Getting a flu shot each year is a straightforward yet impactful way to protect your health. Many people experience mild, temporary reactions after receiving the vaccine. These reactions are a normal indication that your body is building protection against the influenza virus.
Typical Reactions to the Flu Shot
After receiving a flu shot, it is common to experience side effects. The injection site, typically on the upper arm, may become sore, red, or slightly swollen. Beyond the injection site, individuals might also notice systemic symptoms. These can include a low-grade fever, a headache, muscle aches, and fatigue. These responses are typically mild.
How Long Symptoms Last
Most side effects from a flu shot are mild and resolve quickly, usually within one to two days. Symptoms often begin within hours of vaccination. Soreness or swelling at the injection site typically lasts less than two days. Headaches and body aches also commonly appear on the first day and subside within two days. A low-grade fever, if present, also tends to resolve within a day or two.
Distinguishing Normal Reactions from More Serious Concerns
It is important to differentiate between typical, mild reactions to the flu shot and symptoms that may indicate a more serious concern. Normal side effects like injection site soreness, low-grade fever, and muscle aches are usually mild and temporary, generally improving within a couple of days. However, certain symptoms warrant immediate medical attention.
Seek medical care if you experience signs of a severe allergic reaction, which are rare but possible. These signs can include difficulty breathing, wheezing, hives, swelling around the eyes, lips, or throat, paleness, weakness, or a fast heartbeat. These severe reactions would most likely appear within minutes to a few hours after the vaccine is given. Additionally, if a fever exceeds 102.2°F (39°C) or lasts more than 48 hours, or if you develop a severe headache, confusion, or persistent dizziness, contact a healthcare professional.
Understanding Why Reactions Occur
The temporary feeling of being unwell after a flu shot is a normal part of your body’s immune system responding to the vaccine. Vaccines work by introducing your body to a non-infectious form of the influenza virus, which cannot cause the flu itself. This prompts your immune system to produce antibodies, which are proteins designed to recognize and fight off future infections from the actual flu virus.
When your immune system activates to build this protection, it can lead to a mild inflammatory response. This inflammation causes symptoms like soreness at the injection site, muscle aches, and a low-grade fever. These reactions indicate that your body is successfully learning how to defend itself against the flu.