The flu shot is safe. It has been administered billions of times over more than 60 years, and the safety data behind it is among the most extensive for any vaccine. Serious reactions are exceptionally rare, occurring in roughly 1 to 2 people per million doses. The most common side effects are mild: a sore arm, low-grade fever, or muscle aches that resolve within a day or two.
The Flu Shot Cannot Give You the Flu
This is the single most persistent concern, and the biology makes it impossible. Injectable flu vaccines are made in one of two ways: with influenza viruses that have been killed (inactivated) or with just a single protein from the virus’s surface (recombinant vaccines). Neither contains a live virus capable of replicating in your body. You cannot get the flu from a flu shot.
What you can get is a mild immune response. Your body recognizes the vaccine components as foreign and mounts a reaction, which is exactly the point. That process can cause low-grade fever, fatigue, or achiness for 24 to 48 hours. These symptoms are your immune system building protection, not an infection taking hold. It’s also possible to catch a cold or even the flu from community exposure around the same time you got vaccinated, which reinforces the misconception.
Common Side Effects
Most people experience nothing beyond soreness at the injection site. When side effects do occur, they typically include:
- Pain or redness at the injection site
- Headache
- Muscle aches
- Low-grade fever
- Fatigue or general malaise
These are temporary, usually lasting one to two days. They occur at similar rates across all age groups. For adults 65 and older who receive the higher-dose or adjuvanted versions of the vaccine (designed to produce a stronger immune response in aging immune systems), injection-site soreness and muscle aches may be slightly more common. A post-licensure clinical trial found that activity-limiting pain at the injection site was no higher with the adjuvanted vaccine than with the high-dose version, and overall safety profiles were similar between the two.
Rare but Serious Reactions
Severe allergic reactions (anaphylaxis) after a flu shot occur at a rate of about 1.35 per million doses. To put that in perspective, you’re far more likely to have a serious allergic reaction to a food you eat on any given day. Vaccination sites are equipped to recognize and treat anaphylaxis immediately, and reactions almost always happen within minutes of the shot.
A condition called Guillain-Barré syndrome, where the immune system temporarily attacks nerve cells and causes muscle weakness, has a small association with flu vaccination. If the risk exists, it amounts to one to two additional cases per million doses administered. GBS is also triggered by infections themselves, including influenza, so the net risk calculation still favors vaccination for the vast majority of people.
Safety in Pregnancy
Flu shots have been given to pregnant women for more than 50 years. The CDC recommends vaccination during any trimester, and the evidence behind that recommendation is substantial.
One of the largest studies on the topic, conducted through the Vaccine Safety Datalink and spanning three flu seasons (2012 through 2015), found no increased risk of miscarriage among women who received the vaccine during pregnancy. A separate study using the same data system found no increased risk of stillbirth. Research published in 2017 showed that babies born to women vaccinated during their first trimester had no increased risk of major birth defects. Rates of premature delivery and small-for-gestational-age infants were identical between vaccinated and unvaccinated groups in another large study covering five flu seasons.
Pregnancy complications like preeclampsia and gestational hypertension also showed no increase in vaccinated women compared to unvaccinated women in data spanning 2002 to 2009.
Safety in Young Children
Children can receive the flu vaccine starting at 6 months of age. A large safety study of children aged 6 to 23 months, published in JAMA, found no significant association between the inactivated flu vaccine and any medical event in either the short-term (3-day) or extended (42-day) follow-up window. In fact, most medical visits after vaccination occurred less frequently than expected, a pattern researchers attribute to the “healthy vaccinee effect,” where children tend to be vaccinated when they’re already well.
What About Thimerosal?
Thimerosal is a preservative that contains a form of mercury called ethylmercury. It’s used only in multi-dose vials of the flu vaccine to prevent bacterial contamination each time a dose is drawn from the vial. This is a different compound from methylmercury, the type that accumulates in fish and can be harmful at high levels. Ethylmercury is processed and cleared by the body much more quickly.
Data from numerous studies show no evidence of harm from the low doses of thimerosal in vaccines. If thimerosal concerns you, single-dose vials, prefilled syringes, and the nasal spray version of the flu vaccine do not contain it. These thimerosal-free options are widely available.
Egg Allergies Are No Longer a Barrier
For years, people with egg allergies were told to take special precautions before getting a flu shot, since most flu vaccines are manufactured using eggs. That guidance changed. Beginning with the 2023-2024 flu season, no additional safety measures beyond standard practice are recommended for people with egg allergies, regardless of how severe their past reactions to eggs have been. People with egg allergy can receive any flu vaccine appropriate for their age. Non-egg-based options also exist for those who prefer them.
How Safety Is Monitored Year After Year
Flu vaccine safety doesn’t rest on old data alone. The federal government runs four complementary surveillance systems that continuously track vaccine safety. The most visible is the Vaccine Adverse Event Reporting System (VAERS), a national early warning system where anyone, patients, parents, or healthcare providers, can report health problems that occur after vaccination. VAERS is designed to detect “signals,” meaning unusual patterns or spikes in reported events.
When a signal appears, scientists investigate it through more rigorous systems like the Vaccine Safety Datalink, which draws on medical records from millions of patients to determine whether a reported problem is actually caused by the vaccine or just coincidental. This layered approach means that even very rare side effects are likely to be caught and studied. The flu vaccine undergoes this scrutiny every single year because its formulation changes annually to match circulating strains.