It is normal to experience some mild side effects, such as a headache or low-grade fever, after receiving the flu shot. This is by no means a cause for concern.

The flu is a contagious infection caused by a virus. The virus that causes the flu spreads through droplets expelled from the mouth and nose during coughing, sneezing, and talking. People with the flu may have upper and lower respiratory tract symptoms.

Epidemics of the flu occur yearly in the fall and winter months in regions with temperate climates. To reduce the spread of the flu and the associated morbidity and mortality, doctors recommend that eligible people get vaccinated each year.

Here, learn more about why people who got the flu shot yesterday are sick today.

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The flu shot may give people certain side effects, which are usually mild. People may report soreness, redness, and tenderness where they received the injection.

According to the Centers for Disease Control and Prevention (CDC), other side effects people report may include:

These effects may last 1–2 days. They are mild and much less intense than the symptoms of the flu.

The CDC report that participants in clinical studies experienced similar side effects if given a placebo or the flu shot except for increases in soreness and redness at the injection site. People who received a placebo experienced similar rates of body aches, fever, cough, runny nose, and sore throat.

Sometimes people may develop a serious allergic reaction to the flu vaccine, but this is rare. People with an allergy to a component of the flu shot may experience symptoms within a few minutes to a few hours after vaccination.

People do not get the flu from the flu vaccine. Injectable flu vaccines contain inactivated flu viruses that cannot cause the condition. The nasal flu vaccine contains processed live viruses, and these cannot transmit the flu either.

Trained medical personnel inject the vaccine into the deltoid muscle, in the uppermost part of the arm. If they do not apply proper technique and inject the vaccine into the shoulder joint, a person may develop a shoulder injury.

Other factors to consider when administering the flu vaccine include the size of the needle. The most appropriate length of the needle will depend on the size and build of the person receiving the vaccine.

Long needles can penetrate the muscle and reach the bone. Short needles may only penetrate the skin and inject the vaccine into the subcutaneous fat. When the needle does not reach or bypasses the muscle, the vaccine may not be as effective, and it may cause unnecessary discomfort.

Some people may develop flu-like symptoms after immunization. A fever is common in children and infants after vaccination. Doctors state that a fever after vaccination may be due to the immunization or may occur coincidentally with an infection.

A body temperature of 38–39°C helps strengthen the immune system response after vaccination. If a person develops a low-grade fever after vaccination, this means the immune system is active.

People may still get the flu despite having received the vaccine. A 2018 study showed that flu vaccinations prevent flu-associated admissions to the intensive care unit. Vaccines can also reduce the risk of severe flu symptoms in people who get vaccinated.

Learn more about the flu shot here.

Not everyone develops side effects after receiving the flu shot.

Among the most commonly reported side effects are:

  • soreness, redness, tenderness, or swelling at the injection site
  • low-grade fever
  • headache
  • muscle aches
  • nausea

A person who receives the nasal flu vaccine may experience additional side effects, such as:

  • runny nose
  • wheezing
  • headache
  • vomiting
  • muscle aches
  • fever
  • sore throat
  • cough

Like with other vaccinations and injections, some people may faint. It is also possible to develop an allergic reaction, but it is rare.

Some evidence suggests a possible association between the flu vaccine and Guillain-Barré syndrome (GBS). GBS occurs when the immune system attacks the nerves located outside the central nervous system.

However, some studies do not confirm this association. While GBS may occur after someone becomes sick with the flu, this is rare. Despite its rarity, GBS is more common after flu illness than flu vaccination. Doctors suggest that there is no link between nasal flu vaccine spray and GBS.

Usually, side effects from vaccination resolve without complications and last about 1–2 days.

A person should speak with a doctor if they experience:

  • a high fever
  • behavior changes
  • difficulty breathing
  • hoarseness or wheezing
  • swelling around the eyes or lips
  • hives
  • paleness
  • weakness
  • fast heartbeat or dizziness

People experiencing a severe allergic reaction to or serious side effects from the flu vaccine should call 911 to get to the nearest hospital as soon as possible. Anyone who develops the flu should also speak with a doctor.

The flu may present as mild to severe depending on several factors. It may also produce the following symptoms:

  • cough
  • fever
  • sore throat
  • muscle pain
  • headache
  • light sensitivity
  • eye pain
  • runny nose
  • congested eyes

In more serious flu infections, people may require respiratory intervention to ensure they are getting enough oxygen and removing carbon dioxide. Severe flu symptoms include:

Typical side effects after receiving the flu vaccine include soreness, tenderness, and redness at the site of the injection.

Some people, particularly children and infants, may develop a low-grade fever. This may be a sign that the immune system has responded to the vaccination.

Side effects after receiving the flu vaccine are rare and last only a few days. The side effects of the vaccination are much less severe than the symptoms of the flu.

People who develop a high fever, behavioral changes, or allergic reactions following administration of a flu shot should seek emergency medical attention.