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The flu shot can cause mild side effects that are sometimes mistaken for flu. For example, people sometimes experience a sore arm where the shot was given. The soreness is often caused by a person’s immune system making protective antibodies in response to being vaccinated. These antibodies are what allow the body to fight against flu. The needle stick may also cause some soreness at the injection site. Rarely, people who get the flu shot have fever, muscle pain, and feelings of discomfort or weakness. If experienced at all, these effects usually last 1 to 2 days after vaccination and are much less severe than actual flu illness.
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Seasonal flu is a contagious respiratory illness caused by flu viruses. It spreads between people and can cause mild to severe illness. In some cases, the flu can lead to death. In the United States, flu season occurs in the fall and winter. Seasonal flu activity usually peaks in January or February, but it can occur as early as October and as late as May.
The best way to protect yourself and your family from getting the flu is to get a flu vaccine every year. It is recommended that everyone 6 months of age and older should get the flu vaccine. Seasonal flu vaccines have a very good safety track record. The flu vaccine is available by shot or nasal spray.
While everyone who is at least 6 months of age should get a flu vaccine this season, it’s especially important for some people to get vaccinated. Those people include the following:
A complete list is available at People Who Are at High Risk of Developing Flu-Related Complications.
How well the flu vaccine works (or its ability to prevent flu illness) can range widely from season to season. In general, recent studies have supported the conclusion that flu vaccination benefits public health, especially when the flu vaccine is well matched to circulating flu viruses.
Over the last 50 years, seasonal flu vaccines have had very good safety track records. Over the years, hundreds of millions of Americans have received seasonal flu vaccines. The most common side effects following flu vaccinations are mild. The Centers for Disease Control and Prevention (CDC) and the Food and Drug Administration (FDA) closely monitor for any signs that flu vaccines are causing unexpected adverse events and are working with state and local health officials to investigate any unusual events.
Possible side effects include:
The intradermal flu shot may cause other additional mild side effects including toughness and itching where the shot was given. If these problems occur, they begin soon after the shot and usually last one to two days. Life-threatening allergic reactions are very rare. If any unusual condition occurs following vaccination, seek medical attention right away.
No, a flu shot cannot give you the flu. Flu vaccines that are administered with a needle are currently made in two ways: the vaccine is made either with flu vaccine viruses that have been ‘inactivated’ and are therefore not infectious, or with no flu vaccine viruses at all (which is the case for recombinant influenza vaccine). In randomized, blinded studies, where some people got flu shots and others got saltwater shots, the only differences in symptoms was increased soreness in the arm and redness at the injection site among people who got the flu shot. There were no differences in terms of body aches, fever, cough, runny nose or sore throat.
There are several reasons why someone might get flu-like symptoms even after they have been vaccinated against the flu.