Are myths stopping you from getting a flu shot? (Photo: Thinkstock)We're in the thick of cold and flu season, and the U.S. Centers for Disease Control and Prevention estimates that seasonal flu shots prevented 5 million cases of influenza last year and helped keep 40,000 people out of the hospital in 2011. Still, more than half of the population of the United States avoids getting an influenza vaccine each year, usually because they're afraid that the flu shot itself will give them the flu.

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It's one of the biggest myths about the flu shot out there. "It's impossible to get the flu, and it's impossible to spread the flu" from the injection, Dr. Dennis Cunningham, an infectious disease specialist at Nationwide Children's Hospital in Ohio, told LiveScience.com

The confusion may come from the fact that some of the vaccine's side effects—low-grade fever, body aches, and soreness at the injection site—feel like flu-like symptoms. But, "the soreness is often caused by a person's immune system making protective antibodies to the killed viruses in the vaccine," the CDC says on its website. "You cannot get the flu from a flu shot."

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Here are a few other flu-shot related myths:

Myth: Seasonal flu is annoying but harmless—it feels like a bad cold, that's all.

Truth: While both are respiratory illnesses, there are several key differences between the flu and a bad cold. If you have a sore throat and a runny or stuffy nose, it's likely that you have a common cold, not the flu. According to the CDC, Fever, headaches, body aches, extreme fatigue, severe congestion, and a dry cough are signs of influenza; the flu can also lead to pneumonia and bacterial infections. Also, while a cold may last a few days, the flu can leave you feeling miserable for weeks. "When you get the flu, you know it," Dr. Christine Hay, assistant professor at the University of Rochester Medical Center, told Web MD. "You feel like you've been hit by a Mack truck."

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Myth: The flu shot gives you complete protection right away.

Truth: There are three different types of influenza—types A, B, and C—which are divided into several subtypes and strains (How many? The CDC will only say "many" and that "new strains of influenza viruses appear and replace older strains"). The seasonal flu shot protects against the three strains that researchers think will be dominant that year. It takes up to two weeks for your body to gain protection after being vaccinated so, if you're exposed to the virus shortly before getting vaccinated or while you're waiting for your immune system to ramp up, or if you encounter another strain of the virus after you've had your shot, you can still end up with the flu. "The vaccine isn't 100 percent effective, it's only about 50 to 70 percent effective," Philip Tierno, director of clinical microbiology and immunology at New York University Langone Medical Center, told ABC News. "But it will mollify the virus, and hopefully the person won't have a severe reaction."

Myth: You don't need the flu shot every year.

Truth: Dominant flu strains change, so researchers must reformulate the seasonal flu shot every year—which means that last year's shot may not protect you from this year's most-prevalent virus. "It's confusing, since the flu vaccine is different from most vaccines, which offer longer-lasting protection," Dr. William Schaffner, chairman of the department of preventive medicine at Vanderbilt University's School of Medicine in Nashville, Tennessee, tells WebMD.com.

Myth: Only elderly people need the flu shot.

Truth: People who are 65 or older, pregnant, or have certain medical conditions such as asthma, diabetes, and chronic lung disease are at high risk for catching the flu. But healthy people who are at low risk still spend time around family members who are much more susceptible to the virus. If you have young children at home, or if you're a caregiver for anyone in a high-risk group, your getting the flu shot helps protect them as well.

Myth: The flu shot is safe for everyone.

Truth: Babies younger than 6 months old and people who have had Guillain-Barre Syndrome should not receive the seasonal flu shot. Neither should anyone who has ever had a severe allergic reaction to eggs (the virus for the vaccine is grown in chicken eggs) or who has had a severe reaction to a previous flu shot. Otherwise, the flu vaccine is safe for people age 6 months and older.

Myth: You can get the flu from a flu shot.

Truth: The flu shot is an inactivated vaccine, which means that the virus on it has been killed. It's enough to trigger your immune system to start making antibodies, but you can't get an actual infection. (The nasal spray version of the shot, contains a weakened form of the active virus, and while you can end up with some flu-like symptoms from it, it still won't give you the flu.)

Myth: If you've already had the flu, you don't need a flu shot.

Truth: "In any flu season, there's usually both Type A and Type B influenza in circulation," Dr. Trish M. Perl, assistant professor of medicine at Johns Hopkins Medical School in Baltimore, tells WebMD. Just because you've caught one type of the virus doesn't mean you're immune from the other. Getting the flu shot could stop you from suffering through a second round.

Myth: Flu shots protect against the stomach flu.

Truth: Those stomach bugs may feel flu-like, but gastroenteritis isn't related to the influenza virus. While seasonal flu can sometimes cause vomiting and diarrhea in children, it's very rare for adults, so the flu shot does not stop you from ending up with a "24-hour flu" or a "tummy bug."

Myth: Only doctors can administer flu shots.

Truth: You can safely get the flu vaccine in many drugstores (including Walgreens, CVS, and Walmart) and health clinics around the country, even without an appointment.

Myth: You shouldn't get the flu shot if you're already sick.

Truth: Check with your doctor first, but most experts agree that as long as you don't have a fever of more than 101 degrees, it's safe for you to get a flu shot even if you're sick.

Myth: If you make it to November without catching the flu, then you don't need the vaccine.

Truth: Flu season usually peaks in February, and flu vaccines are often still available in December and January. There's still time to minimize your chances of getting the flu this season. The CDC website, however, recommends that you get the seasonal shot as soon as it becomes available in your community, even as early as August.