(CNN) -- Misunderstandings and questions about the flu vaccine can confuse people trying to decide whether to get one. Here, based on information from the Centers for Disease Control and Prevention, are several common misconceptions and questions.
1-The shot can give you flu
Not so, says CDC.
The viruses in flu shots are killed during the production of the vaccine, which means they cannot cause infection. The vaccine batches are then tested, with a group of people randomly assigned to get either the vaccine or salt water.
"The only differences in symptoms was increased soreness in the arm and redness at the injection site among people who got the flu shot," CDC found. "There were no differences in terms of body aches, fever, cough, runny nose or sore throat."
2-The flu shot doesn't work
It doesn't work all the time, but it does confer some level of protection. For example, the CDC says preliminary data for the 2010-2011 season show that it was about 60% effective for all age groups combined, and studies for earlier years found protection rates of up to 90%.'
3-Why do some people feel bad after getting a flu shot?
Soreness at the injection site is one reason, but it usually dissipates within two days. It's caused by the immune system making antibodies to the killed viruses in the vaccine that help a person fight off the flu.
The Advisory Committee on Immunization Practices says symptoms, in rare instances, include fever, muscle pain, and discomfort or weakness, which also typically go away after a day or two.
4-Why do some people wind up getting the flu after getting a shot?
It takes two weeks from the time of injection for the vaccine to confer protection. During that time, you are vulnerable.
In addition, a number of other pathogens, including rhinovirus, can cause flu-like illness. Or you may be exposed to a flu virus that is not included in the vaccine. Finally, the flu shot does not always work -- particularly among the elderly and people with weak immune systems. But even among these high-risk groups, the vaccine can prevent complications.
5-Is "stomach flu" the flu?
"Stomach flu" is often used to describe symptoms of nausea, vomiting or diarrhea. But other viruses, bacteria or parasites can cause such symptoms. Though they can sometimes be related to flu (more often in children), they are rarely the primary symptoms of flu.
"The flu is a respiratory disease and not a stomach or intestinal disease," CDC says.
6-Is it better to get the vaccine later in the season to limit risk that its efficacy will wane?
No. The shot lasts an entire flu season, except for some children who may need two doses. The CDC recommends all people older than six months get a flu vaccine.
7-Is it too late to get the vaccine?
No. Vaccination can help as long as flu viruses are circulating. Though CDC recommends you get vaccinated as soon as vaccine is available in the fall, it "can still be protective to get vaccinated in December or later. Influenza is unpredictable and seasons can vary. Seasonal influenza disease usually peaks in January or February most years, but disease can occur as late as May."
8-The flu shot might adversely affect my pregnancy
Flu vaccine is "an essential element of prenatal care" and is recommended for all pregnant women, according to the American College of Obstetricians and Gynecologists. Pregnant women are among the groups at increased risk for flu complications like pneumonia, infections and dehydration.
Though babies cannot be vaccinated until they have reached six months of age, antibodies they received in utero from their mothers may help protect them. But note that the group recommends against the nasal spray -- the live, attenuated version -- for pregnant women.
9-'I've had the flu before and it was no big deal, so bring it on'
Seasonal flu exacts a bigger toll in some years than in others: Between 1976 and 2007, flu was linked to a low of 3,000 to as many as 49,000 fatalities in the United States, with more than 200,000 hospitalizations.
There are two main reasons: The viruses that circulate in one year may differ from those that circulate in another. And, of course, people change from year to year, meaning that your response to a viral infection one year may not be the same as your response in another.
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