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That response is the point of the vaccine, and it is also what creates most side effects:

• Local inflammation at the injection site causes soreness, redness and swelling.
• Immune signaling chemicals circulating in the body can cause a low fever, aches, fatigue or a headache for a short time.

Feeling a little tired or achy does not mean the vaccine “worked better,” and feeling nothing does not mean it failed. People respond differently, and both experiences are normal.

The Most Common Flu Shot Side Effects

The table below summarizes the reactions people report most often after an injectable flu vaccine, and how long they typically last.

Sore arm

The sore arm is the classic flu shot side effect. The muscle where the vaccine was injected becomes tender, and raising your arm or sleeping on that side may hurt for a day or so. Some people notice a small, warm, red patch or a little firmness under the skin.

What helps: Keep the arm moving gently rather than holding it still. A cool, clean compress on the area can ease tenderness. Many people find that using the arm normally the rest of the day reduces stiffness by the next morning.

Low-grade fever, aches and fatigue

A temperature slightly above normal, along with body aches, chills or tiredness, can show up the evening of the shot or the next day. This is your immune system activating, not the flu itself. It usually clears in one to two days.

What helps: Rest, fluids and a light schedule. Ask your provider or pharmacist whether an over-the-counter pain or fever reliever is appropriate for you, especially if you have kidney disease, liver disease, stomach ulcers, take blood thinners or are pregnant.

Headache and mild nausea

Some people get a dull headache or a queasy stomach the day after vaccination. These typically pass quickly. Staying hydrated and eating normally usually helps.

Feeling lightheaded or fainting

Fainting after any injection is more common in teens and young adults and is usually a reaction to the procedure itself rather than the vaccine. That is why many clinics ask you to sit for a few minutes after your shot. If you have fainted with shots or blood draws before, tell the staff so you can be vaccinated while seated or lying down.

Can the Flu Shot Make You Sick?

This is the question we hear most, and the short answer is no, a flu shot cannot give you the flu.

Injectable flu vaccines are made either with inactivated (killed) virus or with a single protein from the virus, and neither can cause an influenza infection. So why do some people feel sick afterward? A few reasons:

1. The normal immune response. The low fever and aches described above can feel flu-like, but they are short and mild and are not the infection.
2. Timing and coincidence. Flu shots are given in fall, the same season colds, COVID-19, RSV and other respiratory viruses start circulating. Catching an unrelated virus around the time of your shot is common.
3. Exposure before protection kicks in. It takes about two weeks after vaccination for your body to build protection. If you were exposed to flu just before or right after your shot, you could still get sick.
4. Strain mismatch. No vaccine is perfect. In some seasons, circulating strains differ from the vaccine strains, and vaccinated people can still get influenza. Even then, vaccination is associated with lower rates of severe illness and hospitalization.

What about the nasal spray? The nasal spray flu vaccine contains weakened live viruses that are designed not to cause the flu. It can cause a runny nose, nasal congestion, sore throat or mild fever for a short time. It is not recommended for everyone, including pregnant people and certain people with weakened immune systems, so ask your provider which type fits you.

How Long Do Flu Shot Side Effects Last?

For most people, side effects begin within 6 to 24 hours of the shot and resolve within one to two days. A sore arm can linger a little longer in some people, but it should steadily improve, not get worse.

As a simple rule of thumb:

• Day 0 (shot day): Arm tenderness starts. Possibly tired by evening.
• Day 1: Peak of any aches, low fever or fatigue. Arm may be most sore.
• Day 2: Most symptoms fading.
• Day 3 and beyond: You should feel back to normal.

If symptoms are getting worse after 48 hours, if a fever climbs high or lasts more than two days, or if redness at the injection site is spreading, contact a healthcare provider. Those patterns are not typical vaccine reactions and may point to something else, such as an unrelated infection.

What to Avoid After a Flu Shot

There is no long list of restrictions after a flu vaccine. Most people can return to work, school and normal activities right away. A few practical tips:

• Do not skip the waiting period if your clinic asks you to sit for 15 minutes. It protects you if you feel faint.
• Avoid rubbing or massaging the injection site hard in the first few hours, which can increase soreness. Gentle movement is fine.
• Do not push through a hard workout if you feel feverish. Light activity is fine; rest if you feel run down.
• Do not take extra doses of pain relievers beyond the label directions. Follow your provider’s advice.
• Drink enough fluids and get a normal night’s sleep.

Some people ask whether they can drink alcohol after a flu shot. There is no specific interaction, but alcohol can worsen dehydration and fatigue if you already feel achy, so moderation is sensible.

Can You Get a Flu Shot If You Have a Cold?

If you have a mild illness, such as a cold with a runny nose, without a fever, you can usually still get your flu shot. If you have a moderate or severe illness, with or without fever, your provider may recommend waiting until you feel better. This is not because the vaccine is dangerous during an illness, but so it is easier to tell vaccine side effects apart from symptoms of the illness.

If you are unsure, call the clinic before your visit. Our team can help you decide whether to come in now or reschedule for a few days later.

Rare but Serious Reactions: When to Get Help

Serious side effects from flu vaccines are rare. Knowing the warning signs still matters.

