The flu is a respiratory illness caused by the influenza virus. Symptoms of the flu are typically more severe than a cold. They include fever, coughing, body aches, headache, and tiredness that last up to 2 weeks.
Young children, the elderly, pregnant women, and those with weak immune systems may be more likely to suffer dangerous complications.
Proper hand-washing and staying home when sick can help prevent the flu. But health experts say that for the best protection, most people should also get an influenza vaccine, or flu shot, every year.
What is the flu shot?
The flu shot helps the body’s immune system fight off the flu before a person gets sick.
It contains weakened or inactivate flu viruses that make the body think it is being infected without causing illness. These viruses tell the immune system to make special proteins called antibodies.
The flu shot’s effectiveness depends on the age of the person and the strains of flu that are active that year.
The body stores these antibodies and can use them to fight off a future flu infection. As a result, a person who gets the shot may be able to avoid the flu completely, or only get a mild case.
Flu shot effectiveness
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 that are about that year
How health affects the flu shot
The flu shot seems to work better in adults and older children. People over the age of 65 tend to have weaker immune systems, and the shot may be less effective for them. Children under the age of 2 and people with long-term health conditions may also have a lower response to the shot and receive less protection.
Experts still say that the shot offers some protection and should be recommended for these groups, especially because they are most likely to suffer serious complications from the flu.
Matching the shot to the flu
Every year, new strains of the flu spread around the globe. There are hundreds of different strains, but flu shot manufacturers can only include 3 or 4 types in the shot each year.
Medical experts 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 look at 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 shot offers more protection for those who get it. The 2011-2012 flu shot was a good match, and a study in Clinical Infectious Diseases states that it was 71 percent effective that year.
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, Open Forum Infectious Diseases report that the shot was 41 percent effective for younger people, and 56 percent effective for people age 65 and older.
Flu shot side effects
Although side effects are usually very mild, the flu shot can cause pain, redness, or swelling where the shot was given. A few people may also experience body aches or a low fever.
The appearance of hives can signal an allergic reaction to the flu shot.
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:
- Swelling in the face
- Trouble breathing
- Feeling very weak or dizzy
Because the viruses in the shot are weakened or inactivated, the flu shot cannot give someone the flu. However, it is possible to get the flu even after getting a flu shot. This may happen when a person is infected with a strain that was not in the shot, or if a person gets the flu before the shot has had time to take effect.
Those who get a flu shot can not only protect themselves but also those who may be most likely to get very sick or die from the flu. Babies younger than 6 months of age, 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.
Are flu shots safe?
The CDC state that flu shots are safe and have a long history of safety behind them.
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
- Anyone who had a severe allergic reaction to the flu shot in the past
- Those who have had Guillain-Barré syndrome, a rare but severe paralyzing illness
- People who are currently sick with an illness, such as a fever
Egg allergies and the flu shot
Most flu shots contain a small amount of egg protein. Egg-free shots are available for those with severe egg allergies. Studies have shown that people who are allergic to eggs can receive the flu shot without problems.
Pregnant women and the flu shot
The flu shot is safe and highly recommended for pregnant women. It can be given anytime during pregnancy. Pregnant women 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 the mother may pass some of the protection on to her unborn baby. Because babies cannot get the flu shot until they are 6 months old, this benefit may be helpful.
Types of flu shot
Older adults may receive a high-dose version of the flu shot.
Flu shots are available in two doses: standard and high-dose. People under age 65 typically receive a standard dose vaccine. The high-dose version of the shot is recommended for people age 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 smaller 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.
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 and should not be used in the 2016-17 flu season.
When to get the flu shot
Experts recommend people get the flu shot as soon as it becomes available each fall. The shot takes 2 weeks to take effect, and flu season begins as early as October in some cases. But, people may still benefit from getting the flu shot later.
Flu season typically peaks in January or February. However, experts say 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 to protect themselves each flu season, even if the strains in the shot are the same.