Everything You Need to Know About Flu Shots

— -- In addition to dropping temperatures and changing leaves, the start of fall signals the beginning of flu season in the U.S.

Seasonal influenza may be thought of as a common nuisance, but the disease can be dangerous. Last flu season, 970,000 people were hospitalized because of the illness, and the U.S. Centers for Disease Control and Prevention estimated there were 40 million cases.

There is a simple way to diminish the chance of getting the flu or suffering the worst complications from the disease: Get a flu shot.

Who Should Get a Flu Shot?

The CDC recommends that virtually "everyone" 6 months of age or older get a flu vaccine "every season."

Dr. Amy Edwards, a doctor of pediatric infectious disease at Rainbow Babies and Children's Hospital in Cleveland, told ABC News that "young kids" and "adults over 65" especially should make sure they get a flu shot.

In addition, "people with chronic medical problems — such as asthma, diabetes and heart diseases — that put them at a higher risk for complications" when infected with the flu should make sure to get vaccinated, according to Edwards. People who are going to be exposed to those with a higher risk for complications should also make sure to get a flu shot, she added.

"They're at an increased risk for having bad outcomes, including pneumonia and cardiac events, because it puts a lot of strain on the heart. People who have had cardiac events in the last year should really get the flu shot," she said.

Edwards also said that "anybody that doesn't want to take two weeks off for a bad case of the flu" should get vaccinated.

When Should You Get a Flu Shot?

While most health experts advise that earlier is better, the seasonal flu season doesn't pick up until October in the U.S., according to the CDC.

"The CDC recommends October because we never quite know when transmission is going to start," Edwards said.

"I always advocate as soon as possible. In my mind, it is just not worth taking that risk," she added.

What Kind of Flu Shot Should You Get?

In previous years, some doctors recommended getting a nasal spray vaccine, as opposed to the traditional flu shot.

The nasal spray is currently not recommended for use by the CDC because it appears it may not be as effective as preventing the flu as the shot, according to recent studies.

Edwards said she understood the disappointment of many parents with children who do not like getting shots. "Maybe with a little more research, if they can make that vaccine a little stronger, they can bring it back in a few years," she said.

There is a variety of flu shots available, and Edwards recommended talking to your doctor to figure out which one is most appropriate for your lifestyle and medical history.

"For the vast majority of the population, the regular trivalent should be just fine," she said, referring to a vaccine that protects against three strains of flu virus.

People over the age 65 can get flu shots or high-dose vaccines specifically for seniors. These vaccinations have a higher amount of antigen, which helps the immune system develop antibodies.

How Effective Is the Flu Shot?

The effectiveness of a flu shot at preventing the virus "varies by year," according to Edwards.

When making the vaccine, Edwards said, "every year the experts look at what strains have been circulating in the U.S. and what strains have been circulating in other parts of the world."

"There are some years where the vaccine is much better at preventing the flu than other years, but you are always getting some protection," Edwards added.

What Happens If You Contract the Flu?

The flu shot is not a perfect vaccine, and people may contract the virus even if they have received the vaccine. In these cases, experts advise that at-risk patients get antiviral medication to shorten the duration of influenza symptoms.

Dr. William Schaffner, an infectious-disease expert at Vanderbilt University Medical Center in Nashville, Tennessee, said that any people with underlying complications such as heart or breathing problems or above the age of 65 should go to their physician immediately if they show symptoms of the flu.

"Flu is tricky because you can feel mildly ill in the beginning and all of a sudden you crash," he said. Antiviral medication "can ameliorate the severity of the illness, it will make you feel better soon, [and] it will make it less likely you spread disease to others."

Can You Get the Flu From a Flu Shot?

No.