Every fall I hear some version of the same story: "I got the flu shot once, and it gave me the flu, so I never got it again." It usually comes from someone thoughtful who is describing a real experience. They got the shot, and within a day or two they felt lousy. Sometimes they felt lousy for a week.
I take that experience seriously, because it is real. What I want to do here is untangle two things that often get lumped together: the side effects a flu vaccine genuinely causes, and the disease it cannot cause. Both parts of that sentence matter.
Why the vaccine cannot cause influenza
Influenza is caused by a virus that infects your airways and multiplies there. To get the flu, you need a live virus capable of replicating in your body.
Flu shots do not contain one. According to the , injectable flu vaccines are made either from influenza viruses that have been killed (inactivated) and are no longer infectious, or from a single viral protein grown without any virus at all (the recombinant vaccine). A dead virus, or a piece of a virus, cannot cause an infection any more than a photo of a dog can bite you.
The nasal spray vaccine (FluMist) is the one exception that uses live virus, but those viruses have been weakened and "cold-adapted," meaning they are , not in the warmer lungs where influenza does its damage. It can cause a runny nose or sore throat. It does not cause flu illness.
This is not a new claim. The CDC's influenza recommendations said it plainly back in : "Because influenza vaccine contains only noninfectious viruses, it cannot cause influenza. Respiratory disease after vaccination represents coincidental illness unrelated to influenza vaccination."
Yes, there are side effects. Here is what they are.
Saying the shot cannot give you the flu is not the same as saying nothing happens. Things do happen.
The most common side effect by far is a sore arm. Redness or swelling where the shot was given is also common. Less often, people report headache, fever, muscle aches, tiredness, or nausea. When these occur, the they usually begin soon after vaccination and last one to two days. The older clinical guidance is even more specific: whole-body symptoms, when they occur, tend to , and they are more common in people who have never been exposed to the viruses in the vaccine, such as young children.
Serious side effects are rare, but they exist and should not be waved away. Severe allergic reactions occur in about and are treatable. In some seasons, safety monitoring has detected a small increase in Guillain-Barré syndrome, a neurological condition, on the order of ; in most seasons, no increase is found. For context, 3,000 to 6,000 Americans develop Guillain-Barré every year regardless of vaccination, and studies suggest the risk after having influenza itself is higher than the risk after the vaccine.
What happens when half the people get a fake shot
The best way to know which symptoms the vaccine actually causes is to give some people the vaccine, give others a placebo injection, tell no one which they got, and then ask everyone how they felt. That is exactly what a team in Minnesota did in the fall of 1994.
In that , 849 healthy working adults received either the flu vaccine or a placebo. Over the following week:
- Arm soreness was clearly caused by the vaccine: 64% of vaccine recipients reported it, versus 24% of placebo recipients.
- Fever, muscle aches, fatigue, malaise, and headache were not. About 34% of the vaccine group reported at least one of these, compared with 35% of the placebo group. Statistically, there was no difference.
Think about that second number for a moment. Roughly one in three people who got a saltwater injection felt feverish, achy, or run down in the following week. That is the background noise of being a human in October.
The same study found something else useful. The single strongest predictor of feeling sick after the injection, in either group, was having a , which raised the odds of whole-body symptoms more than fourfold. People catch respiratory viruses in the fall. Some of them happen to catch one right after a flu shot.
Larger evidence syntheses do find a small, real effect on fever. The of trials in healthy adults, pooling data from more than 23,000 participants, found that inactivated flu vaccines increase self-reported fever from about 1.5% to 2.3%. Put differently, for every , about one extra person runs a fever. That is a genuine side effect. It is also a long way from "everyone gets sick."
Why you might feel achy anyway: your immune system at work
So where do the aches and fatigue come from in the minority of people who do get them? Not from infection, but from inflammation.
When a vaccine enters your arm, immune cells recognize it as foreign within minutes and release signaling molecules called cytokines. A review in describes how these inflammatory signals, particularly interleukin-6 and C-reactive protein, rise in the blood, generally peak around 24 hours after vaccination, and fall back to baseline within one to three days. Those same molecules are what make you feel feverish and achy when you have a real infection. In the authors' words, the response "mimics the response to natural infection."
That is the whole point. The vaccine is a rehearsal. Your body runs the alarm drill without the fire. A sore arm or a day of feeling off is the sound of that drill, not evidence that the building is burning.
Expectation is part of the story too
There is one more piece of evidence worth knowing about, because it is humbling for all of us. During the COVID-19 vaccine trials, researchers pooled data from more than 45,000 participants and looked specifically at the people who received placebo. After the first injection, such as headache and fatigue. The authors estimated that this "nocebo" response accounted for about three quarters of the whole-body symptoms reported in the vaccine group after dose one.
This does not mean side effects are imaginary. It means that when you expect to feel bad after a shot, you pay more attention to every twinge, and ordinary twinges become "the shot made me sick." That is normal human psychology, not a character flaw.
"But I really did get sick afterward"
I believe you. Here are the most likely explanations, none of which involve the vaccine giving you influenza.
You were already exposed, or exposed too soon after. It takes about after vaccination. Flu caught in that window will feel exactly like flu caught any other time, because it is.
It was a different virus. The flu vaccine . Rhinoviruses, RSV, COVID-19, and dozens of other respiratory viruses circulate in the same months, and plenty of them cause fever and aches that people reasonably call "the flu."
The vaccine worked, but not perfectly. Flu vaccines reduce risk; they do not eliminate it. During the 2024–2025 season, CDC's networks estimated the vaccine reduced the chance of . Interim estimates for the 2025–2026 season from the VISION network were across all ages. Some vaccinated people will still get the flu. The evidence consistently shows they tend to get a milder case.
Those percentages can sound underwhelming until you scale them up. CDC estimates that during the 2024–2025 season, flu caused in the United States, and that vaccination prevented about 10 million illnesses, 180,000 hospitalizations, and 12,000 deaths.
Side effects versus the disease
Here is the comparison that matters:
The CDC's summary is blunt and, I think, fair: the most common reactions to flu vaccines are .
The bottom line
A flu shot can make your arm sore. For a small number of people, it can bring a day of feeling achy or feverish while the immune system does its work. Those are real side effects, and it is fine to plan around them: get the shot on a day when you can take it a little easy, drink water, and know that it passes.
What a flu shot cannot do is give you influenza. It does not contain a virus capable of causing it. When people feel genuinely sick after vaccination, the evidence points to a coincidental cold, a flu exposure that beat the vaccine's two-week head start, a different virus altogether, or an immune response that feels like illness but is not one.
The CDC recommends that everyone six months and older get vaccinated each season, ideally , though vaccination remains worthwhile as long as flu viruses are circulating. If you have questions about which vaccine is right for you, including egg allergies or health conditions, that is a good conversation to have with your clinician or pharmacist.
And the next time someone tells you the shot gave them the flu, you can tell them, kindly: it probably gave you a sore arm and a reminder that your immune system is paying attention. The flu came from somewhere else.