People who got flu vaccine can also be infected with flu virus. Since the protection of the flu vaccination occurs in about two weeks, the disease may be seen in those who encountered influenza microbe before vaccination or before the vaccine provides protection.
In addition, because the influenza virus have different types, the disease may develop in a person who is in contact with a species that is not found within the seasonal vaccine. Even if the person has been vaccinated and a virus type that has been encountered existed in the vaccine, there is also the possibility that the vaccine may fail, especially in people with a weak immune system due to advanced age or chronic illness. The inactivated vaccine itself does not cause flu because it does not contain any live, reproducing viruses.