Why You May Feel Sick After a Flu Shot

You may feel sick after a flu shot, not because it gave you the flu but because of side effects that make you feel unwell. These may include headache, muscle aches, chills, and feeling fatigued. While the vaccine is derived from strains of the influenza virus, they are killed (inactivated) and, therefore, incapable of infecting you.

You can still get the flu after vaccination against it. But if you do, it may be because you were exposed to the flu before or just after you received the shot. You may have also had a weak immune response to the vaccine or be sick with something else entirely.

This article explores the reasons why you may get sick after the flu shot. It also explains how the vaccine may help reduce symptoms and complications if you do end up getting the flu.

Flu Shot Side Effects

Not everyone experiences side effects after a flu shot, but they do occur. Side effects are similar in both adults and children and may include:

• Pain at the vaccine site, which may make it hard to raise or lower your arm
• Feeling fatigued and generally unwell
• Headache, chills, and muscle aches
• A low-grade fever

Flu-like symptoms such as fatigue, muscle aches, and fever typically affect less than 1% of those who get the flu vaccine.

Fever is more common in children than adults.

Allergic Reactions to the Flu Shot

It is possible to have an allergic reaction to the flu shot, but this is extremely rare. Serious allergic reactions are thought to occur only 1.3 times for every 1 million vaccines given. 

Symptoms of anaphylaxis, the most severe type of allergic reaction, may include:

• Trouble breathing
• Swelling around the lips and eyes
• Hives
• Wheezing
• Rapid heartbeat
• Dizziness or weakness
• Pale skin

These symptoms are most likely to occur within a few minutes to hours after you receive the vaccine. 

If you think you're having a severe allergic reaction to the flu shot, seek emergency medical help at once. After you have recovered, you should report the reaction to the Vaccine Adverse Event Reporting System (VAERS).

How Effective Is the Flu Shot?

The flu shot's effectiveness varies from season to season. Typically, people who have received a flu shot are around 40% to 60% less likely to get the flu. This number will be lower in years when the vaccine is not well-matched to the dominant flu strains.

It is important to remember that even if you do get sick after you've been vaccinated, you're likely to experience milder symptoms and recover more quickly than if you had not been vaccinated.

You will also be at less risk for developing serious complications such as:

• Pneumonia 
• Inflammation of the heart
• Worsening heart disease
• Inflammation of the brain
• Multiple organ failure
• Sepsis, a whole-body response to inflection

Children under the age of 5, pregnant people, people over the age of 50, and those with weakened immune systems are more at risk for developing flu-related complications.

Why Can You Still Get the Flu After a Flu Shot?

An annual flu shot offers you the best protection against the influenza virus and is recommended for most individuals 6 months and older. Still, it is possible to get the flu after you've had the shot.

There are many reasons why this might happen. The effectiveness of the shot depends on several factors including the timing of the shot and your body's immune response.

You Haven't Built Up Immunity Yet

It takes two weeks to build up your immunity to influenza after you get the shot. Immunity is how well your body is able to fight off illnesses.

If you get the flu within two weeks of getting the shot, you were probably exposed to the virus around the time you got your vaccine.

You Have a Different Illness

The flu shot does not protect against these flu-like illnesses:

• The common cold
• Pneumonia, although it may protect you from this infection as a complication of the flu
• Bronchitis, which is an infection or irritation of the airways
• Stomach flu

Each of these illnesses can cause symptoms that may be confused with the flu.

The Right Strain of Flu Isn't in the Vaccine

The flu shot provides protection against three or four specific strains, or types, of influenza. These strains are selected by researchers based on the likelihood of their prevalence in the upcoming season.

Because the flu virus mutates, or changes, new vaccines have to be made every flu season.

Despite their best educated guesses, researchers and public health officials may get it wrong. If that year's illness-causing strains of influenza are not included in the vaccine, people who get the flu shot may still get the flu.

You Didn't Respond Fully to the Vaccine

Your immune system may not respond fully to the vaccine, meaning you may still get the flu. This may be true if:

• You have an underlying medical condition that causes a weakened immune system, such as cancer or diabetes.
• The vaccine wasn't stored properly and, as a result, is not as effective.
• The vaccine wasn't given properly and, as a result, is not as effective.

Even if you don't respond fully to the vaccine, you are still less likely to have serious complications from the flu.

You're Over the Age of 65

Anyone over the age of 65 is considered high risk for the flu and associated complications. This means that the flu is more likely to lead to very serious illnesses and even death in this age group.

Despite the fact that older individuals' immune systems may not respond fully to the vaccine, it is still recommended that they get vaccinated every year.

Because of this, the CDC recommends that those over the age of 65 get a "high dose" flu vaccine. This type of vaccine is intended to give this age group a better immune response to vaccination and, therefore, better protection against flu.

Studies show that individuals age 65 and older who got the flu shot:

• Reduced the risk of flu-related doctor visits by 24%
• Reduced flu-related hospital visits by 33%
• Reduced the risk of associated illnesses by 60%

Common Myths

People sometimes avoid getting the flu shot because of misinformation. Some of the most common myths about the flu and the flu vaccine include:

• The flu is not serious: Most people recover from the flu in a few weeks, but for some people, the flu can lead to life-threatening complications. These complications can even happen in young, healthy adults.
• The flu shot doesn't work: It is possible to get the flu after you've been vaccinated, but this doesn't mean the shot doesn't work. In some years, the vaccine is less effective because it is not well-matched to all the circulating viruses. Even so, you will likely experience a milder illness if you have been vaccinated.
• The flu shot can give me the flu: The flu shot does not contain a live virus, so it won't give you the flu.
• Pregnant people shouldn't get vaccinated: It is safe to get a flu shot during pregnancy. In fact, it is especially important for you to get the flu shot if you are pregnant. This is because people who are pregnant are at greater risk for developing flu-related complications.
  

Summary

It's still possible to get sick after the flu shot, but it isn't because of the shot itself. You may get sick for a variety of reasons including the timing of the shot, how effective the shot is that year, or how well your immune system responds to the vaccine.

It is also possible that you contracted an illness other than the flu.

Vaccinated individuals who do get sick typically have less severe symptoms and complications. This is especially important to keep in mind for high-risk groups such as individuals 65 and older, as well as children.