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.
/ Source: TODAY

As experts warn about a looming "twindemic" — a surge in COVID-19 cases during a bad flu season — doctors stress that it's more important than ever to get a flu shot this year.

The American Academy of Pediatrics recently recommended children get their flu vaccines by the end of October. As of late August, pharmacists in all 50 states are now allowed to give childhood vaccinations (for ages 3 and up) under a new directive from the U.S. Department of Health and Human Services, aimed at making vaccines more accessible.

“This could be a bad flu season, so they want everybody, especially children, to be prepared for it," NBC News medical correspondent Dr. John Torres explained on TODAY. "Children 6 months and above (should) get the flu shot. It’s important that they start getting it now."

Some parents, however, are still wary about the flu vaccine. A national survey by Orlando Health from 2015 found that many parents are not only skeptical about flu shots, but more than half of parents surveyed believed that you can get the flu from the vaccine.

In reality, the flu shot can’t and won’t give you the flu, experts stress.

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

That’s because the vaccine is constructed from a killed virus. Furthermore, there are only a couple of proteins from that dead virus in the vaccine, so nobody’s getting injected with a complete virus.

Those proteins, taken from the surface of the killed virus, are enough to 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 explained, just because two events occur around the same time doesn’t mean they’re connected.

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

Most of us get our flu shots in the fall, noted Dr. Michael Ison, a professor in the division of infectious diseases at Northwestern University’s Feinberg School of Medicine in Chicago. “That is when rhinoviruses are circulating,” he said. “Most people think a cold is just a stuffy nose. 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, said 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," he said, "there’s a good chance someone sitting next to you will be coughing.”

This is not to say you won’t get any symptoms after a flu shot. Here are some side effects people may experience:

Arm pain

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

Fatigue, headaches

“There are a few people who feel a little fatigued after getting the shot,” Schaffner said. “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 before or after being vaccinated your body didn’t have enough time to strengthen its forces against the virus. The shot doesn’t reach full effectiveness until two to three weeks after vaccination, Ison said.

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

“Don’t linger," said Ison. "Run to your health care provider and get vaccinated right away."

This story was updated in September 2020 to include relevant information about the coronavirus.