Flu Shot Side Effects: What's Normal and What Isn't

A sore arm the day after a flu shot is so common that many people treat it as part of the deal. Most reactions to a flu vaccine are like that: mild, expected, and gone within a day or two. A few are not, and it helps to know the difference before you are at home wondering whether a headache or a warm, red patch on your arm is something to worry about.

This article sets out what the CDC describes as normal after a flu shot or the nasal spray vaccine, why the vaccine cannot give you flu, the rare reactions that need care, and how to report a problem. It can help you decide what to do next, but it cannot assess your own reaction. If you are worried, call your doctor or pharmacist.

What are the most common flu shot side effects?

The most common side effects of a flu shot are soreness, redness, tenderness, or swelling where the shot was given, according to the CDC. Some people also get a headache, muscle aches, a low-grade fever, nausea, or tiredness.

These reactions usually begin soon after vaccination and last 1 to 2 days, the CDC says, and most go away on their own within a few days without treatment. Many people who get a flu vaccine every year notice no side effects at all.

A yearly flu vaccine is one of the simpler routine steps covered in our physical health guide, and the CDC recommends one every season for everyone 6 months and older, with rare exceptions. Knowing what a normal reaction looks like makes it easier to spot the rare one that is not.

The CDC also notes that it takes about two weeks after vaccination for protective antibodies to develop. A sore arm or a day of feeling run down is not a sign that the vaccine has failed or that you are getting sick.

What side effects can the nasal spray vaccine cause?

The nasal spray flu vaccine can cause a runny or stuffy nose, sore throat, cough, headache, wheezing, vomiting, muscle aches, and fever, according to the CDC. Like reactions to the shot, these usually begin soon after vaccination and are mild and short-lived.

The nasal spray is made with live flu viruses that have been weakened, or attenuated, so they cannot cause flu illness. The CDC explains that these viruses are cold-adapted: they can multiply only in the cooler temperature inside the nose, not in the warmer lungs.

For many people ages 2 through 49, the nasal spray is an option. It is not recommended for everyone, though, including pregnant women, people with weakened immune systems, and young children with recent asthma or wheezing. The section below on who should talk to a doctor first covers this in more detail.

Because vaccine side effects are short-lived, a cough or sore throat that lingers well beyond a few days is less likely to be from the spray and may be a cold or another virus picked up around the same time. Our guide to home remedies for a stubborn cough covers what may help and when a cough needs a doctor.

Can the flu shot give you the flu?

No. The CDC is clear that flu vaccines cannot cause flu illness. Flu shots are made either with inactivated (killed) virus or with a single protein from a flu virus, so there is no live virus in them that could infect you.

So why do some people feel unwell afterward? The CDC points to several reasons:

• Side effects: a low-grade fever, headache, and aches can feel like the start of an illness, but they are much milder than real flu and fade within a day or two.
• A different virus: flu season overlaps with colds and COVID, and the flu vaccine protects only against flu. Our side-by-side look at cold, flu, and COVID symptoms explains how they overlap.
• Timing: you may have been exposed to flu shortly before your shot, or during the two weeks or so it takes your body to build protection.
• A mismatch: many flu viruses circulate, and you can still get sick if you meet one that is very different from the viruses in the vaccine.

The CDC also notes that flu vaccines vary in how well they work, but vaccination has been shown to reduce how severe illness is in people who still get sick. If you do come down with flu soon after your shot, you can still pass it on, so it is worth knowing how long the flu is contagious and when it is reasonable to go back to work or school.

Is it a normal side effect or something else?

Most reactions are normal if they are mild, start soon after vaccination, and fade within a few days. The table below sets common reactions against the ones that need a call or emergency care. It is a guide, not a diagnosis.

What you notice	Is it expected?	What to do
Sore, red, tender, or swollen arm where the shot was given	Yes, the most common reaction	A cool, wet washcloth and keep moving the arm; it usually fades in 1 to 2 days
Headache, muscle aches, tiredness, or nausea	Yes, common and mild	Rest; ask a pharmacist about an over-the-counter pain reliever if needed
Low-grade fever	Yes, in some people	Fluids and rest; call your doctor if it is high or does not settle
Runny nose, sore throat, or cough after the nasal spray	Yes, common with the spray	Usually mild and short-lived
Feeling faint or dizzy, vision changes, or ringing in the ears right after the shot	Can happen after any injection	Tell the vaccine provider right away and sit or lie down
Redness or tenderness at the site that gets worse after 24 hours	Not typical	Call your doctor
Side effects that are not going away after a few days	Not typical	Call your doctor
Hives, swelling of the face, lips, or throat, wheezing, or trouble breathing	No, possible severe allergic reaction	Call 911
New muscle weakness in the weeks after vaccination	No, rare	See a doctor promptly; call 911 if breathing is affected

How can you ease a sore arm after a flu shot?

A sore arm usually settles on its own within a day or two, and a few simple steps can make it more comfortable in the meantime. The two most useful are a cool compress and keeping the arm moving.

• Keep the arm moving. Gentle, regular use of the arm may ease soreness more than holding it still.
• Use a cool compress. A clean, cool, wet washcloth over the spot can soothe soreness and swelling.
• Check before taking a pain reliever. Ask your doctor or pharmacist whether an over-the-counter pain reliever is right for you afterward. Do not give aspirin to children or teenagers.
• Choose your arm. You can ask for the shot in your nondominant arm so any soreness gets in the way less.
• Rest if you feel run down. Mild tiredness or aches for a day are common and usually pass without treatment.

