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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 7 '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 7 '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.
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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. – fredsbend Apr 7 '18 at 18:38
  • I think to improve this answer, you should lift the two sources CDC provides and put them in this answer. – fredsbend Apr 7 '18 at 18:39
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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 7 '18 at 23:28
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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 8 '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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protected by Sklivvz Apr 7 '18 at 14:54

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