The most common side effects of the seasonal flu vaccination are redness or soreness at the injection site in around 10% to 40% of cases. Isolated symptoms, such as a high temperature, muscle pain or feeling slightly unwell are observed in 5% to 10% of cases. They are harmless and subside after a few days without any treatment.

If people have an allergy, in very rare cases they may experience a rash, oedema or breathing difficulties, and in extremely rare cases an immediate allergic reaction (anaphylaxis). In one in every million vaccinated people, a temporal association is observed between the autoimmune disease Guillain-Barré syndrome and the flu vaccination. However, GBS occurs much more frequently with flu.