make the vaccine, and the strains of flu that are going around. There are sophisticated systems in place for monitoring circulating strains of flu around the world, so it is usually possible to predict which strains are most likely to cause trouble with considerable accuracy.
Protection is generally excellent against the strains from which the vaccine is made, but it is possible to still catch the flu after a flu jab.
No vaccine offers complete protection, but the good news is that any illness will almost certainly be milder than if you had not been vaccinated, and recovery should be faster. We strongly recommend getting the flu vaccine to protect against flu.