we regularly face this issue every flu season. After we recommend the flu vaccine to a patient, they look us in the eye and say, “Thanks but no thanks. We got the flu vaccine years ago and caught the flu two days later. We was in bed for days!” we trust our patients, and I’m sure they were sick and in bed after the flu shot. But no matter how hard we try, we can’t convince them that their illness was a coincidence.
The simple truth is that the flu shot cannot cause the flu. The injectable flu vaccine is a killed-virus vaccine and is thus unable to give a recipient the flu. However the intranasal flu vaccine is an attenuated, live-virus vaccine and theoretically could give a recipient a mild case of the flu. The more common side effect, however, is merely nasal congestion.