Flu shot side effects 2026–27: common reactions and when to worry

Common flu shot side effects

The most common flu shot side effects are a sore arm, mild fever, muscle aches, tiredness, and headache, and they usually fade within about two days.

Flu vaccines work by stimulating your immune system to recognize and fight the influenza virus. This immune response sometimes causes minor symptoms, indicating that your body is building protection. Common side effects typically appear within hours or a day after the shot.

Common mild side effects

• Soreness or redness at the injection site: This is the most frequent reaction, experienced by many people. The area might be tender, swollen, or warm to the touch.
• Mild fever: A low-grade fever can occur as your immune system responds to the vaccine.
• Muscle aches and fatigue: Some people feel tired or have minor muscle soreness after the shot.
• Headache: A mild headache may accompany other symptoms.

Adults 65 and older should preferentially get a high-dose (Fluzone High-Dose), recombinant (Flublok), or adjuvanted (Fluad) flu vaccine. If none of these is available, any other age-appropriate flu vaccine should be given.

All flu vaccines for the 2026-27 US season are trivalent (they protect against three flu strains: H1N1, H3N2, and B/Victoria). All three components were updated, including a new H3N2 component targeting subclade K (J.2.4.1, a recent branch of the H3N2 virus), the strain that drove the 2025-26 season. Our guide covers what's new in the 2026-27 flu shot.

How long flu shot side effects last

These effects usually last less than two days and do not require medical treatment. Over-the-counter pain relievers and plenty of rest can help ease discomfort.

When reactions may be more noticeable

In some cases, people may experience more pronounced reactions, especially if they have never received a flu shot.

An egg allergy alone does not call for extra precautions, and people with egg allergy can get any age-appropriate flu vaccine. If you have concerns about the flu shot or its side effects, ask a doctor or pharmacist before your appointment.

Can the flu shot make you tired?

Yes, the flu shot can make you feel tired, and fatigue (unusual tiredness or low energy) for a day or two is a common reaction. Your immune system is building protection against the virus, and that work can leave you drained, sometimes along with a mild fever or achy muscles. The tiredness usually starts within hours to a day after the shot and fades on its own.

A few habits can make it easier:

• Plan a lighter day: Get your shot when you can rest afterward if you need to.
• Drink fluids and eat regular meals: Your body has more to work with when it is fed and hydrated.
• Go to bed on time: Extra sleep supports your immune response better than pushing through.

Tiredness that lasts longer than a couple of days or keeps getting worse is not typical vaccine fatigue. The same goes for exhaustion that comes with a fever above 102°F (39°C), confusion, or trouble staying awake. These signs could point to an unrelated illness, so have a healthcare professional check them. If you have trouble breathing or swelling of the face or throat, call emergency services right away. Tiredness can also be one of the early flu symptoms, so noting when it started and how long it lasts can help you decide whether to call a healthcare professional.

Side effects by vaccine type (high-dose, adjuvanted, nasal, mRNA)

Side effects can differ a little depending on which flu vaccine you get.

• High-dose and adjuvanted shots (65 and older): Fluzone High-Dose and Fluad may cause more arm soreness, redness, or swelling than a standard-dose shot. The CDC suggests giving other vaccines in the opposite arm when you get them at the same visit. The soreness usually fades within the same one to two days as other flu shot reactions.
• FluMist and FluMist Home (nasal spray): There is no injection, so no arm soreness. The most common side effects are a runny or stuffy nose, sore throat, and a fever over 100°F.

FluMist Home is a needle-free nasal spray vaccine that you order online and have shipped to you. For the 2026-27 season it is available in the 48 contiguous states. Adults ages 18 to 49 can give it to themselves, and a caregiver can give it to children ages 2 to 17. People who are pregnant or have a weakened immune system should not use it.

• mFlusiva (a newer mRNA flu vaccine for adults 50 and older): FDA approved mFlusiva (Moderna), the first mRNA flu vaccine, on Aug 5, 2026 for adults 50 and older (full approval for ages 50-64, accelerated approval for 65 and older). It is not yet in CDC's vaccine recommendations and ACIP has not voted, so insurance coverage may vary. Check the product's current prescribing information or ask your pharmacist for the listed side effects before your appointment. This article does not list them, and they may not match those of other flu shots.

Whichever vaccine you receive, the warning signs in the "When to worry" section below apply the same way.

