Flu Shot Side Effects You Might Not Know About

The flu vaccine is a safe and effective way to protect yourself and others from influenza, but like any medical treatment, it can come with some side effects. The majority of side effects are mild and typically resolve on their own within a few days, and rarely require further evaluation such as X-rays or additional tests

The Side Effects of Flu Vaccine

The side effects of flu vaccine are usually mild and temporary, including arm soreness, fatigue, or low-grade fever. Severe reactions are rare but possible. Most people who get the flu shot don’t experience anything more than minor discomfort. Serious complications like allergic reactions occur in fewer than one in a million doses.

6 Most Common Flu Vaccine Reactions

Not everyone reacts the same way to the flu vaccine, but some symptoms occur more frequently than others. These are usually signs that your immune system is responding to the vaccine, which is a good thing.

After receiving the flu shot, it’s common to notice:

• A sore or tender arm at the injection site
• Redness or swelling around the injection area
• Mild muscle aches or headache
• Feeling tired or run-down
• Low-grade fever or chills

These effects generally start within a few hours and fade after one to two days. These are not signs of infection but normal side effects of the flu vaccine.

If you receive the nasal spray version of the vaccine (FluMist), your side effects may include:

• Runny or stuffy nose
• Mild sore throat
• Occasional cough
• Slight decrease in appetite (especially in children)

Why Do Side Effects Happen After a Flu Shot?

Side effects happen because your immune system is learning how to fight the flu. The vaccine introduces inactivated or weakened virus components (or in the case of newer versions, mRNA or recombinant proteins), prompting your body to build antibodies. This process can trigger temporary inflammation or symptoms that mimic a very mild flu — but it is not the flu itself.

These immune responses are more likely in:

• First-time flu shot recipients
• Young children with no prior flu exposure
• Adults with stronger immune reactions

In most cases, side effects of flu vaccination mean the vaccine is working.

Can You Be Allergic to the Flu Vaccine?

While rare, serious allergic reactions can occur. These typically happen within a few minutes to an hour after receiving the shot. If you’re allergic to ingredients in the vaccine, such as egg proteins (though many flu vaccines are now egg-free), you may be at a slightly higher risk.

Signs of an allergic reaction include:

• Hives or skin rash
• Shortness of breath
• Swelling of the face or throat
• Fast heartbeat
• Dizziness or fainting

If you experience these symptoms, seek emergency medical attention immediately.

Are There Any Long-Term Side Effects?

In the vast majority of cases, no. However, there is a very small risk of a neurological condition called Guillain-Barré Syndrome (GBS), which has been observed in approximately 1-2 cases per million vaccinated individuals. The connection is not fully understood, and the risk is far lower than the complications that can arise from getting the flu itself.

Important notes about long-term effects:

• There is no evidence linking the flu vaccine to chronic health problems.
• Monitoring systems like VAERS (Vaccine Adverse Event Reporting System) exist to track all possible side effects.
• If you have a history of neurological conditions, consult a healthcare provider before vaccination.

4 Side Effects of Flu Vaccine in Children

Children, especially those under five, may experience stronger reactions to the flu vaccine compared to adults. Their immune systems are still developing, which makes them more responsive to antigens in the vaccine.

Common pediatric side effects include:

• Irritability or fussiness
• Sleepiness or increased nap time
• Decreased appetite
• Mild fever

In rare cases, febrile seizures can occur in children with high fevers, although these are typically harmless and resolve without lasting effects.

Parents should keep an eye on their child after vaccination and consult a provider if they notice unusual symptoms lasting more than 48 hours.

How to Manage Flu Vaccine Side Effects

Managing the flu vaccine’s side effects is generally straightforward. Most symptoms resolve on their own, but here are a few ways to stay comfortable:

• Apply a cool compress to the injection site if sore or swollen
• Take acetaminophen or ibuprofen (only if needed and after checking with your doctor)
• Stay hydrated and get plenty of rest
• Monitor for any unusual or worsening symptoms

If you have a severe reaction or symptoms that persist beyond two to three days, visit your local urgent care center for evaluation.

Who Should Avoid the Flu Vaccine?

Most people benefit from flu vaccination, but some exceptions apply. You should speak with your doctor if:

• You’ve had a severe allergic reaction to a previous flu vaccine
• You have a history of Guillain-Barré Syndrome
• You’re feeling moderately or severely ill on the day of vaccination
• Your child is under six months old (too young to receive the shot)

Your provider may recommend an alternative type of vaccine or delay until you’re feeling better.

Flu Vaccine Side Effects That May Affect Daily Routine

Some side effects from the flu vaccine may temporarily affect your daily routine, but they’re usually mild and short-lived. After getting the shot, you might experience soreness or swelling at the injection site, low-grade fever, muscle aches, or fatigue. These reactions are normal signs that your body is building protection.

When to See a Doctor for Flu Vaccine Side Effects

While most flu shot side effects are manageable at home, some scenarios require medical attention:

• Symptoms lasting longer than 72 hours
• High fever above 102°F
• Signs of infection at the injection site (redness, pus, warmth)
• Trouble breathing or chest tightness
• Persistent vomiting or dizziness

If you’re not sure whether your symptoms warrant a visit, it’s better to err on the side of caution — especially if you have underlying conditions or are immunocompromised.

Should You Be Concerned About Flu Shot Side Effects?

The flu vaccine is one of the most effective tools in preventing influenza-related hospitalizations and deaths. While side effects of flu vaccine are possible, they are almost always mild and short-lived. Serious reactions are extremely rare and manageable when caught early. Educating yourself about what’s normal — and what isn’t — can help reduce anxiety and encourage more informed healthcare decisions.

Need Quick Relief or Professional Advice? Visit Us Today

Whether you’re dealing with flu vaccine side effects or simply need a quick check-up, Centers Urgent Care is here to help. We offer fast, high-quality urgent care services for both children and adults — with minimal wait times and compassionate care. Visit one of our urgent care centers in NYC to get personalized treatment, professional guidance, and peace of mind when you need it most. Contact us now to schedule your visit or walk in today.