Each year, health officials zealously launch flu shot campaigns, aimed at vaccinating people against the influenza virus. But unlike other viruses, such as those causing measles and mumps, the flu virus mutates too quickly for the vaccine, which is why we have to get the shot every year.
To make a seasonal flu vaccine, every year scientists intensely study the health prospects in the Southern Hemisphere during its winter, which is still summer for us. Based on the viral strains circulating in that region, the scientists predict the flu strains that would make its way to our neck of the woods (the Northern Hemisphere) come winter. Because there are several strains of the influenza virus, what scientists predict will circulate can sometimes be different than what really does circulate. In such mismatch scenarios, the protection rate of the flu shot is lowered.
Of note, scientists are working to create a universal flu vaccine, which would recognize a common genetic portion of the flu virus that is immutable. This means no matter how the virus mutates from season to season, strain to strain, a single shot could protect us all.