You may experience symptoms like fatigue, fever, and muscle aches after getting the flu shot because your body is responding to the vaccine. Rarely, these symptoms may be from the flu or complications.

It’s common to feel symptoms of sickness in the days following an injectable flu vaccine (flu shot).

In most cases, these are signs that your body is producing antibodies to help protect you against the flu virus. Flu shots don’t contain a live flu virus, which means they cannot give you the flu.

However, it’s possible to have flu-like symptoms in the days following the vaccine if you contracted the flu virus right before getting your flu shot. In rare cases, feeling unwell may be a symptom of a more serious complication.

Read on to learn more about the causes of feeling sick after the flu shot and how you can manage it.

It’s relatively common to experience mild side effects for 1 to 2 days after receiving the flu shot. The most common side effects include:

Emergency symptoms

In rare instances, more serious side effects of the flu shot may occur within a few minutes of getting the vaccination. This is usually due to an allergic reaction.

It’s important to get immediate medical attention if you or someone you’re with experiences:

  • trouble breathing
  • wheezing
  • vision changes
  • hives or a skin rash
  • paleness
  • swelling of the lips, eyes, or tongue
  • severe dizziness or fainting (not related to fear of needles)

It’s possible that your feelings of sickness after receiving the flu shot are, in fact, due to having the flu. But these symptoms will not be caused by the flu vaccine itself.

It can take up to 14 days after receiving the flu shot to be fully protected from it.

So, if you contracted the flu virus in the days before getting a flu shot, you may feel sick after a flu shot due to flu symptoms, not the vaccine. Flu symptoms take up to 4 days to appear after the virus enters your respiratory tract.

That said, it’s very unlikely that this situation will occur.

Yes, it’s still possible to contract the flu virus if you have received the flu shot. The flu shot is 40% to 60% effective at preventing the flu, depending on the year, population, and strain match.

That said, it’s important to note that the flu shot greatly reduces the severity of illness.

A 2021 study found that adults with an influenza-associated hospitalization who had a flu vaccine had a 26% lower risk of intensive care unit (ICU) admission and a 31% lower risk of death from the flu compared to those who did not receive the vaccine.

Guillain-Barré syndrome (GBS) is a rare autoimmune condition that occurs when your immune system mistakenly attacks the healthy cells in your nervous system.

According to the CDC, symptoms may develop within a few days or weeks after receiving the flu shot. The most common symptoms include a tingling sensation and muscle weakness that begin in the feet and move up the body, affecting the muscles in your arms, face, and lungs.

According to the CDC, fewer than 2 people in 1 million people in the United States who have had the flu shot developed GBS.

Anybody who receives a flu shot may experience mild side effects and feelings of sickness for 1 to 2 days afterward. Remember, if you’re not feeling well after a flu shot, it doesn’t mean that you have the flu.

Rather, it’s a typical immune response, which helps teach your immune system to recognize and quickly respond to the flu virus should you encounter it in public spaces.

That said, the CDC notes that some people cannot get the flu shot due to having a higher risk of developing severe and possibly life threatening complications. This includes people who:

  • are less than 6 months old
  • have an egg allergy
  • have an allergy to any other ingredients in the vaccine, such as gelatin and antibiotics
  • have experienced a severe allergic reaction to a flu shot in the past

It’s important to speak with a doctor if you’re unsure about whether you have risk factors that may increase your risk of developing complications.

If you’re feeling sick after receiving a flu shot, be sure to get plenty of rest. It can be helpful to treat the situation as if you do have the flu, which means it’s best to try:

You can also try an over-the-counter pain reliever to ease headaches or joint pain.

In most cases, people may feel sick or unwell for 1 to 2 days after getting a flu shot due to possible side effects like sore arm, fatigue, headache, or nausea.

While these can be unpleasant, it’s normal to experience these symptoms because your immune system is naturally creating antibodies to help protect your body against the flu virus.

In very rare cases, feelings of sickness may be related to a complication like anaphylaxis or GBS.

It’s important to get immediate medical attention if you experience symptoms like difficulty breathing, wheezing, or muscle weakness that starts in the feet and quickly moves up.

A doctor could help develop an appropriate treatment plan for you.