I had a flu shot, and then I thought I had the flu - cough, red eyes, sore throat, fever, etc. Can a flu shot give you the flu?

Question

I had a flu shot, and then I thought I had the flu - cough, red eyes, sore throat, fever, etc. Can a flu shot give you the flu?

Answer

The flu vaccine is made of inactivated particles of the flu virus, called virions, and so it can't cause the flu. But sometimes, rarely, it produces symptoms very similar to the flu. It's called the occulo-respiratory syndrome.

The most commonly reported symptoms of this syndrome are respiratory (cough and wheeze, reported in 81% of cases, followed by red eyes (78%), and general symptoms - fever, chills, muscle aches, and fatigue - in 76% of sufferers. These symptoms can start as soon as 4 hours after the shot, and last for 24 hours.

Researchers looked for the cause of these symptoms, and came to the conclusion that they were associated with a high proportion of clumped unsplit virions in the vaccine. But how these clumps produce the symptoms remains a mystery. What's important is that they aren't nearly as bad as the real flu, they last a much shorter time, and only occur in a few people who get shots.