The most common side effects of the flu shot are soreness at the injection site, fatigue, headache, and low-grade fever. These typically show up within a day or two and resolve on their own within a few days. Serious reactions are extremely rare, occurring in fewer than 1 in a million doses.
Common Side Effects
The flu shot triggers your immune system to build defenses against influenza, and that immune response is what causes most side effects. Your body is doing exactly what it’s supposed to do. The most frequently reported reactions include:
- Arm soreness, redness, or swelling at the injection site, usually lasting a couple of days
- Fatigue
- Headache
- Low fever
- Cough
- Muscle aches
These are mild and short-lived. They are not the flu. The standard injectable flu vaccine contains inactivated virus, so it cannot give you influenza.
How Long Side Effects Last
Most people notice side effects within the first 24 hours after vaccination. Arm soreness tends to peak on the first day and fades within two days. Systemic symptoms like fatigue, headache, and low fever generally clear up within a few days. If you’re still feeling off after a week, that’s worth a call to your doctor, as it’s likely unrelated to the vaccine.
High-Dose Vaccines for Older Adults
If you’re 65 or older, you were likely offered a high-dose flu vaccine, which contains a stronger dose of the active ingredient to produce a more robust immune response in aging immune systems. The tradeoff is that side effects are somewhat more common. Soreness at the injection site, headache, muscle aches, and fatigue all occur at higher rates with the high-dose version. These side effects most often appear during the week after vaccination and follow the same pattern of resolving on their own.
Nasal Spray Side Effects
The nasal spray vaccine uses a live but weakened form of the virus, so it produces a different set of side effects than the shot. In children, the nasal spray can cause a runny nose, wheezing, headache, vomiting, muscle aches, low fever, and sore throat. In adults, it’s usually limited to a runny nose, headache, and cough. These begin soon after vaccination and are typically mild and brief.
Side Effects in Children
Children experience the same common side effects as adults: soreness, fatigue, low fever, and fussiness. One concern parents sometimes hear about is febrile seizures, which are brief seizures triggered by a rapid rise in body temperature. Research on this has been mixed. Some studies found a small increased risk in children between 12 and 23 months, particularly when the flu shot was given at the same time as other routine vaccines. Other studies, including one that followed 45,000 children aged 6 months through 23 months over more than a decade, found no association. Febrile seizures after vaccination remain rare, and while they’re frightening to witness, they don’t cause lasting harm.
Serious but Rare Reactions
Severe allergic reactions (anaphylaxis) to the flu vaccine happen at a rate of roughly 1.6 per million doses. Signs include difficulty breathing, swelling of the face or throat, hives, rapid heartbeat, and dizziness. These reactions almost always occur within minutes to a few hours of vaccination, which is why you’re asked to wait briefly at the pharmacy or clinic after your shot.
Guillain-Barré syndrome, a condition where the immune system attacks nerve cells and causes muscle weakness, has been associated with the flu vaccine at an estimated rate of 1 to 3 cases per million vaccinations. When it does occur, it typically develops within six weeks of vaccination. For context, influenza infection itself also carries a risk of Guillain-Barré syndrome, and at a higher rate than the vaccine.
Egg Allergies and the Flu Shot
If you have an egg allergy, you can still get a flu vaccine. The CDC recommends that everyone 6 months and older with an egg allergy receive their annual flu shot. You can get any flu vaccine appropriate for your age, whether it’s egg-based or not. As of the 2023-2024 season, no additional safety precautions beyond what’s standard for any vaccine are recommended, regardless of how severe your egg allergy has been in the past.
Managing Side Effects
Your first instinct might be to reach for ibuprofen or another anti-inflammatory painkiller, but research from the University of Rochester Medical Center suggests you hold off. Nonsteroidal anti-inflammatory drugs like ibuprofen, aspirin, and naproxen can dampen your body’s production of the antibodies the vaccine is designed to create. The recommendation is to avoid these medications for one to two days before and up to a week after vaccination when possible.
Instead, try these approaches:
- Apply a cool compress to your sore arm
- Gently move and stretch the arm to improve blood flow
- Stay hydrated and rest
- Wear loose-fitting clothing over the injection site
If you take daily aspirin for heart disease or a prescription anti-inflammatory for chronic pain, talk to your doctor before stopping those medications. The advice to skip painkillers applies mainly to otherwise healthy people reaching for over-the-counter relief for mild, temporary discomfort.