The flu, or influenza, is a highly contagious viral infection that attacks the nose, throat, and lungs. Millions of Americans get the flu every year, especially during the height of flu season each winter. The flu can cause sore throat, runny nose, body aches, fatigue, fever, and chills, and often feels like an especially bad cold.
Unlike most colds, the flu can also lead to vomiting and diarrhea, and can cause serious respiratory problems, organ failure, and even death. There is no cure for influenza, and newer medications only serve to limit symptom duration and severity.
Vaccinations are our best protection against the harm that flu can cause to individuals and to communities. Both the U.S. Centers for Disease Control and Protection (CDC) and the World Health Organization (WHO) recommend that everyone older than six months get a flu vaccine each year.
Over the past half-century, hundreds of millions of Americans have safely received flu vaccines, and the safety of flu vaccines has been extensively researched. Each year, flu shots prevent millions of illnesses and doctor’s visits in the United States and worldwide.
Like any vaccine, there is the potential for people to develop mild side effects from flu shots, as well as rare cases of more severe reactions. In this article, I’ll explain how flu shots work and overview their potential side effects.
I’ll also discuss how to tell if you’re having an allergic reaction to a flu shot, and when you should talk to your doctor about your flu shot side effects.
What is the Flu Shot?
While it’s possible to get the flu after you’re vaccinated, it’s still worthwhile to get the shot because it can make any flu symptoms you do experience much milder. The flu vaccine works by introducing inactive versions of several flu strains to the body’s immune system.
The immune system then develops antibodies that will fight against the flu virus later on, in the event that you become exposed to influenza during flu season. The majority of flu vaccines are shots that are given with a needle, but there is also a needle-free nasal spray vaccine called FluMist.
The form of the influenza virus used in the flu vaccine cannot cause the flu. The virus in these vaccines is either dead (in the case of injected vaccines), or altered and made inactive (in the case of FluMist).
You may experience mild flu-like symptoms after a flu shot because of your body’s immune response, but these reactions are not caused by the vaccines giving you a flu infection. You also cannot spread any of these symptoms or side effects to others after getting the vaccine.
That’s because there is no active, live flu virus in any form of the vaccine.
Possible Flu Shot Side Effects
All medications and vaccines come with the chance of side effects. Usually, flu vaccine side effects are mild. They appear soon after vaccination, then go away on their own after a few days.
In fact, randomized, blind studies have found that patients tend to display the same frequency of side effects, regardless of whether they are injected with salt water, or with real flu shots. (The only significant difference was that flu shots led to higher rates of arm soreness and redness.)
The most common side effects that people experience after flu vaccines are all much less severe than actually getting sick with the flu.
Injection site reaction
After receiving a flu shot, you may experience pain, redness, or swelling in the area on your body where it was injected. This is one of the most common flu vaccine side effects. This kind of swelling is not an allergic reaction that can then spread throughout your body.
Instead, it’s a more “localized” immune system response that occurs wherever a needle punctures the skin and injects a vaccine serum into the body.
Dizziness or fainting
During the vaccination process, fainting is most often triggered by pain or anxiety. You should tell your healthcare provider if you start to feel dizzy or hear a ringing in your ears while you’re getting a shot.
You should also let them know if you have a history of fainting while getting shots. You might be able to get your shots lying down, or in a more secure sitting position. When some people faint, they may also twitch or jerk around.
This reaction is not a seizure, and it doesn’t cause permanent damage. It is merely a response that some people have to losing consciousness. The biggest danger when fainting is that you will injure yourself while falling.
Fever is one of the more common side effects of getting a flu shot. Fever usually appears in the first few days after getting the shot, and goes away just as quickly. This kind of fever comes from your body’s natural immune response to the flu vaccine, as it begins to produce protective antibodies against influenza.
Fever is also a symptom of the flu itself. But the fever that sometimes occurs as a side effect of a flu vaccine is different. It is not caused by the body fighting off an ongoing, contagious infection or illness.
Post-vaccine fevers are usually milder than the fevers caused by the actual flu virus, and should clear up in a few days.
Body or muscle aches are another common side effect of the flu vaccine. The aches themselves are a kind of muscle inflammation that is experienced as a persistent soreness in the body. It is a byproduct of the immune system’s response to the presence of the vaccine in the body.
