While some vaccines provide immunity to illnesses for years or even a lifetime, flu shots are given annually. This helpful animated video explains the two factors that contribute to this. First, there are more than 100 influenza viruses, and the most common subtypes change from year to year.
Secondly, the flu virus mutates quickly, even forming hybrids strains.
To combat this, researchers collect samples all year round so that virologists can design next season’s flu vaccine. Twice annual meetings held by the WHO determine which four strains will be targeted by the next vaccine. These expert predictions are almost always correct. However, researchers still want to develop a universal flu vaccine that would cover all strains and possible hybrids. Until that point, scientists and doctors will continue to track influenza infections and
Once the strains are decided on, vaccines are created containing antigens that are the same as those on the flu cell. Patients receive the vaccine, and their bodies recognize antigens as invaders, which triggers an immune reaction, which explains the side effects one has after receiving any vaccine. The body creates antibodies that it can then call on after contracting the influenza virus.