Family Practitioner Questions Flu Shot

Why do I have a fever after my flu shot?

I get my flu shot every year. I've never gotten a fever from it before until now. What's wrong? Could this just be coincidence?

19 Answers

It may be a slight result of vaccination doing its job. It also may be a coincidence. It also depends on how high the fever is. You do not mention that reading.
Be sure to mention to your primary care doctor, but if the fever was low or you just felt a little warm for awhile, it is probably the vaccine doing what it is meant to do.

Take Care and God Bless
The flu shot formulations are slightly different every year. Fever is a common side-effect from the flu shot and may last 2-3 days. There is no need for concern as long as the fever is less than or equal to 101F AND as long as over-the-counter antipyretics (like acetaminophen) help.
Sorry for the late response. It could be a coincidence, esp. if it has remitted by now.
It could be a coincidence, and you happened to get a different viral infection at the same time. However, they did place a new strain into this year's vaccine, and you make be appropriately reacting to the vaccine to make antibodies against the circulating strains.
According to the CDC, the flu shot can cause mild side effects such as muscle and joint soreness, redness or swelling at the injection site, a low-grade fever and body aches. These symptoms should go away within two days. These side effects are mild and short-lasting when compared to symptoms you would feel with a bad case of the flu. Remember, because the flu vaccine is composed of the "dead" virus, you cannot get the flu from a flu shot.
This is a good question! Fever is the body’s natural response when it encounters an invader. This means your immune system kicked into gear. It is not uncommon for you to not experience a fever previously, because you may have other underlying invaders, such as the virus that causes the common cold, that your immune system is fighting, at the same time you got your vaccine. If your fever remains after a few days, or goes above 102 degrees Fahrenheit then seek medical attention. Be sure to drink plenty of fluids and rest when you can.

Dr. Kelly Walker DNP, FNP-C, APRN
It could be a coincidence or you could be having a mild reaction. If you don't develop any further symptoms, I would not allow it to prevent you from having another flu vaccination next year, although you might take a dose of tylenol with it as a prophylactic measure!
Slight fever and possibly also aching symptoms some folks get after any vaccination, not only the flu shot, is an indication that your immune system is mobilizing a response and generating antibodies, which is the purpose of the vaccine in the first place. This is NOT undesirable, but a natural consequence of an immune system being challenged. This can mimic an infection, as the exact same response is triggered by infections, and is the reason some people claim they get the flu from a flu shot -- WRONG! The symptoms are similar, though much milder, and there is no infection possible from the flu shot.
Immunizations depend on the body's immune system to respond. Part of this response can be local warmth or redness, mild malaise, or even low-grade fevers. OTC medication usually helps with the symptoms, and they are usually transient. If they last more than 24-48 hours or get progressively worse, get checked out.
According to the Center for Disease Control, mild side effects from the flu shot include soreness, redness or swelling at the injection site, low-grade fever and aches. For children, side effects from the flu nasal spray can include runny nose, wheezing, headache, vomiting, muscle aches and fever.

These side effects are not due to the flu virus as is often a misconception. They are most likely to occur in the first 1-2 days and then stop. Studies show that only ~5% of people have fever and muscle aches.
Very often a healthy person can tolerate getting the flu, but they. can give it to one of the high risk groups mentioned above. This is the logic behind all people getting this important immunization!
The flu shot can cause low grade fever lasting 1-2 days. This is not to be a cause of alarm. Since the shot contains inactive virus particles you cannot get the flu from the vaccine. If the fever does persist you may want to see a physician as it is likely unrelated to the vaccine and could be sign of another infection unrelated to the vaccine.

Richard Aballay
The flu vaccine is an attenuated virus which means that it is just like the regular flu virus but it has some changes so it not as virulent as the native strain. It does not cause you as many symptoms as the real virus but your body still mounts a reaction to fight the virus and it is your body that creates the fever just like a regular flu. It is normal to feel like you have some of the symptoms of a flu after getting the flu vaccine.
The vaccine gives you a small dose of the viruses that cause the flu so you may have a reaction i.e. Fever It should respond to Tylenol and rest and fluids
The flu vaccine is like giving yourself
a small dose of the flu so your immune system is going to work on creating resistance. Some vaccines are active and others are inactive but they basically create immunity the same way.
It is possible to develop a low-grade fever after any vaccination - this can sometimes be a sign that your body is mounting an immune response to the vaccine. While you can't get sick from a flu shot (they don't contain live influenza virus), some people do feel feverish or achy for a few days afterward. It can also be coincidence - there are always other viral infections circulating in the community!
Either you were in the process of getting ill at the time of your shot and it is just a coincidence, or it is a sign that your body is mounting an immune response to the shot, which is a good thing. In any case keep a close eye on it, and see your doctor if it doesn't resolve in a couple of days or worsens.
Best regards,
Dr. McGill
Low grade fever is not unusual after a flu injection, along with some weakness and local muscle soreness. It is thought to be due to the vaccine inducing an immune reaction, which can manifest as slight fever. It will last only a couple of days and Tylenol or Ibuprofen should help control those symptoms .
If you have high fever over 101F, you need to seek medical attention as soon as possible.
The flu vaccine rarely causes serious adverse effects. The most common reaction to the flu shot is mild soreness at the site of injection which is well-tolerated. Flu-like symptoms, such as fever and muscle aches, occasionally occur six to 12 hours after receiving the flu shot and last one to two days. If your symptoms do not resolve or if symptoms worsen, follow-up with your regular doctor.