The side effects of the flu shot and nasal spray, and when your symptoms may be the sign of an allergic reaction

• The side effects of the flu shot are soreness at the injection site, headache, mild fever, and nausea.
• The side effects of the nasal spray are runny nose, wheezing, sore throat, headaches, fever, and nausea.
• Most side effects are mild, so look out for symptoms like trouble breathing, weakness, and dizziness, which may be the sign of an allergic reaction.
• This article was reviewed by Jason R. McKnight, MD, MS, a family medicine physician and clinical assistant professor at Texas A&M College of Medicine.

The flu shot has some side effects like soreness around the injection site and headaches, but these are short-lived and should not deter you from getting vaccinated.

On rare occasions, the flu shot could trigger an allergic reaction that needs to be treated by a doctor.

Here's what you need to know about the side effects of the flu shot and how to tell the difference between a normal response and an allergic reaction.

The side effects of the flu shot

Some of the most common side effects of the flu vaccine include:

• Soreness, redness, a small rash, or swelling at the site of injection
• Headache
• Mild fever (less than 101 °F)
• Nausea or stomach pain

Side effects typically begin shortly after injection and should only last a few days, according to the CDC.

"Headache, fever, and sore muscles are not a sign of allergic reactions," says Michael McNeil, MD MPH, Team Lead for Vaccine Safety Datalink at the CDC's Immunization Safety Office. "These reactions can occur as a result of the patient's immune system responding to a vaccine."

Most of these side effects are unavoidable, but to avoid arm soreness, try taking ibuprofen two hours before you receive your vaccination.

And while some people believe you can catch influenza from the flu shot, this is not true, since the flu vaccine does not contain any active viral particles.

Side effects of the nasal spray

Getting a nasal spray flu vaccine may help you avoid some side effects of the flu shot, like arm soreness, but it comes with its own share of side effects.

Possible side effects from the nasal spray vaccine include:

• Runny nose
• Wheezing
• Sore throat
• Headaches
• Fever
• Nausea

Nasal vaccines are safe for most individuals between the age of 2 and 49, according to the CDC. But some groups who should not get the nasal spray include:

• People with a weakened immune system
• Those suffering from chronic diseases, such as asthma or type 1 diabetes
• Children ages 2-17 who are taking medications such as aspirin and salicylate-containing therapies
• Pregnant women
• Find a complete list of who should avoid the nasal spray vaccine on the CDC's website

Allergic reactions to the flu shot

Though the side effects listed above are normal reactions to the flu vaccine, it's important to monitor your response to the vaccine to make sure that you are not having an allergic reaction. Though allergic reactions are rare — they occur in an estimated 1.3 of every 1 million vaccinations — they can be serious.

Symptoms of a serious allergic reaction to a flu vaccine include:

• Trouble breathing
• Hoarseness or wheezing
• Pallor/paleness
• Weakness
• Fast heartbeat
• Dizziness

According to McNeil, these symptoms usually occur "within a few minutes to a few hours after receiving the shot and they require immediate medical evaluation and treatment."

The flu shot, like all vaccines, contains several components that can cause an allergic reaction. Most allergic reactions are caused by an allergy to egg protein, gelatin, or other additives in the vaccine. While the flu shot contains trace amounts of egg, research has shown that most people with mild egg allergies can receive influenza vaccines.

Serious non-allergic reactions

In extremely rare cases, the influenza vaccine can trigger Guillain-Barré syndrome (GBS), a neurological disorder that causes paralysis.

It is estimated that only one-or-two individuals develop GBS for every 1 million people vaccinated. GBS can occur days or weeks after a vaccination, and is experienced as an acute muscle weakness, difficulty controlling eye muscles, or difficulty swallowing.

Anyone presenting these symptoms should contact their doctor.

The bottom line

The flu vaccine — as a shot or nasal spray — can come with side effects including headache, fever, and nausea. However, it's still important to get vaccinated each year.

"Flu vaccines have a good safety record," says McNeil "Hundreds of millions of Americans have safely received flu vaccines over the past 50 years, and there has been extensive research supporting the safety of flu vaccines."

Influenza is a dangerous disease, particularly for the young, the elderly, and those with compromised immune systems.

Getting a flu shot will reduce your chance of catching the disease yourself, or passing it to those that are vulnerable. While there are potential side effects, they are minor and short-lived for most people and should not influence your plans to get a flu shot.

Related articles from Health Reference:

• How to prevent the flu, according to doctors
• Yes, the flu is contagious. Here's when you're most likely to spread the virus
• You can't get the flu from the flu shot, but there are side effects
• How long the flu should last and when you should see a doctor
• The flu vaccine is not 100% effective but you should still get it every year