Key Takeaways
- The most common side effect is soreness, redness, or swelling at the injection site.[1]
- Side effects are generally mild and resolve on their own within one to three days.[1]
- The flu shot cannot give you the flu; it contains inactivated virus or a single viral protein.[5]
- Serious reactions like severe allergy and Guillain-Barré syndrome are very rare.[2]
- People with egg allergy can receive any flu vaccine appropriate for their age.[4]
- Seek emergency care for hives, facial swelling, or difficulty breathing after the shot.[1]
Every fall, tens of millions of Americans get a flu shot, and a meaningful number of them feel something afterward: a sore arm, a little fatigue, maybe a low fever. Most of that is completely normal, and none of it is the flu. The CDC recommends the vaccine for everyone 6 months and older each season.[9] This guide explains what to expect after the shot, why it happens, how long it lasts, and the difference between a normal reaction and one that needs medical attention.
It is the companion to our Flu Season guide, which covers when to get vaccinated and how well the shot works. Here we focus on the side effects themselves.
Common Side Effects: What's Normal
The most common flu shot side effects are local reactions at the injection site, plus a handful of mild whole-body symptoms:[1]
| Side effect | How common | How long it lasts |
|---|---|---|
| Soreness, redness, or swelling at the injection site | Very common | Usually 1 to 2 days[1] |
| Headache | Common | Resolves within a day or two[6] |
| Muscle aches | Common | Resolves within a day or two[6] |
| Fatigue | Common | Resolves within a day or two[6] |
| Low-grade fever | Occasional | Short-lived, usually under 24 hours[6] |
| Nausea | Occasional | Short-lived[6] |
Many people report no side effects at all. When they do occur, they are a sign that your immune system is responding to the vaccine, not that you are getting sick.[1]
Can the Flu Shot Make You Sick?
No. This is the most persistent myth about the flu shot, and it is worth stating plainly: the flu shot cannot give you the flu. Injectable flu vaccines are made with inactivated (killed) virus or a single viral protein, neither of which can cause infection. The nasal spray vaccine contains weakened live virus that cannot cause flu illness in healthy people.[5][8]
So why do some people feel under the weather after the shot? The mild fever, aches, and fatigue are your immune system building protection, and they are far milder than actual flu. It is also easy to catch an unrelated cold or other virus around the same time you get vaccinated, which can make it feel like the shot made you sick when it did not.[5]
Sore Arm and Injection Site Reactions
A sore arm is by far the most common side effect, and it is nothing to worry about. The muscle reacts to the injection itself, causing localized soreness, redness, and sometimes mild swelling at the site.[1] It usually appears within hours and fades over one to two days.
A cool compress and gently moving the arm help. You can also use the arm normally; keeping it still tends to make stiffness worse, not better.[6]
How Long Do Side Effects Last?
Most flu shot side effects begin within hours to two days of the shot, peak on the first day, and fade within one to three days.[1] The timeline below shows the typical pattern:
If symptoms persist beyond a few days, or if they get worse instead of better, that is worth discussing with a clinician, since it may be an unrelated illness rather than a vaccine reaction.[1]
Why Side Effects Happen
Side effects are a sign that the immune system is doing its job. When the vaccine introduces an inactivated virus or viral protein, your body recognizes it as foreign and mounts a response, producing antibodies that will protect you if you encounter the real virus later. The soreness, mild fever, and fatigue are byproducts of that immune activation, not an infection.[1]
This is also why side effects tend to be mild and brief: the immune response is short-lived, and it resolves once protection is established, typically within a few days.
How Common Are They?
Side effects follow a clear gradient, from very common and mild to extremely rare and serious. The chart below puts that range in perspective:
The key point is that the common reactions are mild and self-limited, while the serious ones are rare enough that most clinicians will never see a case.[2]
Rare and Serious Side Effects
Serious side effects from the flu shot are rare. The table below contrasts what is normal with what warrants medical attention:[7]
| Reaction | Normal (no action needed) | Seek medical care |
|---|---|---|
| Injection site | Soreness, redness, mild swelling for 1 to 2 days | Severe pain, spreading redness, or swelling that worsens after 2 days |
| Fever | Low-grade, under 24 hours | High fever or fever lasting more than a couple of days |
| Allergic reaction | Not applicable | Hives, facial or throat swelling, difficulty breathing, fast heartbeat, dizziness |
| Neurologic | Not applicable | Weakness or tingling in the limbs (possible Guillain-Barré syndrome) |
Guillain-Barré Syndrome: What the Evidence Shows
Guillain-Barré syndrome (GBS) is a rare neurological condition in which the immune system attacks the nerves, causing weakness and sometimes paralysis. It is one of the things people worry about most when considering the flu shot, so it is worth looking at the actual numbers.[2]
GBS occurs in about 1 to 2 out of every 100,000 people in the United States each year, and most cases are not related to vaccination. Some studies have found a possible small association between the injectable flu vaccine and GBS, estimating fewer than 1 to 2 cases per million people vaccinated, while other studies have found no association at all.[2]
Here is the context that matters: the risk of GBS after influenza infection itself is higher than the risk after vaccination. Getting the flu is a far more common trigger for GBS than getting the flu shot.[3]
Allergic Reactions and Egg Allergy
Severe allergic reactions to the flu shot, including anaphylaxis, are very rare and usually occur within minutes to hours of vaccination. Signs include hives, swelling of the face or throat, difficulty breathing, a fast heartbeat, and dizziness. These require immediate emergency care.[7]
If you have an egg allergy, the guidance has changed in your favor. Since the 2023-2024 season, the CDC recommends that people with egg allergy may receive any flu vaccine appropriate for their age and health, without additional precautions beyond those recommended for anyone else. Studies show egg-allergic people are not at increased risk of severe reactions to egg-based vaccines.[4][10]
The one true contraindication is a history of a severe, life-threatening allergic reaction to a previous dose of flu vaccine or to a vaccine component. If that applies to you, discuss alternatives with your clinician.[7]
How to Manage Side Effects
Most side effects need no treatment and pass on their own. If they are bothersome, a few simple steps help:[6]
- Cool compress on the injection site for soreness or swelling.
