Updated October 6, 2026
Quick answer: The most common flu shot side effects are soreness, redness, or swelling where the shot was given, and sometimes a low-grade fever, headache, muscle aches, nausea, or tiredness. They usually start within a day and fade within 1 to 2 days. The flu shot cannot give you the flu. Call 911 for signs of a severe allergic reaction, such as trouble breathing or swelling of the face or throat, which are rare and usually begin within minutes to a few hours.
Walk in for your flu shot, no appointment needed, at 797 S Arroyo Pkwy, Pasadena. We are open Monday to Friday 8:00 AM to 8:00 PM and Saturday and Sunday 9:00 AM to 5:00 PM, and we take our last patient 30 minutes before close.
Common flu shot side effects
- Soreness, redness, or swelling where the shot was given (the most common)
- Low-grade fever
- Headache
- Muscle aches
- Nausea
- Tiredness
These reactions are signs that your immune system is responding to the vaccine. They are usually mild and do not need treatment.
How long do flu shot side effects last?
Most side effects begin within hours to a day after the shot and fade within 1 to 2 days. A sore arm can feel worse the day after the shot before it improves. If you are feeling worse instead of better after 2 days, or you have a fever that lasts longer than that, something else may be going on, such as a cold, COVID-19, or another infection, and it is worth being seen.
Can the flu shot make you sick?
No. Flu shots are made with inactivated (killed) virus or with a single protein from the virus, so they cannot cause the flu. So why do some people feel sick afterward?
- Side effects. A day of aches, tiredness, or a low fever is your immune system at work, not the flu.
- Another virus. Flu shot season overlaps with colds, RSV, and COVID-19, so catching one of these around the same time is common.
- Exposure before protection builds. It takes about two weeks after the shot to be protected, so you can still catch the flu during that window.
- No vaccine is perfect. The flu shot lowers your risk of getting sick and of serious illness, but it does not prevent every case.
The nasal spray flu vaccine is different: it contains weakened live viruses that cannot cause flu illness but can cause a runny nose, congestion, or sore throat for a day or two.
Feeling sick after a flu shot? What helps
- Rest, and drink plenty of fluids.
- Put a cool compress on a sore arm, and keep moving the arm gently.
- An over-the-counter pain reliever such as acetaminophen or ibuprofen can ease aches or a low fever. Follow the label, and ask a provider first if you have a medical condition or take other medicines.
Can you get a flu shot with a cold?
Usually, yes. According to the CDC, a mild illness such as a cold is generally not a reason to delay your flu shot. If you have a moderate or severe illness, with or without a fever, it is usually best to wait until you have recovered. Tell our staff how you are feeling, and the provider will help you decide.
When to get care
Call 911 for signs of a severe allergic reaction: trouble breathing, wheezing or hoarseness, swelling of the face, lips, or throat, hives, a fast heartbeat, dizziness, or weakness. These are rare and usually begin within minutes to a few hours after the shot.
Walk in or call us for a fever or aches that last more than 2 days, symptoms that are getting worse instead of better, redness, warmth, or pain at the injection site that keeps spreading, shoulder pain that does not improve, or new weakness or tingling in your legs.
Get your 2026-2027 flu shot
This season's flu vaccines were updated to match the viruses expected to circulate, and the CDC recommends a yearly flu vaccine for everyone 6 months and older without a reason to avoid it. September and October are ideal, but later is still worthwhile. Our 2026-2027 flu season guide covers timing and what is new this year.
Frequently Asked Questions
Can the flu shot make you sick?
No. Flu shots are made with inactivated virus or a single protein from the virus, so they cannot give you the flu. Some people get mild side effects such as a sore arm, low-grade fever, or aches for a day or two, which is the immune system responding.
Is it normal to feel sick after a flu shot?
Feeling a little run down, achy, or feverish for a day or two is a common, normal reaction. Feeling sick for longer, or getting worse, usually means another illness, such as a cold or COVID-19, and is a reason to be seen.
How long do flu shot side effects last?
Most side effects start within a day of the shot and fade within 1 to 2 days.
Can you get a flu shot with a cold?
Usually, yes. A mild illness such as a cold is generally not a reason to delay. If you have a moderate or severe illness, with or without a fever, it is usually best to wait until you feel better. Tell our staff how you feel and the provider will advise you.
When should I worry about a reaction to the flu shot?
Call 911 for signs of a severe allergic reaction, such as trouble breathing, swelling of the face or throat, hives, a fast heartbeat, dizziness, or weakness. These are rare and usually start within minutes to a few hours. See a provider for a fever or aches that last more than 2 days, redness or pain at the injection site that keeps spreading, or new weakness or tingling in the legs.
Can I walk in for a flu shot?
Yes. Walk in during business hours, no appointment needed, or save your spot online. We are open Monday to Friday 8:00 AM to 8:00 PM and Saturday and Sunday 9:00 AM to 5:00 PM, and we take our last patient 30 minutes before close.