Severe allergic reaction (anaphylaxis)

Signs usually appear within minutes to a few hours of vaccination and may include:

• Difficulty breathing or wheezing
• Swelling of the face, lips, tongue or throat
• Hives all over the body
• A fast heartbeat
• Dizziness or weakness

If you have any of these signs, call 911 or go to the nearest emergency department immediately.

Guillain-Barré syndrome (GBS)

GBS is a rare neurological condition in which the immune system affects the nerves, causing weakness or tingling that usually starts in the legs. Some studies have found a very small possible increased risk after flu vaccination, on the order of one or two additional cases per million people vaccinated. Influenza infection itself is also linked to GBS. If you develop new weakness, tingling or difficulty walking in the weeks after vaccination, seek medical care promptly.

Shoulder injury from improper injection

Rarely, a shot given too high on the shoulder can cause ongoing shoulder pain and reduced range of motion. This is related to injection technique rather than the vaccine itself. Shoulder pain that does not improve after several days should be evaluated.

Who Should Talk to a Provider Before Getting a Flu Shot?

Most people 6 months and older are advised to get a flu vaccine every season. You should talk with a provider first if you:

• Have had a severe allergic reaction to a previous flu vaccine or one of its ingredients
• Have had Guillain-Barré syndrome within six weeks of a previous flu vaccine
• Are moderately or severely ill right now
• Are pregnant and want to discuss timing (flu vaccination is recommended during pregnancy)
• Are 65 or older and want to know about higher-dose or adjuvanted options

What about egg allergies? Current CDC guidance states that people with egg allergies can receive any flu vaccine that is otherwise appropriate for their age and health. You do not need extra precautions beyond those recommended for any vaccine.

Side Effects in Children

Children can have the same mild reactions as adults: a sore arm or leg, fussiness, a low fever or tiredness. Young children receiving a flu vaccine for the first time may need two doses given at least four weeks apart, and your provider will tell you if your child needs a second dose.

Call your child’s provider if a fever is high, lasts more than two days, or if your child seems very unwell, will not drink, or is unusually sleepy. Call 911 for any trouble breathing or signs of an allergic reaction.

Side Effects in Older Adults

Adults 65 and older are often offered a high-dose, adjuvanted or recombinant flu vaccine because these options are designed to produce a stronger immune response in older bodies. These vaccines may cause slightly more arm soreness or short-term achiness than standard-dose vaccines. Those reactions are still usually mild and short.

Flu Shot Side Effects vs. Actual Flu: How to Tell

If your symptoms include a cough, sore throat, high fever and severe aches lasting more than two days, you may have a respiratory infection unrelated to your vaccine. Our guide to COVID vs. flu symptoms can help you sort through what you are feeling, and testing can confirm the cause.

Is the Flu Shot Worth the Side Effects?

For most people, yes. A day of sore arm and mild tiredness is typically far easier than influenza, which can cause a week or more of illness and, for some people, serious complications such as pneumonia, hospitalization or worsening of chronic conditions like asthma, diabetes and heart disease. Flu vaccination also helps protect the people around you, including babies too young to be vaccinated and older family members.

The timing of your shot matters, too. Read when to get your flu shot for the ideal window, and if you have missed it, it is usually not too late.

Getting Your Flu Shot at Sunny Side Medical Center

Sunny Side Medical Center offers vaccinations, including seasonal flu shots, for children, teens and adults at our clinic at 3228 W. North Ave. in Chicago. We serve patients from Logan Square, Hermosa, Humboldt Park, Avondale and nearby neighborhoods.

When you come in, our team will:

• Review your health history and any previous vaccine reactions
• Help you choose the flu vaccine that fits your age and health
• Answer questions about side effects and what to expect
• Ask you to wait briefly after your shot so we can make sure you feel well

We welcome in-network and out-of-network insurance and self-pay patients. Coverage varies by plan, so contact us to confirm your benefits. Walk-ins may be available for vaccinations; call ahead to check.

Ready to get protected? Book an appointment or call 773-292-9660.

Frequently Asked Questions

How long does a sore arm last after a flu shot?

Usually one to two days. It should improve steadily. If pain, redness or swelling gets worse after 48 hours or spreads, contact a provider.

Can the flu shot make you sick?

No. Injectable flu vaccines cannot cause influenza. Short-term aches, tiredness or a low fever are the immune system responding, not an infection.

Is it normal to have a fever after a flu shot?

A low-grade fever for up to a day or two can be normal. A high fever, or a fever that lasts longer than two days, should be checked by a provider.

What should I avoid after my flu shot?

Avoid strenuous exercise if you feel feverish, avoid hard massage of the injection site, and follow label directions for any over-the-counter medicine. Otherwise, normal activities are fine.

Can I get a flu shot if I have a cold?

With a mild cold and no fever, usually yes. If you are moderately or severely ill, your provider may suggest waiting until you recover.

How long does flu shot protection last?

Protection builds over about two weeks and tends to decrease over the following months, which is one reason the vaccine is recommended every season.

Do flu shot side effects mean the vaccine is working?

Side effects show your immune system is responding, but having no side effects does not mean the vaccine failed. Both are normal.

Related guides: when to get your flu shot