- Move the arm normally rather than keeping it still.
- Rest and fluids if you feel tired or achy.
- Over-the-counter pain relievers like acetaminophen or ibuprofen can be used for discomfort, though routine use to prevent side effects is not recommended.
To be clear about the difference, here is how a normal vaccine reaction compares to actual flu:
| Feature | Flu shot reaction | Actual flu |
|---|---|---|
| Onset | Within hours to 2 days of the shot | Sudden, often within hours |
| Fever | Low-grade, if any | High, often 101-104°F |
| Body aches | Mild | Severe |
| Duration | 1 to 3 days | 5 to 7 days or longer |
| Respiratory symptoms | None | Cough, sore throat, congestion |
When to See a Doctor
Most flu shot side effects resolve on their own. Seek care if you experience any of the following:[1]
- Signs of a severe allergic reaction: hives, swelling of the face or throat, difficulty breathing, fast heartbeat, or dizziness
- A high fever or a fever that lasts more than a couple of days
- Severe or worsening pain, redness, or swelling at the injection site
- Weakness or tingling in the limbs
- Symptoms that get worse instead of better after a few days
Severe allergic reactions are a medical emergency. If you have trouble breathing or swelling of the face or throat, call 911.
Frequently Asked Questions
The most common side effects are soreness, redness, or swelling at the injection site, along with headache, low-grade fever, muscle aches, and fatigue. These are generally mild and resolve on their own within a few days.[1]
No. Flu shots are made with inactivated (killed) virus or a single viral protein, so they cannot cause flu illness. The nasal spray vaccine contains weakened live virus that cannot cause flu in healthy people.[5]
Most side effects begin within hours to two days of the shot, peak on the first day, and fade within one to three days. Soreness at the injection site usually resolves within one to two days.[1]
Arm soreness is the most common side effect and is a normal immune response to the vaccine. The muscle reacts to the injection, causing localized soreness, redness, and swelling that usually resolves within a day or two.[1]
Very rarely. Some studies have found a possible small association, estimating fewer than 1 to 2 cases of GBS per million people vaccinated, while other studies have found no association. GBS is far more common after influenza infection itself than after vaccination.[2]
Yes. Since the 2023-2024 season, the CDC recommends that people with egg allergy may receive any flu vaccine appropriate for their age and health, without additional precautions beyond those recommended for anyone.[4]
Signs of a severe allergic reaction (anaphylaxis) include hives, swelling of the face or throat, difficulty breathing, a fast heartbeat, and dizziness. These are very rare and usually occur within minutes to hours of vaccination; seek emergency care immediately.[7]
For a sore arm or mild aches, a cool compress and moving the arm help. Over-the-counter pain relievers like acetaminophen or ibuprofen can be used for discomfort, but routine use to prevent side effects is not recommended.[6]
Fatigue after vaccination is a normal immune response as your body builds protection. It is usually mild and resolves within a day or two, and is not a sign that you are getting the flu.[1]
A low-grade fever is an occasional, mild side effect that resolves within a day or two. It is not the flu. A high or persistent fever is unusual and worth discussing with a clinician.[6]
Seek care for signs of a severe allergic reaction such as hives, swelling of the face or throat, or difficulty breathing. Also contact a clinician for a high fever, severe or worsening arm pain, or symptoms that do not improve within a few days.[1]
High-dose and adjuvanted flu vaccines for adults 65 and older can cause slightly more injection-site soreness and mild systemic symptoms than standard-dose shots, but the reactions are still generally mild and short-lived.[7]
References
- Centers for Disease Control and Prevention (CDC). Flu Vaccine Safety. https://www.cdc.gov/flu/vaccine-safety/index.html
- Centers for Disease Control and Prevention (CDC). Guillain-Barré Syndrome and Flu Vaccine. https://www.cdc.gov/flu/vaccine-safety/guillainbarre.html
- Centers for Disease Control and Prevention (CDC). Guillain-Barré Syndrome (GBS) and Vaccines. https://www.cdc.gov/vaccine-safety/about/guillain-barre.html
- Centers for Disease Control and Prevention (CDC). Flu Vaccines and People with Egg Allergies. https://www.cdc.gov/flu/vaccines/egg-allergies.html
- Centers for Disease Control and Prevention (CDC). Misconceptions about Flu Vaccines. https://www.cdc.gov/flu/prevent/misconceptions.htm
- Centers for Disease Control and Prevention (CDC). Possible Side Effects from Vaccines. https://www.cdc.gov/vaccines/basics/possible-side-effects.html
- Centers for Disease Control and Prevention (CDC). Clinical Guidance for Seasonal Influenza Vaccine Safety. https://www.cdc.gov/flu/hcp/vax-summary/vaccine-safety.html
- Centers for Disease Control and Prevention (CDC). Key Facts About Seasonal Flu Vaccine. https://www.cdc.gov/flu/vaccines/keyfacts.html
- Centers for Disease Control and Prevention (CDC). Who Should and Who Should NOT Get a Flu Vaccine. https://www.cdc.gov/flu/prevent/vaccinations.htm
- Advisory Committee on Immunization Practices (ACIP). Prevention and Control of Seasonal Influenza with Vaccines: Recommendations, 2025-26 Influenza Season. MMWR. https://www.cdc.gov/mmwr/volumes/74/wr/mm7432a2.htm