Some people who receive the flu shot may experience side effects. Often, these side effects are minor and temporary. Healthcare professionals recommend that those eligible for the flu vaccine get it annually.

The common side effects of the flu shot are much less severe than the symptoms of the flu and are unlikely to cause complications. However, in rare cases, people may experience serious side effects or allergic reactions to the flu shot.

This article will explain both the minor and serious side effects of the flu shot and how it works.

Person holding forehead because of a headache due to a flu shot.Share on Pinterest
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Some people may experience side effects after receiving the flu vaccine. These may differ from person to person and depending on the form of the vaccine.

Injectable flu shot

The side effects that most commonly occur with the injectable flu shot are local soreness, discoloration, and swelling at the injection site. Other possible side effects of the flu shot include:

Although it is uncommon, some people, especially children, may develop a low grade fever after vaccination, which is a response of the immune system. The cells of the immune system, which include lymphocytes and leukocytes, function better at a slightly higher body temperature.

The presence of a low grade fever after receiving a vaccination is a sign that the immune system is working. Anyone experiencing this symptom can treat it with over-the-counter medication.

Sometimes, people may faint after receiving an injection, including the flu shot. However, this is uncommon, and it is typically a reaction to the injection process rather than the flu shot specifically.

While not all experts agree, some evidence points to a small association between the flu shot and Guillain-Barré syndrome. Guillain-Barré syndrome occurs when the immune system attacks the nervous system. It is a rare condition but occurs more frequently in people who have had the flu shot.

Experts calculated the risk of developing Guillain-Barré syndrome after getting the flu shot. They estimated that the condition affects 1–2 people in every million who get the flu shot.

People should note that this complication is very rare and that the association is not well-documented.

Nasal flu vaccine

The nasal flu vaccine contains a live, attenuated virus that may cause some mild symptoms without leading to the flu. The side effects may include:

  • a runny nose
  • wheezing
  • headaches
  • vomiting
  • muscle aches
  • fever
  • a sore throat
  • a cough

Experts suggest that these side effects occur because the nasal vaccine contains live, weakened flu virus, as opposed to the inactivated virus in the flu shot.

Researchers have not seen an association between Guillain-Barré syndrome and the nasal vaccine.


Although people may experience side effects after receiving the flu shot or nasal vaccine, the side effects are less severe than the symptoms of the flu. Complications, hospitalization, and death can result from a flu infection, even in otherwise healthy people.

Conversely, flu vaccines are safe for most people. Over the past 50 years, hundreds of millions of people in the United States have received the flu shot without serious effects or complications.

The objective of flu vaccination programs is to limit the spread of the flu and establish herd immunity to protect those who cannot receive the flu vaccine. To establish herd immunity, a high percentage of people would need to get the vaccine, but this does not occur.

In the U.S., people may receive either a trivalent, inactivated influenza virus (TIV) vaccine or a live, attenuated, cold-adapted influenza virus (LAIV) vaccine. The TIV vaccine contains components of the killed virus, whereas the LAIV vaccine contains living, weakened virus.

Neither the TIV vaccine nor the LAIV vaccine causes the flu, but they both stimulate the immune system to provide protection against this illness.

Vaccination is an effective measure for preventing the flu and limiting its spread. People may also develop immunity by getting sick with the flu. However, as the flu can be serious, vaccination is the preferred route to immunity.

The two types of flu viruses that contribute to the annual flu epidemic are influenza A and influenza B.

You can learn more about these two types of influenza here.

Proteins called surface hemagglutinin surround the flu virus. These proteins attach to the cells in the human body and allow the virus to infect the person.

People develop immunity to the flu virus when the body creates antibodies against the surface hemagglutinin protein. When a person has exposure to the virus, antibodies against the surface hemagglutinin protect them by stopping the virus from attaching to the cells in the lungs and causing infection.

Vaccine manufacturers develop immunizations with this concept in mind. By exposing people to the killed or weakened viruses, the immune system can produce antibodies against the surface hemagglutinins.

Every year, people should contact a doctor or pharmacist to check whether they can receive the flu shot. The Centers for Disease Control and Prevention (CDC) recommend that anyone aged 6 months and above receive the annual flu vaccine.

However, some people should not get the flu shot, including those:

  • under 6 months of age
  • at known risk of life threatening allergic reactions to flu vaccines or ingredients in the vaccine
  • with severe egg allergies
  • with a history of Guillain-Barré syndrome

Doctors and other healthcare professionals can help people determine whether the flu shot is safe for them.

In rare cases, people may experience concerning side effects after receiving the flu shot, and they might require medical attention. Side effects that may be a cause for concern include:

  • high fever
  • behavioral changes
  • signs and symptoms of a severe allergic reaction

Severe allergic reactions are also rare, but people should look out for:

  • difficulty breathing
  • hoarseness or wheezing
  • swelling around the eyes or lips
  • hives
  • paleness
  • weakness
  • rapid heartbeat

These effects typically occur within minutes or hours of receiving the vaccine. Anyone experiencing these severe side effects should seek emergency medical attention.

Some people may experience mild side effects after receiving the flu shot. However, these side effects are significantly less severe than the symptoms of the flu. Even otherwise healthy people are at risk of complications, hospitalization, and death if they catch the seasonal flu.

In rare situations, people may experience severe side effects or allergic reactions. These effects typically occur immediately after vaccination, and doctors can manage them with medication. Anyone who has a severe reaction to a flu vaccine will not be eligible to receive another one in the future.

The more vaccinated people in the population, the less the virus spreads from person to person. Herd immunity protects people who cannot receive the flu shot.

The CDC recommend that people who can get the flu shot get vaccinated yearly.