Can The Flu Shot Cause The Flu?

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Photo by Diego Cupolo via Flickr. Creative Commons License
Photo by Diego Cupolo via Flickr. Creative Commons License

Jenny T. Asks:  I’d love to see a detailed discussion on why the flu vaccine can not possibly cause the flu. This is the number one excuse I hear over and over again.

Our Answer: Hi Jenny, that is indeed a concern we’ve heard and it’s one that we think we can address pretty well. It helps to know how flu vaccines are made. The injectable flu vaccines (shots) are all either inactivated (killed) viruses or recombinant (only fragments of a virus, not whole viruses). So there is absolutely no chance of them causing flu illness because they aren’t live and cannot replicate.

The FluMist consists of live, attenuated viruses, similar to MMR vaccine. FluMist vaccine viruses are cold-adapted and temperature-sensitive, which means they have been altered to the point that they cannot replicate at body temperature. They are also attenuated so that they cannot produce influenza-like illness even if they did manage to replicate a small amount in the body. So in a healthy person, there is no way any of the flu vaccines can cause flu illness. In some special populations, it is recommended (for extra precautionary measures) to get the flu shot instead of the FluMist.

People may indeed get the flu soon after getting the vaccine but that could be due to 1. being infected with wild-type flu virus just prior to getting the vaccine or within 2 weeks of getting the flu vaccine, which may be before the body has produced flu-fighting antibodies, or 2. having a bad cold that the person attributes to the flu but is not actually caused by the flu virus. The flu vaccine does not protect against the common cold, bronchitis, or other illnesses that may be mistaken for the flu. And 3. it is also possible to catch a different strain of the flu virus that was not covered by the vaccine, however that would still be due to wild-type flu virus and not caused by getting the flu vaccine. In fact, even if one gets the flu after having been vaccinated, symptoms are typically less severe than if the person did not get the flu vaccine.

Here’s some more detail on how the flu vaccines are made.

Here’s a good comparison of symptoms between influenza and the common cold.

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