The flu is a respiratory illness caused by the influenza virus. Symptoms of the flu are typically more severe than those of a cold. However, a vaccine is available that can keep the effects and risks of the flu at bay.

This common infection can also be life-threatening in some cases. According to the Centers for Disease Control and Prevention (CDC), hospitals admit 200,000 people each year for flu-related problems.

Young children, older adults, pregnant people, and individuals with weak immune systems may be more likely to experience dangerous complications.

A flu shot contains weakened or inactivated flu viruses that bring the immune system into action without causing illness. These viruses instruct the immune system to make special proteins called antibodies.

The body stores antibodies and can use them to fight off a future flu infection. As a result, a person might be able to avoid the flu completely after receiving the shot, or only get a mild case.

Keeping an eye on hygiene and staying home when sick can help prevent the flu. However, most people should also get an annual flu shot.

This article explains the facts about how they work, their level of safety, any side effects.

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The flu shot is completely safe.

The CDC states that flu shots have a long history of safe use.

The shot is recommended for people age 6 months and older, with only a few exceptions.

The following people should talk to their doctor before getting the shot:

  • those who are allergic to any ingredient in the flu shot or have had severe allergic reactions to the vaccine in the past
  • those who have had Guillain-Barré syndrome, a rare but severe paralyzing illness
  • people with a current illness, such as a fever

Egg allergies and the flu shot

Most flu shots contain a small amount of egg protein, and egg-free shots are available for those with severe egg allergies.

However, the American Academy of Allergy Asthma and Immunology (AAAAI), the American College of Allergy Asthma and Immunology (ACAAI), and the American Academy of Pediatrics (AAP) all agree that people with egg allergies can receive the flu shot without experiencing a severe reaction.

Pregnant women and the flu shot

The flu shot is safe and highly recommended for women during pregnancy.

It can be given anytime during pregnancy. Women who are pregnant may be more likely to have serious complications of the flu due to a higher strain on the heart, lungs, and immune system.

A study in the New England Journal of Medicine suggests that a mother might pass some of the protection of the flu shot on to her fetus. As infants cannot get the flu shot until they are 6 months old, this can help protect them during the interim period.

The effectiveness of the flu shot can vary widely from year to year and depends on two main factors:

  • the health and age of the person getting the shot
  • how well the shot matches the flu strains active in that year

How health affects the flu shot

The flu shot seems to function better in adults and older children. People over the age of 65 years tend to have weaker immune systems, and the shot might be less effective for them.

Children under the age of 2 years and people with long-term health conditions might respond less to the shot and receive weaker protection. A person can receive the shot from the age of 6 months onward.

The CDC recommends yearly shots for children under the age of 2 years and over the age of 65 years, especially as they are most likely to experience serious complications from the flu.

Matching the shot to the strain

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The vaccine must match whichever flu strains are active in that season.

Every year, new strains of the flu spread around the globe. There are hundreds of different strains, but the manufacturers of influenza vaccines can only include 3 or 4 types in the shot each year.

Medical researchers must narrow it down to the strains that are most likely to make people sick. A few months before flu season arrives, researchers study the flu strains that were most common the year before.

They also examine strains that are spreading in other parts of the world. They use this data to predict which strains of flu will affect people during the upcoming flu season.

Sometimes, experts can accurately predict which strains of flu will spread, and the shot is considered a good "match." When this happens, the vaccine offers more protection for those who receive it.

The 2011 to 2012 flu shot was a good match, and a study in Clinical Infectious Diseases states that it was 71 percent effective that year.

However, in other years, the shot may be a poor match. This happens when flu predictions are inaccurate or the virus changes before flu season begins.

Even when the virus is a poor match, however, the shot may still be helpful. During the 2014-2015 flu season, for instance, one of the viruses mutated, leading to a less effective flu shot match. It was also considered to be a particularly severe flu season.

Despite these problems, the journal Open Forum Infectious Diseases reports that the shot was 41 percent effective for younger people, and 56 percent effective for people age 65 years and older.

Although side effects are usually very mild, the flu shot can cause pain, redness, or swelling at the site of injection. A small number of people may also experience body aches or a low fever.

In rare cases, the flu shot can cause a severe allergic reaction. When this happens, it usually occurs within minutes or hours after the shot is given. The following are signs that require emergency treatment:

  • wheezing
  • swelling in the face
  • hives
  • trouble breathing
  • feeling very weak or dizzy
  • paleness

As viruses in the shot are weakened or inactivated, the flu shot cannot give someone the disease. However, it remains possible to contract flu even after getting a flu shot.

This may happen as a result of infection with a strain that was not in the shot, or if a person gets the flu before the shot has taken effect.

Those who get a flu shot do not only protect themselves, but also those who might be more likely to experience more symptoms or die from the flu.

Infants younger than 6 months, people with long-term health conditions, and older adults may be less likely to get the flu when the people around them get the flu shot.

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Scientists have developed several types of flu vaccine.

Flu shots are available in two doses: Standard- and high-dose.

People under 65 years of age typically receive a standard-dose vaccine. Doctors designed the high-dose version of the shot for people aged 65 and older.

Most flu shots are trivalent, which means they contain three strains of flu. Newer vaccines are being developed with four strains, known as quadrivalent shots. Both are considered safe and effective, and the CDC recommend getting whichever type is available.

An intradermal shot is available for those who have a fear of needles. It uses a needle that is 90 percent shorter than the standard shot and is injected just under the skin instead of into the muscle. It is approved for people aged 18 to 64 years.

Although a nasal spray version of the flu shot has been available in recent years, a CDC vaccine advisory group reported that it is not effective.

Experts recommend people get the flu shot as soon as it becomes available each fall.

The shot takes effect in about 2 weeks, and flu season begins as early as October in some cases. However, people may still benefit from getting the flu shot later, as flu season typically peaks in January or February.

Getting the flu shot in the late winter and early spring months may still offer protection.

The flu shot is effective for about a year. This means people need a new shot for protection each flu season, even if the strains in the shot are the same.