Flu Vaccine: What Possible Reactions or Side Effects?

Expert answer:

The possible adverse reactions or side effects of influenza vaccine are:

  • Pain, redness and stiffness at the injection site: It occurs in 15% to 20% of people who take the flu vaccine and usually disappear spontaneously within 48 hours;
  • Abscess: They are usually associated with a secondary infection or technical errors of vaccine application;
  • Fever, malaise and muscle pain: They occur in less than 1% of people vaccinated. They may appear 6 to 12 hours after application and persist for 1 or 2 days. They are more common in individuals who have not had prior contact with the influenza vaccine antigens. Antigens are the substances responsible for the formation of specific antibodies in the body. In the case of the influenza vaccine, the antigens are dead viruses;
  • Anaphylactic reactions (hypersensitivity): They are extremely rare and can be caused by any component of the vaccine. Some influenza vaccines may have a minimal amount of egg protein, which can trigger immediate allergic reactions in people with a severe allergy to this protein.

It is important to remember that flu vaccine does not cause flu. Not even a "gripe". The viruses used in the vaccine are dead and are not capable of causing any infection.

The flu vaccine is safe and well tolerated by the vast majority of people. However, there should be some precautions in certain situations:

  • In case of moderate or severe febrile illness, it is recommended to postpone vaccination until the complete cure of the disease, so that the manifestations of the disease are not attributed to the vaccine;
  • People who are allergic to chicken eggs that only have hives after eating eggs can take the flu vaccine provided safety measures are taken. In this case, the person should be observed for at least 30 minutes after vaccination, in a place with conditions of care in case of anaphylactic reaction.

The family doctor or general practitioner can clarify any doubts about the flu vaccine and warn of its possible side effects.