Common flu shot side effects include pain, redness or localized swelling, as well as fever, muscular pain and headache. These normally emerge within a few hours of the injection.

Reactions are mild and generally improve within 2 to 3 days without any specific treatment. However, there are some measures you can take to help relieve specific side effects from vaccination.

A more serious allergic reaction may present with a high fever, accelerated heart rate, shortness of breath, weakness, or body paralysis. In these cases, you should seek medical attention immediately.

The most common side effects from the flu shot include:

1. Localized pain

Reactions that are localized to the site of injection, like pain, redness or swelling, are commonly experienced following the flu shot. These side effects can be noted for up to 72 hours after injection, and are more common in those over the age of 65.

How to relieve: You can apply a cold compress to the arm where the injection was administered for the first 24 to 48 hours. If the pain is very intense, if a red rash emerges, or if you have difficulty moving the arm, you should seek medical attention immediately.

2. Headache or muscular pain

A headache or general muscular pain can emerge 6 to 12 hours following vaccination. It usually improves within 2 days. In addition, some people may experience joint pain.

How to relieve: You should rest and drink plenty of fluid. If the pain is very intense, you can take analgesics, like acetaminophen, as directed by your doctor.

3. Fever

Another common side effect of the flu shot is a mild fever, with a temperature up to 38ºC (or 100.4ºF). This is normal following a vaccination, and can be accompanied by other symptoms like chills, fatigue and excessive sweating. These symptoms are usually temporary and emerge 6 to 12 hours following infection, and last for up to 2 days.

How to relieve: You should drink plenty of fluids to ensure hydration and rest as much as possible. If these symptoms cause moderate discomfort, you can take analgesics or fever medication, like acetaminophen, as indicated by your doctor. Learn about other ways to treat a fever at home.

Less common side-effects

Although they are more rare, the flu shot can cause serious side effects that require immediate medical attention, like:

High fever, over 39ºC (or 102.2ºF)
Itching or hives, that can emerge throughout the body
Severe allergic reaction or anaphylactic shock, which can occur immediately following the injection or within hours of administrations. This reaction presents with symptoms like increased heart rate, shortness of breath, swelling of the lips, tongue or throat, dizziness and cardiac arrhythmias
Guillain-Barré syndrome, which presents with symptoms like weakness and body paralysis. It can develop in people with a history of this illness, which is why any history should be reported prior to receiving the injection.
Blood vessel swelling in the kidneys, lungs or heart, which can cause symptoms like general malaise, fatigue, fever, lack of appetite and weight loss
Glandular swelling in the neck, armpit or groin due to changes in the lymph system that can cause swollen lymph nodes.

If you notice any of these more severe side effects, you should call 911 or proceed to the closest hospital.