Side effects of the flu shot


Influenza vaccination can cause a number of side effects. Most are based on a local or systemic inflammatory response to the vaccine and last a maximum of two to three days. As a rule, they do not involve any longer-lasting complications. More serious side effects are usually caused by allergies.

These can be directed against various ingredients of the flu shot. Allergies can also manifest as local reactions, in the worst case they trigger anaphylactic shock.

These are the typical side effects of a flu shot

The typical side effects of a flu vaccine are based on the local response of the body to the vaccine. This includes the redness of the puncture site as well as a swelling. Also pain at the puncture site and / or in the muscle into which the vaccine was injected are not uncommon. Diarrhea can also be caused by a flu shot.

If the entire body responds to the vaccine, it comes to tiredness, fatigue, fever and sometimes also body aches. Allergies to vaccine ingredients may also trigger allergic reactions, including severe life-threatening anaphylactic shock.


The redness after an influenza vaccination usually occurs directly at and around the puncture site. It is based on a local immune reaction of the body against the flu vaccine. In particular, many immune cells are washed into the area of ​​the puncture site, because there is a very high dose of the vaccine. In order to get all the immune cells there, an increased blood circulation is necessary.

This increased blood flow is also noticeable on the superficial smallest blood vessels, so that a reddening of the skin comes about. The reddened spot is usually warmed up as well. The overheating is also due to the immune response. The body tries to reach a temperature that damages the invading virus. So it should be weakened and the immune defense be relieved.


The swelling is usually a local phenomenon at the injection site, which lasts for about two to three days. Mostly, the tissue around the puncture site is not only swollen, it also feels much firmer than the surrounding tissue.

The swelling comes from the body's local reaction to the flu vaccine. In order for the vaccine, which is classified by the body as a potentially threatening pathogen, can be adequately attacked by the immune system, a variety of immune cells is necessary.

These usually get through an increased blood flow to the injection site. However, together with the cells, a lot of fluid is also transported into the tissue, causing it to swell there.


The pain after the flu vaccine usually only affects the puncture site for a short time. In most cases, however, the muscle in which the influenza vaccine was injected is also weakened for a few days. In addition, every movement and tension of the muscle is painful. The feeling is comparable to a strong sore muscles.

This side effect of the flu vaccine is also based on the desired effect that the immune system deals with the vaccine and combats this. It comes both on the surface and deeper in the muscle to an inflammatory reaction ( see also : muscle inflammation), which is caused by the immune response to the flu vaccine.

In the process, messenger substances are released, which are supposed to cause further immune cells. However, these messenger substances simultaneously trigger a signal on the pain-conducting nerve fibers, which arrives in the brain and is registered there as pain.

Inflammation at puncture site

Inflammation at the puncture site is desired to some extent because it reflects the immune response to the flu shot. Therefore, it can lead to the typical five symptoms of inflammation: redness, swelling, overheating, pain and limited functionality of the affected tissue. However, slight contamination may also result in proper inflammation of the puncture site which is not due to the vaccine.

So, for example, superficial skin germs (especially the bacterium Staphylococcus aureus) can penetrate into deeper skin layers and cause inflammation there due to an unclean disinfection of the skin. This usually lasts longer and manifests itself more as a reaction to the vaccine.


It is possible to be allergic to various ingredients of the flu vaccine. The most common is the allergy to chicken protein. This plays an important role, since the flu vaccines are produced on the basis of fertilized chicken eggs and therefore may contain traces of egg white.

An allergic reaction, on the other hand, can take any form from mild symptoms consisting only of a slightly increased localized inflammatory response, from a rash with itching to a dangerous anaphylactic shock.

Anaphylactic shock

Anaphylactic shock is the worst form of allergic reaction to an allergen. Contact with the allergen - in the case of the flu shot, this is one of the many ingredients of the vaccine - triggers an extreme immune system reaction. There is an excessive release of various messenger substances and immune signals.

In the case of anaphylactic shock, this leads to a destabilization of the circulation up to a veritable circulatory shock, which manifests itself by an enormously low blood pressure and a greatly increased pulse. In addition, the mucous membranes in the airways can swell so much that it comes to life-threatening breathlessness.


Influenza vaccination can also cause symptoms such as diarrhea in addition to local and systemic inflammatory responses. This side effect is ultimately due to the immune response. When recruiting the immune system against the flu vaccine, many different messengers are released. These lead to changes in the balance in the hormone balance.

Some of the affected hormones are responsible, inter alia, for the regulation of the water balance. If only little water is absorbed by the hormones around the intestine and a lot of water is released from the body, this leads to the typical thin and watery stool. However, by definition, diarrhea is only referred to when both consistency and stool frequency have changed significantly.

Fever after vaccination

After the influenza vaccination, systemic reactions of the immune system occur in addition to local inflammatory reactions. The fever is one of the most effective defense mechanisms of the body. The immune system identifies the processed viruses from the flu vaccine as potentially threatening pathogens.

Since most pathogens are mainly composed of proteins, ie proteins, their function is highly dependent on the prevailing ambient temperature. Therefore, the body tries to make the environmental conditions for the pathogens as negative as possible and increases the body temperature. This is how the fever comes about as a side effect of the flu vaccine. It usually lasts for a maximum of two to three days.

Head and body aches (muscle aches)

Headache and body aches are a typical symptom of the flu. In an attenuated form, they can also occur with the flu vaccine. The influenza vaccine is carried out with processed and thereby attenuated influenza viruses, so that the body gets to know the surface structure of the pathogens. It activates the immune system just as it would with the right flu.

However, since it is an attenuated pathogen, the physical response is not nearly as pronounced as with a flu. Nevertheless, symptoms such as fever, fatigue, headache and body aches can occur. Muscle pain is also typical, and this is particularly due to the muscle in which the flu vaccine was injected.

Guillain-Barre Syndrome

Guillain-Barré syndrome (GBS) is an inflammatory nerve disorder. The nerves, for example, can be temporarily damaged after an infection. Of these, especially the nerve roots that leave the spinal cord are affected by the inflammatory changes.

The main symptom is paralysis, which begins in the legs and continues to rise. Due to the inflammatory cause of the disease, GBS was also under discussion as a potential consequence of the flu shot. However, no association was found between the GBS and the vaccine.

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