Don't believe us? Take it from the Centers for Disease Control and Prevention: "No, a flu shot cannot cause flu illness." Most people know that vaccines work by exposing the body to some form of the bug you're protecting against, and that's where the misconception may come from. It's true that early vaccines contained the real virus—the first smallpox vaccine involved exposing people to actual pus from a boil, after all—but vaccines have come a very long way since then.

Related: The Bizarre Story Of The Very First Vaccine

The flu vaccine you get at the doctor's office today comes in one of several types. The injection contains either an inactivated form of the virus, which can't make you sick, or no flu virus at all. The nasal spray does contain live flu virus, but the virus is weakened so that it can't cause flu either. There have been randomized, blinded studies that gave some people flu shots and some people salt-water shots, and the only difference between the two groups was that those with the flu shots experienced a bit of tenderness in the arm that got the shot. According to the CDC, "There were no differences in terms of body aches, fever, cough, runny nose or sore throat."

So why do we all know somebody who swears they got sick right after getting a flu shot? Correlation. Most people get their flu shot during flu season, which means they could be exposed to the flu at any time. It takes the flu shot two weeks to fully protect you from the virus, but only 1–4 days for the influenza virus to start showing symptoms. If you get sick, you'll naturally think about what you did to catch the virus, and that flu shot may be at the front of your mind. But correlation does not imply causation; in other words, just because one thing happens around the same time as another thing doesn't mean that one caused the other.

Related: You Can't Boost Your Immune System, And You Really Don't Want To

Of course, this is all assuming it's actual influenza that you caught. The flu vaccine doesn't protect against the common cold, bronchitis, or stomach flu—and it doesn't cause those things, either. Those illnesses are also common during flu season, so any symptoms you may feel after you get a flu shot could be from coming down with one of them. But one thing is for certain: your symptoms didn't come from the flu shot, so don't be afraid to get yours. It'll go far in protecting you from a serious illness, and will help protect those at risk around you who can't be vaccinated.

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