Flu Shot Side Effects: What To Expect After Your Jab

Flu season is upon us again. 

With up to 8% of people in the United States falling sick with the flu each season, getting your flu shot against the Influenza virus is an important consideration. 

The CDC recommends that, with few exceptions, everyone above six months of age gets an annual flu jab. 

But, as with every other medical intervention, the flu vaccination comes with side effects, risks, and benefits to consider. 

In this article, we tell you everything you need to know about flu shot side effects and answer some frequently asked questions.

What are the side effects of the flu shot?

The flu vaccine is available either as an injection or as a nasal spray. It is generally very safe. 

Side effects, such as pain, headache, and muscle aches, are typically milder than flu symptoms, and they tend to resolve after a day or two. 

These side effects are a normal part of your immune response to the vaccine. In other words, the shot is teaching your immune system how to react to the virus. 

Possible side effects of the flu jab include:

1) Soreness at the injection site

Soreness or pain at the needle injection site is a common side effect of most vaccines, particularly those – like the flu shot – that are injected into a muscle. 

The soreness occurs because your body sees the puncture as an injury and sends pain signals to your brain to inform you of the damage. 

It is a perfectly natural response that should resolve quickly as the inflammation stops and your body heals the area.

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2) Redness

Like soreness, redness is another common side effect of injectable vaccines. It occurs because your immune system is responding to the damage caused by the injection to your skin. 

Blood vessels around the area get engorged to allow more flow of blood and immune cells. 

In addition to redness, this increased blood flow can lead to warmth, and fluid leaking out of your blood vessels can lead to swelling of the area.

3) Fever

It is possible to have a low-grade fever (less than 101 degrees) after your flu shot. This is just another response of your immune system to the vaccine. 

A high fever is unlikely and may be a sign that you have a flu infection (the vaccine is not 100% effective, and it is possible to get a flu infection even after taking it) or another type of infection.

4) Muscle aches

Muscle aches are another common side effect you can get after a flu vaccination as a result of the immune response. 

They commonly occur more around the injection site, but they can also be more generalized.

5) Headache

Just like muscle aches, some people get headaches after flu vaccines. Fortunately, it is generally mild and resolves quickly.

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6) Fatigue or weakness

Because your immune system is working hard to produce antibodies, you may feel more tired than usual after your flu shot.

7) Fainting

Feeling dizzy or even outright fainting is a possible but uncommon side effect of the flu jab. 

In fact, fainting is a possibility with many vaccines. It is most likely due to the process of getting an injection and not because of the vaccine itself.

Fainting itself doesn’t cause much harm, but the falls that occur as a result can cause injury. 

To avoid this, healthcare providers normally ask you to sit down when getting the flu shot and will want you to stay around for some time after the jab.  

8) Allergic reaction

Some people may experience serious side effects or adverse reactions like an allergy to the vaccine. 

Signs of severe, life-threatening allergy include:

  • Difficulty breathing
  • Swelling around the eyes and mouth
  • Hoarse voice
  • Wheezing
  • Rash
  • Fast heartbeat

This is rare and usually occurs shortly after the vaccine is administered (within a few minutes to hours). 

If you have any of the above symptoms, contact the emergency services as soon as possible.

If you have a severe egg allergy, you may react to flu vaccines because most are made with egg-based technology. 

Let your healthcare worker know if you have an egg allergy, any other known allergies, or if you have had any severe allergic reactions to vaccines.

9) Guillian Barre Syndrome

According to the CDC, some studies have shown a possible association between the injectable flu vaccine and Guillian Barre Syndrome (GBS). 

GBS is a neurological condition in which a person’s immune cells attack their nervous system. 

The estimated risk is very low, however (one or two cases per one million vaccinated people).

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Does everyone have an adverse reaction to the flu vaccination?

No, not everyone will get an adverse reaction to the flu vaccination. Just like some people get only sniffles when they have a flu infection while others get seriously ill, everyone reacts differently to the flu vaccine.

Can the flu shot give you the flu?

No. The flu jab cannot give you the flu. The injectable vaccines contain either an inactivated virus or substances designed to be similar to the virus. 

The nasal sprays do contain live viruses, but they have been altered in a way that they cannot cause infections.  

Is it normal to feel unwell or get sick after the flu jab?

Yes, you may feel more tired than usual and get headaches, muscle aches, or a low-grade fever. All these are common side effects of the flu shot.

How long after a flu vaccine can you expect side effects?

The side effects usually occur within the first day or two after you receive the flu shot. They usually go away after two days. 

If symptoms persist, it may mean that you have some form of infection, and you should contact your healthcare provider.

Fainting usually occurs within minutes of getting the vaccine. If it occurs much later, it may be due to another cause, and you should seek medical advice.

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When to see a doctor if you are concerned

You may get the flu shot and not get any side effects, or the symptoms may be mild. Sometimes, they may be severe. 

You should see a doctor when:

  • You experience symptoms that persist for more than two days.
  • You have signs of a severe allergic reaction after taking the shot, such as difficulty breathing, swelling of the eyes and mouth, severe rashes and itching, wheezing, or voice hoarseness.


The flu vaccine aims to offer protection against flu infections, but its effectiveness may vary for different individuals.

It’s important to consult with healthcare professionals to understand individual considerations. 

As with any other vaccine, flu shots can cause side effects. Common side effects of flu jabs include pain and redness at the injection site, headache, and muscle aches.

These effects are generally mild and often resolve within a short period, typically two days.

If symptoms persist, it may be a sign of an infection, and you need to contact your doctor as soon as possible.  

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  1. CDC. Key Facts About Influenza (Flu).
  2. CDC. Prevent Seasonal Flu.
  3. CDC. Flu Vaccine Safety Information.
  4. CDC. Flu Vaccine and People with Egg Allergies.

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