Being a little sore after your jab is a small price to pay to prevent influenza.

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Flu vaccines, like so many things in life, are not one-size-fits-all. In fact, there are several different kinds of influenza vaccines being offered this year. The vaccine may be given by injection or nasal spray. There are special dosages for the youngest recipients and the oldest. The vaccine can be made using an inactivated (killed) virus or an attenuated (weakened but live) virus. They may be produced using eggs to grow the virus…or not.

flu shot side effects
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But these various versions of the flu vaccine are more alike than different. Because the flu virus can and does mutate, and more than one strain can circulate at any given time, scientists try to predict which strains of the virus will be most active for the upcoming flu season. And then the flu vaccine is prepared to specifically protect against those viruses.

"This year all the manufacturers are producing vaccines that protect against four different strains of the influenza virus," William Schaffner, MD, medical director of the National Foundation for Infectious Diseases and professor of preventive medicine and infectious diseases at Vanderbilt University School of Medicine, tells Health. A vaccine against four strains of virus is called quadrivalent.

But even though the vaccines are offering the same protection, they're not all made the same way or intended for the same people. And that means the side effects may be slightly different, too. Which vaccine you should get depends a lot on your age, but also on other factors like health conditions and allergies. It's especially important for everyone 6 months old and up to get vaccinated against influenza this year since we're in the middle of a pandemic of another serious respiratory illness, and hospital resources are already stretched thin.

Why do flu vaccines cause side effects?

All vaccines can cause side effects, and the vast majority of these symptoms are completely normal. "Side effects are basically telling you that your immune system is working," says Michael Knight, MD, a primary care physician and assistant professor of medicine at George Washington University Medical Faculty Associates in Washington, DC. "The immune cells are rushing to where the vaccine was injected to react to it." Local soreness at the site of the injection, in fact, is the most common side effect after any vaccine jab.

Flu shots have been around since the 1930s and are considered extremely safe. According to the US Department of Health and Human Services (HHS), most side effects of the flu vaccine are mild and go away in a few days.

Read on to find out what you might experience after you get your flu shot. (One thing you absolutely can not get from the shot is a case of the flu!) For the vast majority of people, none of these possible—and mostly mild or rare—side effects are reasons not to be protected against influenza.

What are the main side effects of the 'regular' flu shot?

A standard-dose flu shot for people ages 6 months to 64 years old contains an inactivated (killed) influenza virus. CDC says the most common side effects from this vaccine include:

  • Pain, swelling, or redness where the shot was given
  • Headache or muscle aches
  • Fatigue
  • Mild fever

"Almost everybody gets a sore arm," says Dr. Schaffner. "It's usually gone after a few hours, though some people still have a sore arm the next day. And there's always a small percentage of people, around 3%, who have a feeling of fatigue, aches and pains, headache the next day."

These are side effects that can occur from any injected vaccine, and they mean that your immune system has been activated. "But just because you experienced side effects doesn't necessarily mean you had a good immune response," explains Dr. Knight. And the reverse is also true: Not having these side effects doesn't mean your immune system isn't responding to the shot. Reactions vary from person to person.

What are the rare side effects?

Fortunately, serious side effects from the flu vaccine are very rare, says HHS. One is a very small increased risk of Guillain-Barré Syndrome (GBS), a neurological disease.

"Guillaume-Barre syndrome occurs after about one in a million doses of vaccine," says Dr. Schaffner. "If someone has GBS within six weeks of receiving the flu vaccine, they shouldn't get the vaccine again." Most people fully recover from GBS. And as the CDC points out, there's research to suggest that the risk of developing GBS is actually higher after getting the flu than it is from getting the vaccine.

Severe allergic reactions are "extremely rare," affecting fewer than 1 or 2 people in a million, says HHS. Per the CDC, signs of a severe reaction can include:

  • Difficulty breathing
  • Hoarseness or wheezing
  • Hives
  • Paleness
  • Weakness
  • Rapid heartbeat
  • Dizziness

Anytime someone experiences such symptoms, call 911 or head to the nearest hospital.

Senior flu shot side effects

There are two different vaccines that are made specifically for people age 65 and older. One shot contains the exact same dose of antigen (the part of the virus that triggers the immune response) as the injection for younger adults, with the addition of an adjuvant—an ingredient added to help spark a stronger immune response. The other shot is a high-dose version of the vaccine, with four times the amount of antigen.

Regardless which of these vaccines is administered, the common side effects are the same for all of the inactivated flu vaccines: sore arm, perhaps some muscle aches, or a mild fever. "All injectables have similar side effects," says Dr. Schaffner. The rare serious side effects are the same too.

Nasal spray vaccine side effects

The nasal spray vaccine is available for non-pregnant people ages 2 to 49 who don't have serious, underlying illnesses, says CDC. It's mostly used in pediatrics, though there are certainly adults who decide they'd rather skip the needle if they can. This vaccine isn't just an inhaled version of the injectable. Instead it contains a weakened—not killed—version of the flu virus to stimulate the immune system.

"This vaccine evokes real protection because the flu viruses multiply in the nose and the back of the throat," says Dr. Schaffner. The reason this vaccine doesn't cause the illness it's designed to protect, he explains, is that the virus is specially engineered so that it dies as soon as it's exposed to the slightly higher temperatures inside the body.

The most common side effects for this vaccine, according to the CDC, are:

  • Runny nose or nasal congestion
  • Wheezing
  • Headache

Other possible side effects are vomiting, muscle aches, fever, sore throat, and cough. According to the CDC, "if these problems occur, they usually begin soon after vaccination and are mild and short-lived."

An egg-free vaccine option

Most flu vaccines are still made using eggs to grow the vaccine cells. Understandably, people with egg allergies have been nervous about getting the vaccine. "But what we've learned is that egg allergy plays an extraordinarily small role in reactions to the vaccines," explains Dr. Schaffner, who says that the vast majority of people with egg allergies will have no problem with the flu vaccine. "The only people now who are cautioned about having an influenza vaccine because of allergies are those who've had a severe, anaphylactic reaction in the past."

Now even those people can get the flu vaccine. A mostly egg-free vaccine became available in 2016, according to the CDC. And in this 2021-2022 flu season, for the first time, there is now a completely egg-free vaccine made by growing the virus in animal cells rather than chicken eggs.

You'll need to speak with a health care provider to determine which of the available flu vaccines is right for you. While there's no way to know how you'll feel afterwards, "the best predictor of what side effects you'll experience is your previous reaction to the shot," says Dr. Knight.

Even if you have to endure an achy arm for a day or so, it's only temporary. Why risk a case of the flu, which can lead to serious complications? As the CDC states, getting the flu shot is a lot safer than getting the flu.

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