- Flu vaccines will help protect you against the influenza virus.
- The flu shot cannot give you the flu, though the side effects may feel very similar to a minor version of the virus.
- Possible side effects largely depend on what type of flu vaccine you receive.
- This article was reviewed by Tania Elliott, MD, who specializes in infectious diseases related to allergies and immunology for internal medicine at NYU Langone Health.
- Visit Insider’s homepage for more stories.
Like most medical products, flu vaccines can have side effects.
“Side effects are not a bad thing,” Dr. Ian Nelligan at Stanford Health Care tells Insider. “It is a reminder that one’s immune system is responding appropriately to the vaccine.” He added that “side effects of the flu vaccine are much more mild than the symptoms of an influenza infection.”
If you get the flu shot, common side effects include, “soreness, redness, or swelling might develop at the injection site,” Dr. Charles Peters of the Mayo Health Clinic System tells Insider. Other common side effects include achiness or a low-grade fever of less than 101 degrees Fahrenheit.
Other side effects that are less common include a high fever, above 101 degrees Fahrenheit and, in rare cases, a neurological disorder called Guillain-Barré syndrome, which occurs when the nerves outside of the brain and spinal cord are damaged causing numbness, reflex loss, and paralysis — though paralysis is often temporary.
Fewer than two cases for every million people vaccinated get Guillain-Barré syndrome, according to the Centers for Disease Control and Prevention.
If you opted for the nasal spray instead, Peters says you might suffer from a runny nose, wheezing, headache, vomiting, muscle aches, fever, sore throat, or cough.
But regardless of whether you get the shot or nasal spray, the flu is a serious illness. It infects tens of millions of people in the US each year and kills tens of thousands. An annual flu shot is the most effective way to prevent an infection from the flu virus, which is why it’s recommended by physicians.
Can the flu shot give you the flu?
Contrary to popular belief, the flu vaccine does not give you a minor version of the flu.
That’s because “the flu shot is either made from ‘killed’ virus or from proteins of the virus, not the virus itself,” Dr. Neena Xavier, a clinical assistant professor at the University of Alabama at Birmingham, tells Insider. “So you cannot get the flu from the flu vaccine if you get it as a shot.”
Nasal sprays, on the other hand, do contain live viruses. But the viruses are specially designed to be too weak to infect someone who has a normal immune system with the flu.
How does the flu shot work?
The flu vaccine exposes your immune system to something very similar to the flu virus you might encounter during flu season.
“Once you are exposed to it, your body makes antibodies, things that recognize the virus and then will fight it off quickly so it doesn’t have a chance to reproduce in your body,” Xavier says.
“Side effects occur because when a person’s immune system is activated it produces antibodies to the flu. You get the symptoms you normally do when your immune system is activated. The virus is not causing it — your body’s fight mechanism is.”
Xavier adds: “The common side effects can happen to anyone regardless of age, gender, or medical condition.”
How long do flu shot side effects last?
Regardless of which type of vaccine you get, shot or nasal spray, side effects for both will usually last less than two days.
“The common side effects can happen to anyone regardless of age, gender, or medical condition,” Xavier says.
But if side effects are more severe or last longer than usual, you should seek medical attention. For example, sometimes the vaccine can trigger severe allergic reactions, within minutes to hours of getting the shot.
“If you feel throat tightening or difficulty breathing after a flu shot, seek medical help immediately,” Xavier says. “In addition, the flu shot, on rare occasions, can cause a nerve disease called Guillain-Barré syndrome, which is a weakness in your legs that travels up your body. If you experience weakness in your legs, not just aches, then contact your doctor immediately.”