How common are flu shot side effects?

With fall well underway and winter just around the corner, chances are your doctor, pharmacist, and maybe even your mom are telling you it's time to roll up your sleeve for a flu shot. If you're inclined to ignore them, we get it: No one really enjoys being jabbed with a needle.

But many people aren't primarily dissuaded by a quick pinch; instead, they're convinced that the vaccine has too many side effects and too few benefits to be worth the trouble. Are they right?

While everyone (ideally with the help of their doctor) has to do their own risk-benefit analysis, it's crucial to start with some sound info. Here are some key facts to help you decide.

About 65 percent of people have some side effects.

That number comes straight from the Centers for Disease Control and Prevention, and yes, it sounds high. But we're not talking about serious side effects here. Instead, it refers to local reactions, like a little pain or redness at the injection site, or the temporary sensation of your arm being heavy. (This surprisingly simple thing could boost your flu shot's effectiveness.)

If you have an egg allergy, you could have an allergic reaction.

Most flu vaccines are manufactured with the help of egg protein. If a bite of a runny omelet causes you to break out in hives or makes your throat start to close up, the flu shot could prompt the same response. That said, talk to your doctor: Many people who think they're allergic to eggs really aren't (maybe you've outgrown a childhood allergy), said Nicole Bouvier, MD, associate professor of infectious diseases and microbiology at the Icahn School of Medicine at Mount Sinai. And if you're really, truly allergic you can sidestep this problem by asking your doctor if he or she stocks a specific type of flu vaccine called FluBlok, which is not made with eggs.

Serious side effects are extremely rare.

It's true that if you're allergic to something (like eggs) in the flu shot you could end up with a life-threatening reaction called anaphylaxis. It's also theoretically possible to develop a neurological condition called Guillian-Barre Syndrome (GBS), though it's not clear that the shot actually causes GBS. And the likelihood of developing it is very rare: about 1 or 2 people out of every million who are vaccinated get GBS—as do about 3,000 to 6,000 people in the U.S. every year, whether or not they got any shots.

It's impossible to get the flu—or even a run-of-the-mill cold—from the vaccine.

"You might develop a low-grade fever and muscle aches, or feel a little lethargic," especially if you get the vaccine for the first time after skipping it for many years, said Bouvier. But this isn't a mild version flu: It's just an immune system reaction, and it means that your body is responding appropriately to an invader.

In fact, it's impossible to get the flu (or any respiratory infection) from the vaccine for a few reasons: First, assuming you're getting the shot (and not the nasal spray), you're being given an inactivated (a.k.a. dead) virus.

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"It's killed and broken up into little pieces," Bouvier explained. "And even if one tiny virus was to survive all that, you're injecting it into a muscle [in your arm], and it can't travel from your muscle to your lungs."

On average, the flu shot is about 65 percent effective.

How well the vaccine works varies from year to year, depending on how well scientists have matched the strains in the shot with the viruses that are circulating in a given season.

"There's a lot of educated guessing going on," Bouvier admitted.

That said, the flu can be miserable (and in some cases life-threatening), and the shot offers to cut your risk pretty substantially. (Here's how to avoid the flu, beyond just getting a vaccine.) Bonus: If you get vaccinated and still end up with the flu, your case won't as bad. "No, it's not 100 percent effective, but it does reduce complications," said Bouvier.

Some people definitely need to be vaccinated.

The CDC's high-risk list includes people who are especially vulnerable to flu complications like pneumonia, bronchitis, or even death. Pregnant women, kids under age 5, adults 65 and up, and people with chronic conditions like asthma and heart disease are all on the list. If you're very obese (BMI of 40 or higher), you're also in this vulnerable group.

"We're not sure why, but during the 2009 pandemic we learned that obese people were dying from the flu at higher rates compared to normal weight people of the same age," said Bouvier.

Think about protecting others, not only yourself.

Not in the high-risk category? Health experts still want you to get the shot.

"We basically say that everyone above the age of 6 months should be vaccinated," said Bouvier.

The reason: Herd immunity. Let's say you're a pretty healthy 30- or 40-something and you get the flu. Your odds of surviving are very high, though you'll feel lousy. Even if you're willing to deal with a week or two of misery, during that time you're also apt to expose other people—including those who are extra vulnerable to complications.

"What I tell people is to think about whether there's someone in your life—like a grandparent, person with asthma, or new baby—who you would not want to get the flu," said Bouvier. "If you don't want to be the person to give it to them, you should get vaccinated."

This article first appeared on Prevention Magazine.