Common Flu Shot Side Effects To Know

Flu vaccines, like so many things in life, are not one-size-fits-all. In fact, there are several different kinds of influenza vaccines being offered each year. The vaccine may be given by injection or nasal spray. There are special dosages for the youngest recipients and the oldest. The vaccine can be made using an inactivated (killed) virus or an attenuated (weakened but live) virus. They may be produced using eggs to grow the virus…or not.

But these various versions of the flu vaccine are more alike than different. Because the flu virus can and does mutate, and more than one strain can circulate at any given time, scientists try to predict which strains of the virus will be most active for the upcoming flu season. And then, the flu vaccine is prepared to protect against those strains.

But even though the vaccines offer the same protection, they're not all made the same way or intended for the same people. And that means the side effects of flu shots may be slightly different.

Which vaccine you should get depends greatly on your age and other factors like health conditions and allergies. However, with few exceptions, it's important for everyone 6 months old and up to get vaccinated against influenza yearly.

Why Do Flu Vaccines Cause Side Effects?

All vaccines can cause side effects, and most of these symptoms are entirely normal.

"Side effects are basically telling you that your immune system is working," said Michael Knight, MD, a primary care physician and assistant professor of medicine at George Washington University Medical Faculty Associates in Washington, DC. "The immune cells are rushing to where the vaccine was injected to react to it."

In fact, local soreness at the injection site is the most common side effect after any vaccine jab.

Flu shots have been around since the 1930s and are considered extremely safe. Most side effects of the flu vaccine are mild and go away within a few days.

Read on to find out what you might experience after you get your flu shot. (One thing you absolutely can not get from the shot is a case of the flu!) For the vast majority of people, none of these possible—and mostly mild or rare—side effects are reasons not to be protected against influenza.

Flu Shot Side Effects

A standard-dose flu shot for people ages 6 months to 64 years old contains an inactivated (killed) influenza virus. The most common side effects from this vaccine include:

• Pain, swelling, or redness where the shot was given
• Headache or muscle aches
• Nausea
• Fatigue
• Mild fever

"Almost everybody gets a sore arm," William Schaffner, MD, medical director of the National Foundation for Infectious Diseases and professor of preventive medicine and infectious diseases at Vanderbilt University School of Medicine, told Health. "It's usually gone after a few hours, though some people still have a sore arm the next day. And there's always a small percentage of people, around 3%, who feel fatigued, aches, pains, headache the next day."

These side effects can occur from any injected vaccine, meaning that your immune system has been activated. However, not having these side effects doesn't mean your immune system isn't responding to the shot. Reactions vary from person to person.

Rare Flu Shot Side Effects

Fortunately, serious side effects from the flu vaccine are very rare. One is a very small increased risk of Guillain-Barré Syndrome (GBS), a neurological disease.

Guillaume-Barre syndrome develops after one or two out of every million vaccine doses.

"If someone has GBS within six weeks of receiving the flu vaccine, they shouldn't get the vaccine again," said Dr. Schaffner.

While some people who develop GBS suffer permanent nerve damage, most experience a full recovery. In addition, there's research to suggest that the risk of developing GBS is actually higher after getting the flu than it is from getting the vaccine.

Severe allergic reactions are "extremely rare," affecting fewer than 1 or 2 people in a million. Signs of a severe reaction can include:

• Difficulty breathing
• Hoarseness or wheezing
• Swelling around the lips or eyes
  
• Hives
• Paleness
• Weakness
• Rapid heartbeat
• Dizziness

These symptoms generally develop within hours of receiving the vaccine. Any time someone experiences such symptoms, call 911 or contact a healthcare provider immediately.

Flu Shot Side Effects for People 65 and Older

Two different vaccines are made specifically for people aged 65 and older. One shot contains the exact same dose of antigen (the part of the virus that triggers the immune response) as the injection for younger adults, with the addition of an adjuvant—an ingredient added to help spark a stronger immune response. The other shot is a high-dose version of the vaccine, with four times the amount of antigen.

Regardless of which of these vaccines is administered, the common side effects are the same for all inactivated flu vaccines: sore arm, perhaps some muscle aches, or a mild fever.

"All injectables have similar side effects," said Dr. Schaffner. The rare serious side effects are the same too.

Nasal Spray Flu Vaccine Side Effects

The nasal spray vaccine is available for non-pregnant people ages 2 to 49 who don't have serious, underlying illnesses. It's primarily used in pediatrics, though some adults decide they'd rather skip the needle if they can.

This vaccine isn't just an inhaled version of the injectable. Instead, it contains a weakened—not killed—version of the flu virus to stimulate the immune system.

The reason this vaccine doesn't cause the flu is that the virus is specially engineered so that it doesn't multiply once it's exposed to slightly higher temperatures inside the body.

The most common side effects of this vaccine include the following:

• Cough
• Headache
• Runny nose

Children may also experience wheezing, vomiting, muscle aches, sore throat, and low-grade fever.

The CDC says, "if these problems occur, they usually begin soon after vaccination and are mild and short-lived."

An Egg-free Vaccine Option

Most flu vaccines are made using eggs to grow the vaccine cells. Understandably, people with egg allergies have been nervous about getting the vaccine. However, most people with egg allergies will have no problem with the flu vaccine.

The CDC recommends that people who have had severe allergic reactions to eggs in the past receive their flu vaccines under the watch of a healthcare provider who can detect and handle severe allergic reactions.

People who have had severe allergic reactions (such as anaphylaxis) to egg-based flu vaccines are advised against receiving egg-based flu vaccines in the future. Fortunately, they can likely still receive one of the two flu vaccines that are not made using eggs. However, they should receive their vaccines under the supervision of a healthcare provider who can discern and manage severe allergic reactions.

You'll need to speak with a healthcare provider to determine which of the available flu vaccines is right for you.

A Quick Review

Getting a flu shot often means you have to endure an achy arm for a few hours. Some people may also experience side effects like headaches, nausea, and mild fever. Serious side effects are rare.

For most people, flu shot side effects are mild and fleeting, so why risk a case of the flu, which can lead to serious complications? Getting the flu shot is a lot safer than getting the flu.