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Why the flu shot can't give you the flu

Experts say it's impossible to get the flu from the flu shot. But here are some reasons people think the shot makes them sick.
by Linda Carroll /  / Updated  / Source: TODAY

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As this season’s flu roars across the country there are still folks who haven’t gotten vaccinated. Some don’t get the shot because they are convinced that the vaccine actually makes them sick.

When Joe Scarborough of MSNBC's "Morning Joe" tweeted that he had changed his mind about getting a flu shot, a number of his followers responded about getting sick from the vaccine.

It can’t and it won’t, experts say.

It’s biologically impossible for you to catch the flu from being vaccinated, says Dr. William Schaffner, an infectious disease specialist and a professor of preventive medicine at Vanderbilt University.

That’s because the vaccine is constructed from killed virus. Not only that, but also there are only a couple of proteins from that dead virus in the vaccine. So, nobody’s getting injected with a complete virus.

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Those proteins, taken from the surface of the killed virus, are enough rev up our immune system and give it a clear target. But they certainly aren’t enough for the flu virus to reconstitute itself in our bodies.

It might be a cold

For the most part, people are mistakenly assuming that symptoms which coincidentally developed around the time they got the shot were caused by the vaccine. But, Schaffner says, just because two events occur around the same time doesn’t mean they’re connected.

“We all know the rooster crows before dawn,” he explains. “But we don’t think the rooster makes the sun come up.”

Most of us get our flu shots in the fall, says Dr. Michael Ison, a professor in the division of infectious diseases at Northwestern University’s Feinberg School of Medicine. “That is when rhinoviruses are circulating,” he adds. “Most people think a cold is just a stuffy nose,” Ison says. “But you can get really sick from a cold. In fact, many viruses can make you as sick as the flu does.”

There’s also the issue of where you get your shot, says Dr. Richard Zimmerman, a professor of family medicine at the University of Pittsburgh. “If you go to the pharmacy or to the doctor’s office there’s a good chance someone sitting next to you will be coughing,” Zimmerman says.

Which is not to say you won’t get any symptoms after a flu shot.

Arm pain

About 20 percent of people experience soreness in the arm that got the shot, Zimmerman says. “That lasts about a day and then goes away,” he adds.

Fatigue, headaches

“There are a few people who feel a little fatigued after getting the shot,” Schaffner says. “And a few get a headache. These are all part of the body’s immune response to the vaccine. It’s not influenza.”

You already have the virus

If you were exposed to the flu shortly after being vaccinated your body didn’t have enough time to marshal its forces against the virus. The shot doesn’t reach full effectiveness until two to three weeks after vaccination, Ison says.

So, there's no good reason not to get the shot. Even people with egg allergies are now cleared to get it.

The latest stats from the Centers for Disease Control and Prevention show that the flu is widespread in 46 states.

In case you’re thinking that means it’s pointless now to get the shot, Zimmerman points out that the flu in many places comes in two waves—which often contain different strains.

So while you may have somehow avoided being infected in the first wave, you might not be so lucky with the second.

“It’s late, but it’s not too late,” says Schaffner. “Don’t linger. Run to your health care provider and get vaccinated right away. And make a New Year’s resolution to get your flu shot earlier than this year.”

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