Can the Flu vaccine cause Flu?

No, it is a myth that the flu vaccine causes the flu.

Flu vaccinations cannot cause the flu because injected flu vaccines contain inactivated (killed) viruses rather than live viruses, and killed viruses cannot cause infection. The nasal spray vaccine given to children, does however contain a specially modified (“cold-adapted”) live virus: although deemed ‘live’, it cannot survive at normal body temperature and is therefore also incapable of causing infection.

The “myth” probably arises from the fact that some people experience side effects of the vaccination. Side effects are usually mild and include soreness, redness, and/or swelling where the shot was given, and less often, headache, (low grade) fever, nausea, muscle aches, and fatigue, but only last a day or so. Since these can mimic symptoms of flu, confusion may arise. While some people may feel slightly unwell after having the flu jab, the simple fact is that it does not give you the flu. 

Another confounding factor is that colds and flu are more common around the time that many people receive their flu vaccine, so a subset of people will have their flu jab while already in the incubating phase of an infection they have already acquired. It is natural – but incorrect – to suspect a connection.