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I've heard in many places that you can catch the flu from the flu shot. This is in contrast to most other vaccines for some technical reason. I read on Gizmodo today that:

The flu shot does not, and cannot, give you the flu.

Has this been tested? What are the results? Is such a strong statement, that it cannot happen, justified?

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    Folks, I've had to delete three theoretical answers that looked at the composition of vaccines and guessed the effect. We do not give medical advice here. If you want to answer that a medicine has a side effect (or does not), show a relevant study of side effects. Further theoretical answers will be deleted without warning. – Sklivvz Apr 07 '18 at 14:58
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    Please keep all comments on topic (the question). If you don't like the moderation complain using the contact form or go to meta which are the appropriate places. Further comments ignoring basic netiquette will be deleted. – Sklivvz Apr 07 '18 at 16:36

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Quoting from the US Centers for Disease Control and Prevention page Misconceptions about Seasonal Flu and Flu Vaccines:

Can a flu shot give you the flu?

No, a flu shot cannot cause flu illness. Flu vaccines given with a needle are currently made in two ways: the vaccine is made either with a) flu vaccine viruses that have been ‘inactivated’ and are therefore not infectious, or b) with no flu vaccine viruses at all (which is the case for recombinant influenza vaccine). The most common side effects from the influenza shot are soreness, redness, tenderness or swelling where the shot was given. Low-grade fever, headache and muscle aches also may occur.


Some people do get a seasonal flu vaccine and still get sick with flu-like symptoms. The same page explains this:

There are several reasons why someone might get flu symptoms even after they have been vaccinated against flu.

  1. One reason is that some people can become ill from other respiratory viruses besides flu such as rhinoviruses, which are associated with the common cold, cause symptoms similar to flu, and also spread and cause illness during the flu season. The flu vaccine only protects against influenza, not other illnesses.
  2. Another explanation is that it is possible to be exposed to influenza viruses, which cause the flu, shortly before getting vaccinated or during the two-week period after vaccination that it takes the body to develop immune protection. This exposure may result in a person becoming ill with flu before protection from the vaccine takes effect.
  3. A third reason why some people may experience flu like symptoms despite getting vaccinated is that they may have been exposed to a flu virus that is very different from the viruses the vaccine is designed to protect against. The ability of a flu vaccine to protect a person depends largely on the similarity or “match” between the viruses selected to make the vaccine and those spreading and causing illness. There are many different flu viruses that spread and cause illness among people. For more information, see Influenza (Flu) Viruses.
  4. The final explanation for experiencing flu symptoms after vaccination is that the flu vaccine can vary in how well it works and some people who get vaccinated may still get sick.
Lee Daniel Crocker
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David Hammen
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    Can you explain why "flu vaccine viruses that have been ‘inactivated’ and are therefore not infectious"? And possibly how "recombinant influenza vaccine" works without having the virus in it at all? +1 in advance. –  Apr 07 '18 at 18:38
  • I think to improve this answer, you should lift the two sources CDC provides and put them in this answer. –  Apr 07 '18 at 18:39
  • There is a tiny part of your post I don't agree with, and that is that your answer implies an impossibility where there is just an inprobability. The flu vaccine might just use the flu virus in a weakened form (http://www.vaccineinformation.org/how-vaccines-work/), and some human mistake or machine failure might just make it not weakened enough, efficiently infecting the patient. Even though normally the general public does not have to worry about this matter,because this happening is really improbable. I would still like you to include this point of view, since OP might not know how vac. work – SK19 Apr 07 '18 at 21:27
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    @SK19 -- The flu shot does not use a weakened form of the virus. It instead uses an inactivated (i.e., killed) form of the virus, or a recombinant vaccine that contains influenza virus proteins but no influenza virus, live or dead. The nasal vaccination contains live but weakened influenza virus, but this form of vaccination has fallen out of favor due to a reduced efficacy. This question is not about the nasal spray. It is about the flu shot. – David Hammen Apr 07 '18 at 23:28
  • @DavidHammen That takes out my skepticism :D – SK19 Apr 08 '18 at 06:24
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    @SK19 note that even in the case of the weakened virus, reversion to infection has only been observed with the polio vaccine and no other, so it's a risk, but there is no evidence that even the nasal version reverts to disease form. – Sklivvz Apr 08 '18 at 11:49
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    @fredsbend - I'd rather not add those references. The CDC is qualified to give medical advice while I am not. That's why I kept my answer to quoting their medical advice. – David Hammen Apr 10 '18 at 12:24
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    Once again, the expert scientists contradict my own personally observed behavior. I know that anecdotal evidence is frowned upon, but isn't that what skepticism is all about? What to do when a expert makes a claim (flue shots cannot cause the flu) that is false based on personal experience. Certainly, my own lying eyes must be to blame... – Michael J. Nov 14 '18 at 23:09
  • @MichaelJ. I don't take the flu shot because of exactly what I have seen. What do I risk, other than the flu? If I'm convinced the flu shot can give me the flu, when it's ironically supposed to prevent the flu, what other action can I reasonably be expected to take when the risks aren't that high in the first place? But some science dogmatists apparently just expect us to march down to walgreens and get a shot right now. No thanks. –  Sep 09 '19 at 23:11
  • @MichaelJ. - Lying eyes are certainly part of the problem here. Some people are afraid of going to see their doctor, period. Some of them cover this fear up with lies. Even more people are afraid of getting a shot, and some of them cover up this fear with lies. It doesn't take many flat out liars to convince other people that the flu shot makes one get get the flu, despite all of the evidence against this. Being on the receiving end of a lie is extremely contagious. – David Hammen Sep 10 '19 at 08:37
  • @fredsbend - Here's what you risk: – David Hammen Sep 10 '19 at 08:46
  • (1) Wasted time and money because you would be one of the 80% to 95% who never would have contracted the flu in the first place, depending on the severity of the season. You're playing Russian roulette once a year by not getting vaccinated. – David Hammen Sep 10 '19 at 08:56
  • (2) A rather small chance of getting some other disease while your slightly weakened immune system fights off what the perceived threat from the flu shot. This immune system reaction is the point of the vaccination. – David Hammen Sep 10 '19 at 08:56
  • (3) A very small chance that you'll develop an adverse allergic reaction to non-influenza related components in the shot. – David Hammen Sep 10 '19 at 08:56
  • (4) A ridiculously small chance of developing Guillain-Barré syndrome. This has not been ruled out. – David Hammen Sep 10 '19 at 08:57
  • (5) You'll get the influenza vaccination and some time later in that flu season you'll still get the flu. There are multiple forms of the influenza virus. The flu shot is an attempt to vaccinate the population against the forms of the virus that appear to be likely to strike. Sometimes the scientists who guess which forms will hit guess right, other times, not. To make matters worse, influenza viruses are incredibly bad at copying themselves. Even if those scientists did guess right, the virus can mutate itself over the course of an influenza season so as to be immune to the vaccination. – David Hammen Sep 10 '19 at 08:57