How to relieve flu shot side effects

Most side effects ease with rest, fluids, and a few home steps. Here are some tips to help you feel better after your vaccination:

Relieve injection site discomfort

• Apply a cool, damp cloth to reduce soreness and swelling.
• Keep the arm moving to prevent stiffness.
• Avoid heavy lifting or strenuous activity with the vaccinated arm for a day.

Ease fever and muscle aches

• Drink plenty of fluids to stay hydrated.
• Rest as needed to support your immune system.
• Take over-the-counter pain relievers such as acetaminophen or ibuprofen, following the recommended dosage.

Monitor your symptoms

Keep track of how you feel after the shot. Most reactions start within hours to a day and fade within about two days, so a quick check each morning and evening can show whether you are improving. If symptoms improve within a couple of days, there is usually no cause for concern. If they worsen or new symptoms develop, seek medical advice. What to avoid after a flu shot covers more ways to make the next day or two easier. If you're wondering whether you can drink alcohol after the flu shot, see our guide; until then, the rest and fluid tips above apply.

Accessing medical help for flu shot side effects

Not sure if a symptom is normal? A licensed doctor is available by video on Doctronic. A video visit is $39.

Using telehealth for flu shot side effects can help you avoid unnecessary worry and determine if further medical evaluation is needed.

When to worry: signs you need care

While serious side effects from the flu vaccine are very rare, it is important to recognize signs that require prompt medical attention.

Signs of an allergic reaction

Severe allergic reactions, known as anaphylaxis, can occur but are very rare, at roughly 1 to 2 cases per million flu vaccine doses. Symptoms typically develop within minutes to a few hours after the shot and may include:

• Difficulty breathing or wheezing
• Swelling of the face, lips, tongue, or throat
• Rapid heartbeat or dizziness
• Hives or rash spreading beyond the injection site
• Feeling faint or loss of consciousness

If you or someone else experiences these symptoms, call emergency services immediately.

Other concerning symptoms

Beyond allergic reactions, watch for symptoms that persist or worsen over several days, such as:

• High fever above 102°F (39°C)
• Severe headache or neck stiffness
• Persistent vomiting or diarrhea
• Confusion or difficulty staying awake
• Severe muscle pain or weakness

These signs may indicate complications or unrelated illnesses and should be evaluated by a healthcare professional.

Can the flu shot give you the flu?

No, the flu shot cannot give you the flu. Standard-dose, high-dose, and adjuvanted shots contain inactivated (killed) flu viruses, and the recombinant shot (Flublok) is made from a single flu protein produced in a lab, so none of them contain a live virus that can infect you.

The achiness, low fever, or tiredness some people feel afterward is your immune system responding to the vaccine. That reaction is the protection being built, and it usually fades within a day or two. Real flu tends to hit harder and last longer.

A few things can make it seem like the shot caused an illness:

• Timing: You may have been exposed to flu or another respiratory virus just before or after your appointment, before the vaccine had time to work.
• Other viruses: Colds and other respiratory infections circulate during flu season, and the flu shot does not protect against them.
• Strain match: You can still catch flu after vaccination, though the shot may make the illness milder.

The nasal spray is a different type, so check the vaccine type section above for its side effects.

Frequently Asked Questions

Can the flu shot give me the flu?

No. Flu shots contain no live virus; see 'Can the flu shot give you the flu?' above for why some people feel achy afterward.

How long do flu shot side effects last?

Mild side effects like soreness or low-grade fever typically last one to two days. If symptoms last longer or get worse, have a healthcare professional check you.

Who should avoid the flu vaccine?

Most people can safely get the flu vaccine. Those with severe allergies to vaccine components or a history of Guillain-Barré syndrome within 6 weeks of a previous flu vaccine dose should discuss risks with their doctor.

What should I do if I experience a severe reaction?

If you have difficulty breathing, swelling of the face or throat, or other signs of anaphylaxis, seek emergency medical care immediately.

Is it safe to get a flu shot if I am pregnant?

Yes, flu vaccination is recommended during pregnancy to protect both mother and baby from influenza complications.

The Bottom Line

Flu shot side effects are usually mild, including soreness at the injection site, low fever, muscle aches, tiredness, and headache. They typically start within a day and fade within about two days. Call a healthcare professional if symptoms worsen or persist, and call emergency services for trouble breathing or swelling of the face or throat.