Soreness that spreads, or redness and tenderness that get worse rather than better after the first day, is not the usual pattern and is worth a call to your doctor.

What serious reactions can happen after a flu vaccine?

Serious reactions to flu vaccines are rare. The ones the CDC describes are severe allergic reactions, Guillain-Barré syndrome, fainting, and in young children, a small increase in fever-related seizures with certain vaccine combinations.

Severe allergic reaction

A severe allergic reaction, called anaphylaxis, is rare but can be life threatening. According to the CDC, it would most likely happen within a few minutes to a few hours after the vaccine is given. Signs include:

• Difficulty breathing, hoarseness, or wheezing
• Swelling around the eyes or lips, or of the face and throat
• Hives (raised, itchy welts on the skin)
• Paleness or weakness
• A fast heartbeat or dizziness

If you notice any of these, call 911 and get to the nearest hospital. If you are unsure whether a skin change that appears days later is hives or something else, our guide to common types of rashes shows how they differ, but hives with any breathing or swelling symptoms are always an emergency.

Guillain-Barré syndrome

Guillain-Barré syndrome (GBS) is a rare disorder in which the immune system damages nerve cells, causing muscle weakness and sometimes paralysis. Most people recover fully, though some are left with long-term nerve damage.

The evidence linking GBS to flu vaccines has varied from season to season. When an increased risk has been found, the CDC says it has been small, on the order of 1 to 2 additional cases per million doses. The CDC also notes that GBS is more common after flu illness itself than after flu vaccination, and it has not been associated with the nasal spray vaccine.

Fainting

Like other injections, a flu shot can occasionally cause fainting. The CDC thinks this comes from the vaccination process rather than the vaccine itself, and the main danger is injury from a fall. That is why experts recommend sitting or lying down for a shot and being observed for 15 minutes afterward.

Fever-related seizures in young children

In children 6 through 23 months old, the CDC found a higher chance of a seizure caused by fever when a flu shot was given at the same visit as the pneumococcal conjugate vaccine or the DTaP vaccine. The CDC describes the absolute risk as small. Tell your child's provider if your child has ever had a seizure.

Who should talk to a doctor before getting a flu vaccine?

Most people can get a flu vaccine without any special steps, but the CDC asks you to tell your vaccine provider beforehand if any of these apply:

• You have had an allergic reaction after a previous dose of flu vaccine, or you have any severe, life-threatening allergies, such as to gelatin or certain antibiotics.
• You have ever had Guillain-Barré syndrome.
• You are moderately or severely ill. People with a minor illness, such as a cold, can usually be vaccinated, but people who are moderately or severely ill should usually wait until they recover.
• Your child has ever had a seizure.

Children younger than 6 months are too young for a flu vaccine. Someone who had a severe allergic reaction to a flu vaccine should not get that vaccine again and might not be able to get other flu vaccines, so that decision needs to be made with a doctor.

What about egg allergy?

Egg allergy is not a reason to skip a flu vaccine. The CDC says people with egg allergy may get any flu vaccine, egg-based or not, that suits their age and health. Since the 2023 to 2024 season, no extra safety measures are recommended beyond those used for every vaccine, regardless of how severe a past reaction to egg was.

Who should not get the nasal spray?

The nasal spray has a longer list of people who should get a shot instead. According to the CDC, it is not recommended for children younger than 2, adults 50 and older, pregnant women, people with a weakened immune system, children 2 to 4 years old with asthma or wheezing in the past 12 months, children and teens taking aspirin or salicylate medicines, and people who recently took flu antiviral medicine, among others. People 5 and older with asthma, and anyone with a chronic lung, heart, kidney, liver, or metabolic condition, should talk with their provider first.

How do you report a reaction to VAERS?

You can report a reaction after any vaccine to the Vaccine Adverse Event Reporting System (VAERS), an early warning system run jointly by the CDC and FDA to watch for possible vaccine safety problems. Health care providers and vaccine makers are required to report certain events, and patients and caregivers can submit reports too.

Reports can be filed online through the VAERS reporting website or by phone at 1-800-822-7967, and your doctor may file one for you. VAERS is only for reporting reactions. Its staff do not give medical advice, so get care first if you need it.

Separately, the National Vaccine Injury Compensation Program is a federal program that compensates people who may have been injured by certain vaccines. Your doctor or the program itself can explain whether it applies to you.

When should you call 911 or a doctor after a flu shot?

Call 911 or get to the nearest emergency room right away for signs of a severe allergic reaction, which usually appear within minutes to a few hours of the shot:

• Difficulty breathing, wheezing, or hoarseness
• Swelling of the face, lips, eyes, or throat
• Hives, especially with any other symptom on this list
• A fast heartbeat, dizziness, paleness, or weakness
• Muscle weakness that is affecting your breathing

Call your doctor if:

• Redness or tenderness where you got the shot gets worse after 24 hours
• Side effects worry you or do not seem to be going away after a few days
• You or your child develops a high fever or noticeable behavior changes
• You notice new muscle weakness in your legs or arms in the weeks after vaccination

Afterward, report the reaction to VAERS, or ask your doctor to. If you are unsure whether a symptom is a side effect or the start of an illness, a quick call to your doctor or pharmacist can usually settle it.