Why you may feel like garbage after you get the Flu shot

But first things first, whatever you're feeling is not the flu: You can’t actually catch the flu from your flu shot.

That remains one of the biggest myths about the flu vaccine out there. In fact, according to a 2015 study of 1,000 people published in Vaccine, 43 percent believed that getting the flu vaccine could give you the flu.

That’s just wrong.

"The flu shot is a killed flu virus that consists of only half of the virus—the part you need to make an immune response to," says Andrew Pekosz, Ph.D., professor of molecular microbiology and immunology at Johns Hopkins University Bloomberg School of Public Health. "It's also then administered into your arm muscle, which is not a place the flu virus normally goes to. So there is no possibility you can get the flu from the flu shot."

That doesn't mean you won't feel some discomfort from the shot itself, however.

"Most people have a little redness and soreness at the site of the inoculation. These are normal symptoms and are due in part to your body’s immune system reacting to the vaccine," says Pekosz. "Usually these don’t last for more than a day or two."

You can get other symptoms, too. These include a low-grade fever or body aches, according to the Centers for Disease Control and Prevention (CDC). This is normal and could be your body’s early immune response reacting to a foreign substance entering the body.

In fact, 15 percent of healthcare workers who received the H1N1 vaccine in 2009 reported fever, and 23 percent said they experienced generalized pain or low back pain, a separate study in Vaccine found.

Symptoms of the FluMist vaccine—which is administered by a nasal spray—include runny nose or nasal congestion, fever, and sore throat. You can also experience muscle aches, decreased appetite, or lethargy.

But, “the flu mist vaccine isn't recommended this year because it hasn't been very effective over the past few years," says Pekosz.

So it’s possible you can feel some side effects from the flu shot, but these are mild and tend to go away on their own within a few days.

If you’re really feeling sick for a sustained amount of time afterwards, well, you probably just caught another virus that the flu vaccine doesn’t protect you against.

"The flu vaccine protects against influenza virus, but there are a number of other viruses that can cause a flu-like disease,” says Pekosz. "Viruses like human parainfluenza virus, respiratory syncytial virus (RSV) and coronaviruses all circulate in fall and early winter, the time that flu vaccination programs are in full-swing." These viruses and their symptoms usually last two to eight days.

Bottom line: It's normal to feel soreness, redness, tenderness, or even develop a mild fever or body aches during the two days after you get vaccinated—that’s just your immune response, not the flu illness itself.

So there’s no reason to avoid getting the flu shot because you think it’ll make you sick. Unless you have severe or life-threatening allergies to the flu vaccine or any ingredients in it, you should roll up your sleeves for one each year, the CDC says (People with severe allergies, those who have had Guillain-Barré syndrome, or who feel ill should talk to their doctor first).

A day or two—at most—of a little discomfort is a small price to pay for helping prevent getting sick with the flu. While most people will recover from the flu within several days to two weeks, some people can develop complications from the virus, which can include sinus and ear infections, pneumonia, and inflammation of the heart (myocarditis), brain (encephalitis), or muscle (myositis, rhabdomyolysis). It can even be deadly.