Swelling

The swelling is usually a local phenomenon at the injection site, which lasts for about two to three days. In most cases, the tissue around the injection site is not only swollen, it also feels much firmer than the surrounding tissue. The swelling comes from the local reaction of the body to the flu vaccine.

In order for the vaccine, which is classified by the body as a potentially threatening pathogen, to be sufficiently attacked by the immune system, a large number of immune cells are required. These usually reach the injection site through increased blood circulation. Together with the cells, however, a lot of fluid is also transported into the tissue, causing swelling to occur there.

Pain

The pain after the flu vaccination usually only affects the injection site itself for a short time. In most cases, however, the muscle into which the flu 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 severe muscle ache. This side effect of the flu vaccination is also based on the desired effect that the immune system deals with the vaccine and fights it. An inflammatory reaction occurs both on the surface and deeper in the muscle, which is caused by the immune response to the flu vaccine. In the process, messenger substances are released which are supposed to induce further immune cells. These messenger substances, however, simultaneously trigger a signal at the pain-conducting nerve fibers, which arrives in the brain and is registered there as pain.

Inflammation at injection site

Inflammation at the injection site is desired to a certain extent, as it reflects the immune response to the flu vaccination. Therefore, the typical five inflammatory symptoms: redness, swelling, overheating, pain and limited functionality of the affected tissue can occur. However, slight impurities may additionally cause a proper inflammation of the injection site, which is not caused by the vaccine. For example, if the skin is not disinfected properly, superficial skin germs (especially the Staphylococcus aureus bacterium) can reach deeper skin layers and cause inflammation there. This usually lasts longer and is more pronounced than a pure reaction to the vaccine.