Although this immune response may make you feel uncomfortable or achy, this side effect should lift in a few days. These muscle aches are usually milder than the body soreness that’s caused by an actual flu infection.
Life-threatening allergic reactions to the flu shot are rare. When allergic reactions do occur, they most often begin within a few minutes to a few hours of receiving a flu vaccine.
The most common cause of allergic reactions to flu vaccines is egg allergies. In the US, most doses of the flu vaccine are manufactured using a process that involves fertilized chicken eggs.
As a result, some flu vaccines contain trace elements of egg protein. This process has started to change. As of 2017, there is minimal risk for patients with egg allergies.
According to the CDC, most people with mild to moderate egg allergies can safely take any kind of flu vaccine, without experiencing bad reactions or needing extra monitoring. People who have a history of more severe allergic reactions to eggs (symptoms that go beyond hives or itchiness, such as shortness of breath) should only be vaccinated in a supervised medical setting.
People with severe egg allergies are also good candidates for the flu vaccines that are produced every year using alternative (egg-free) production methods. All of these production methods have been shown to yield equally safe and effective vaccines.
People with severe, life-threatening allergies to any ingredient in a flu vaccine other than egg proteins (for example severe allergies to gelatin, or antibiotics), or a past history of severe allergic reactions to a flu vaccine, should not get that vaccine.
Guillain-Barré syndrome (GBS) is a disorder that damages the body’s nerve cells, causing muscle weakness and sometimes paralysis. While most people fully recover from GBS, some people experience longer-term nerve damage.
In rare cases, GBS can cause death. In the United States, an estimated 3,000 to 6,000 people develop GBS each year; most are over the age of 50. Some studies have found a very rare link between GBS and the flu shot; if it exists, the risk is estimated to be between 1 for 2 cases for every million vaccinated people.
GBS is more commonly observed following an actual influenza infection than after the vaccine. GBS has never been associated with the nasal spray vaccine. Some people with a past history of GBS should not get the flu vaccine.
Other possible side effects
For people who opt for the nasal spray vaccine, side effects might also include sore throat, cough, and runny nose. Young children who get their flu shot alongside pneumococcal (PCV13) or DTaP vaccines might have a slight risk of experiencing “febrile seizures.”
These kinds of childhood seizures are more commonly experienced during an actual flu infection than they are as a side effect of the flu vaccine.
How To Tell If You’re Having An Allergic Reaction
Following vaccination, signs of an allergic reaction can include:
- Difficulty breathing
- Hoarseness or wheezing
- Swelling around the eyes or lips
- A fast heart beat or dizziness
These symptoms would most likely emerge within a few minutes to a few hours after a vaccination is given. Although life-threatening reactions are rare, they require immediate medical attention to interrupt the body’s allergic response.
When to See a Doctor
If you are experiencing the signs of a severe allergic reaction, call 9-1-1 and get to the nearest hospital immediately. You should also tell your doctor if any post-vaccine symptoms end up becoming more severe or uncomfortable, or do not clear up after several days.
If you are ever treated for a severe allergic reaction or side effect tied to the flu vaccine, you or your doctor can file a report with the Vaccine Adverse Event Reporting System (VAERS). Your doctor might file this report.
You can also file it yourself through the VAERS website or by calling 1-800-822-7967. You may also be eligible for compensation through the National Vaccine Injury Compensation Program (VICP); claims can be filed at www.hrsa.gov/vaccine-compensation/index.html or by calling 1-800-338-2382.
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Frequently Asked Questions
K Health articles are all written and reviewed by MDs, PhDs, NPs, or PharmDs and are for informational purposes only. This information does not constitute and should not be relied on for professional medical advice. Always talk to your doctor about the risks and benefits of any treatment.
K Health has strict sourcing guidelines and relies on peer-reviewed studies, academic research institutions, and medical associations. We avoid using tertiary references.
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Seasonal Flu Vaccines. (2021).
Fainting (Syncope) after Vaccination. (2020).
Misconceptions about Seasonal Flu and Flu Vaccines. (2021).
Influenza (Flu) Vaccine (Inactivated or Recombinant): What You Need to Know. (2021).
Febrile Seizures Following Childhood Vaccinations, Including Influenza Vaccination. (2015).