Dr. Rae Osborn 20 Oct 2022

For most of us, a flu shot is not a problem but there are cases where an allergic reaction occurs. These reactions can vary from mild annoyances to dangerous effects. Read further to learn about the flu shot and what signs indicate an allergy.

What is a flu shot?

A flu shot is an injection or nasal spray that contains the influenza vaccine. Vaccination is recommended for most people to prevent severe illness from the flu virus. A new flu shot is made available every year. The reason this is necessary is that the virus actually mutates and changes the proteins on the outer coat of the virus. These two proteins of the influenza virus are called hemagglutinin and neuraminidase, abbreviated as H and N, respectively. 

Vaccines target these proteins and if they change, then a pre-existing vaccine no longer works. This is also how flu viruses are named. For instance, H1N1 flu; which is also called swine flu.

Who should get a flu shot?

People older than 6 months can receive a flu shot, although there are exceptions. People in high-risk groups such as those with heart or lung problems are strongly advised to get a flu shot. Note: It is essential you first check with your doctor before having any vaccine since they know your general state of health and if you may have an issue that makes vaccination risky.

Common side effects associated with the flu shot

There are some side effects that most people have which are not actually allergic reactions. These are normal responses; some of these effects may be delayed and appear a day or two after the shot. These side effects are as follows:

  • Pain in your arm: You will likely experience some discomfort and pain if you have the flu shot in injection form. This is why it is smart to use the arm that is not dominant as the site of injection. In other words, if you are right-handed, ask to have the shot in your left arm.
  • Redness and swelling in your arm: The skin of your arm where you have gotten the shot may redden and swell. This is also a common reaction to the flu injection.
  • Headache: You may find your head hurts for a day or two as the flu vaccine starts to trigger your immune system.
  • Aches and pains: General muscle pains and aches are also a common feature after a flu vaccine. This is also usually a sign that your body is mounting an immune response to the vaccine, which is expected.
  • Fever: You may develop a slight fever as your immune system responds to the flu vaccine.

What is an allergic reaction?

An allergic reaction is when your body reacts to some type of antigen you are exposed to, causing an immune response with noticeable symptoms. The immune system is supposed to react to the flu shot because the idea is for your immune system to create a memory of the particular virus or viral strains that the shot is designed for.

By having a memory of the virus your immune system can quickly mount a response if exposed to the actual virus. This means that you can fight off the virus without having any symptoms or only minor symptoms of the flu. The idea is to lessen the severity if you do catch the virus. In fact, this is how vaccination is supposed to work. However, in some people, the reaction is severe and abnormal and they actually have an allergic response.

Signs of a flu shot allergy

The signs of a severe allergic reaction to the flu vaccine are given and discussed below:

  • Edema: You may experience swelling around the lips and eyes. This can worsen causing your throat to swell up making breathing difficult.
  • Dizziness: This is when a person feels unsteady on their feet and the world seems to be spinning around them.
  • Hives: This is a rash on the skin that occurs soon after getting the shot. Hives appear as itchy raised red bumps on the skin.
  • Wheezing: You may find that you wheeze and your voice sounds hoarse.
  • Pale skin: You may experience pallor as the allergic reaction occurs.
  • Tachycardia: This is when the cardiovascular system is affected and the heart rate increases to above normal.
  • Breathing problems:  Any difficulty in breathing is an alarming sign of an allergic reaction that could be progressing to a dangerous state of anaphylaxis.

Allergic reactions usually happen quite soon after receiving the flu shot but there can be delayed reactions, though some of these are simply the side effects that are expected. Soon after getting a flu vaccine, you may see signs of an allergy and in extreme cases, anaphylaxis can occur.

Signs of anaphylaxis

It is rare for a person to experience an anaphylactic attack. However, you should consult with a doctor right away if you experience any severe signs of allergy that could worsen, endangering your life. Anaphylactic reactions are rare and account for about 1.3 in every million doses of the vaccine given. Anaphylaxis typically occurs soon after getting the shot (within 30 minutes); however, there can be a delayed reaction of a couple of hours.

Signs of anaphylaxis include the following:

  • Clammy skin
  • Rapid heartbeat
  • Extreme swelling of the face, eyes, lips, and throat
  • Difficulty breathing
  • Losing consciousness
  • Feeling anxious

Anaphylaxis is dangerous and can rapidly lead to death. A person needs immediate medical care if any of the above signs are noticed.

Flu shot allergy and eggs

A common reason for people having a severe reaction to a flu shot is that they have an egg allergy. Many flu vaccines are produced by growing the virus in chicken eggs. Unfortunately, this means if you have an allergy to eggs you likely will have an allergy to the flu shot. About 2% of children are allergic to eggs, which makes it more of a challenge when it comes to protecting this group from the flu.

People allergic to eggs should rather get an egg-free vaccine or a vaccine that only contains an egg content of fewer than 1.2 μg/ml. Always check with your doctor first if you plan on getting a flu shot and you do have an egg allergy.

Flu shot and allergy to other ingredients

Flu shots also contain other ingredients such as gelatin, which a person may be allergic to. Even if you have never had an allergy before, it can happen which is why you should be watched for 30 minutes after taking the shot to ensure there are no immediate reactions.

Flu shots and Guillain-Barré syndrome

Guillain-Barré syndrome is a neurological condition that can, on rare occasions, happen after receiving a flu shot. In the case of the swine flu vaccine, only about 2 cases per million of Guillain-Barré occurred.

In Guillain-Barré syndrome, the myelin sheath covering the nerve cells of the peripheral nervous system is affected due to an inflammatory response, and the person shows progressive paralysis. Scientists think this could be related to an autoimmune reaction in the person’s body. A person who has a flu shot and then has any unusual weakness and loss of feeling should see a doctor right away.

Treatment options for flu shot allergies

Signs of a minor allergy such as itching at the site and swelling in the arm can be treated using pain killers and anti-histamines. It is very important, though, to seek help if further allergy signs appear, including hives and particularly swelling in the face, and the other symptoms of allergy we have mentioned. An allergic reaction can progress to anaphylaxis which will require emergency care at a hospital. If unsure it is always best to check with your doctor who can advise you further.


A flu shot allergy is not common, and it is even rarer for a person to develop anaphylaxis or Guillain-Barré syndrome. However, you should see your doctor right away if you are worried and notice unusual symptoms after receiving the flu vaccine.

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About the Author:

Dr. Rae Osborn has a Ph.D. in Biology from the University of Texas at Arlington. She was a tenured Associate Professor of Biology at Northwestern State University where she taught many courses for Pre-nursing and Pre-medical students. She has written extensively on medical conditions and healthy lifestyle topics, including nutrition. She is from South Africa but lived and taught in the United States for 18 years.


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