Why the Flu Shot Won't (and Can't) Give you the Flu

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It's that time of year again - time to get the flu shot. In case you are wondering when is the best time to get your flu shot, please read here

Every year, the medical community emphasizes the importance of getting the flu vaccine. And, every year the same excuses pop up as to why people are not going to do it. But, the one excuse that I simply cannot hear anymore is that the flu shot will give someone the flu. 

This one drives me crazy because it is simply not possible. Let me explain why. 

In order to make an effective flu vaccine, what has to be put into the body is the piece of the influenza virus that the immune system recognizes - this is called the antigen. Antigens are generally found on the outside of the virus and can be made into a vaccine in a few different ways. 

The flu vaccine, in particular, can be comprised of a cocktail of antigens that would contain no actual influenza viruses at all. In this case, there is absolutely no way that a person could get the flu from it because it does not contain any actual viruses.

The flu vaccine can also contain influenza flu viruses that have been inactivated. Because we don't consider viruses to be living - we can't say that they are dead either. But, an inactivated virus is like a dead virus. It cannot replicate and that means that it cannot set up an infection. 

Neither of the two ways that the flu vaccine is made could possibly result in a flu infection because there are no active influenza viruses in it. 

For those people who, in the past, have received a flu shot and got the flu a few days later - I have an explanation. It takes several weeks for the immune system to build up protection after the vaccine. Therefore, it is likely that the person encountered the flu virus sometime after receiving the flu shot and before having immunity. Although it looks like the flu shot is the culprit - it is more of a case of mistaken identity. 

More than one randomized, blinded studies have tested this. In two groups of people - one got flu shots and the other got saltwater shots. In the two groups, there were no differences in terms of body aches, fever, cough, runny nose or sore throat. (1) The only differences reported were in soreness of the arm and redness at the injection site. 

So, I get that adding "get the flu shot" to your endless list of things to do may not be your top priority. But, the flu is going to be bad this year and the vaccine seems to be a good match. But, whatever you do - do not use the excuse that the flu shot is going to give you the flu - because it can't. 



(1) Carolyn Bridges et al. (2000). Effectiveness and cost-benefit of influenza vaccination of healthy working adults: A randomized controlled trial.

Kristin Nichol et al. (1995). The effectiveness of vaccination against influenza in healthy working adults. New England Journal of Medicine. 333(14): 889-893.


* Three strains are included in the vaccine this year

  • an A/Michigan/45/2015 (H1N1)pdm09-like virus (updated)
  • an A/Hong Kong/4801/2014 (H3N2)-like virus
  • a B/Brisbane/60/2008-like (B/Victoria lineage) virus