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Flu Shot
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Immune System

Flu Shot

Annual vaccination is the most effective way to protect against influenza and reduce the risk of serious complications.

What is the Flu?

The flu (influenza) is a respiratory infection caused by viruses that infect the nose, throat, and lungs. While most people recover on their own, the flu can be serious or fatal for some. Those at higher risk of complications include people 65 and older, children under age 5, pregnant individuals, and those with certain health conditions.

Benefits of Vaccination

Getting a flu shot each year provides several key benefits:

  • Reduces the likelihood of getting sick with the flu.
  • Lessens the severity of the illness if you do get infected, reducing the risk of hospitalization or complications.
  • Helps prevent chronic health conditions (such as COPD or diabetes) from worsening due to the flu.
  • Protects pregnant individuals and their babies during the first few months of life.
  • Lowers the risk of severe, life-threatening flu in children.
  • Protects those around you, including vulnerable individuals.

How It Works

The flu vaccine triggers the immune system to produce antibodies approximately two weeks after administration. These antibodies provide protection against the specific flu viruses contained in the vaccine.

Recommendations and Timing

The CDC recommends an annual flu shot for everyone 6 months and older. Because flu viruses change constantly and immune protection declines over time, the vaccine is updated and administered every year.

For most people, September and October are optimal times for vaccination. Children requiring two doses should receive their first as soon as it is available, followed by a second dose at least four weeks later.

Safety and Side Effects

Flu vaccines are safe, though mild side effects may occur, including:

  • Soreness, redness, or swelling at the injection site
  • Headache
  • Fever
  • Nausea
  • Muscle aches

While rare, some individuals may experience fainting or serious allergic reactions. Individuals with severe, life-threatening allergies to vaccine ingredients or a history of Guillain-Barre syndrome (GBS) should consult their healthcare provider before vaccination.

Frequently Asked Questions

No, you cannot get the flu from the flu shot. The injectable vaccine contains inactivated (killed) virus or specific viral proteins, not live virus, so it is biologically impossible for it to cause an influenza infection. If you feel unwell after vaccination, it is likely a mild immune response such as soreness at the injection site, low-grade fever, or fatigue, which typically resolves within a day or two. Is it too late to get a flu shot if I am already sick with a cold?::Generally, it is safe to receive a flu shot if you have a mild illness, such as a common cold without a fever. However, if you have a moderate to severe illness with a fever, it is best to wait until you have recovered before getting vaccinated. This ensures that your body can mount an optimal immune response to the vaccine and prevents the confusion of attributing vaccine side effects to your existing illness. How long does it take for the flu shot to start working?::It typically takes about two weeks after vaccination for your body to develop antibodies that provide protection against the flu viruses. This is why health experts recommend getting the shot by the end of October, ensuring that immunity is established before flu activity usually begins to rise in the community. Who should avoid getting the flu shot?::People with a history of severe allergic reactions to a previous flu vaccine or to any component of the vaccine (such as eggs in some formulations, though egg-free options are available) should consult their doctor before getting vaccinated. Additionally, individuals with a history of Guillain-Barre syndrome within six weeks of a previous flu vaccination should discuss the risks and benefits with their healthcare provider.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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