How Long Does It Take for the Flu Shot To Be Effective?

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Flu season comes around every year. And so should your plan to get your flu shot. The shot, also known as the influenza vaccine, is a vaccine that protects against the influenza viruses. All flu vaccines in the United States are "quadrivalent" vaccines, which means they protect against four different flu viruses: an influenza A(H1N1) virus, an influenza A(H3N2) virus, and two influenza B viruses.

Most flu vaccines are given with a needle, usually in the arm, but there also is a nasal spray flu vaccine. Most importantly, the flu vaccine is the best way to reduce your odds of spending time in flu agony.

Why You Should Get the Flu Shot

The flu may be common but is a potentially serious disease. In some people, it can lead to hospitalization and sometimes even death. The Centers for Disease Control and Prevention (CDC) estimates that, annually between 2010 and 2020, the flu has caused:

But it's important to remember that each flu season is different, and flu can affect people differently during different flu seasons. For example, during the 2021-2022 influenza season, due to COVID-19 restrictions including social distancing and masking, the CDC estimated that influenza was associated with 9 million illnesses, 4 million medical visits, 100,000 hospitalizations, and 5,000 deaths.

Also, if you're wondering if you really need the flu vaccine, the answer is 100% yes. The CDC recommends that everyone six months and older get vaccinated every year.

"We always encourage everybody to get vaccinated as soon as possible," said Jean Moorjani, MD, a pediatrician at Orlando Health Arnold Palmer Hospital for Children.

Vaccination to prevent flu and its potentially serious complications is particularly important for people who are at a higher risk of developing serious flu complications. If you are 65 or over, or have certain health factors such as chronic lung disease, asthma, and kidney disease, you are at an increased risk of developing serious complications from flu.

How Long Before the Vaccine Works

To keep yourself safe during the flu season, it's best to get your vaccine as soon as you can each fall. But there is one thing the flu vaccine can't do: safeguard you from the flu immediately. In fact, the vaccine needs some time to work its magic.

"It takes about two weeks after getting the vaccine for your body to build up enough antibodies to protect against the flu," said Dr. Moorjani. That means the flu could still creep up on you during that two-week waiting period.

Since you need to give your body two weeks to build up those flu antibodies, it's crucial you get your vaccine before the end of October, said Dr. Moorjani. No one wants to enter the thick of flu season, which ramps up in November and December, without proper protection.

Other Precautions

Because of the two-week build-up period, it's important to take other flu-preventing precautions. You still need to get enough sleep, maintain a healthy diet, and stick to your workout schedule to keep your immune system in fighting shape in case you do encounter the flu virus.

And of course, wash your hands with soap and water (or use hand sanitizer if you can't find a sink) regularly—especially before you eat or touch your eyes, nose, or mouth, all of which are conduits that allow flu viral particles to enter your body and infect you. Even after the two-week wait, never slack on these anti-flu measures.

How Effective the Flu Shot Is

Flu vaccine effectiveness can vary from season to season. The variation depends in part on the age and health status of the person getting the vaccine and the similarity or "match" between the viruses in the vaccine and those in circulation.

During seasons when flu vaccine viruses are similar to circulating flu viruses, the flu vaccine has been shown to reduce the risk of flu illness by 40% to 60%.

A Quick Review

So, to stay healthy during flu season, don't delay getting your flu vaccine. It can up to two weeks for the flu vaccine to be effective so be sure to get it before the peak of flu season and, in the two weeks after receiving your vaccine, be sure to wash your hands often, eat a well-balanced diet, and exercise to stay healthy.

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  1. Centers for Disease Control and Prevention. Influenza (flu) flu shot.

  2. Centers for Disease Control and Prevention. Burden of flu.

  3. Centers for Disease Control and Prevention. 2021–2022 Estimated Influenza Burden.

  4. Centers for Disease Control and Prevention. How to prevent flu.

  5. Centers for Disease Control and Prevention. Who is at higher risk of flu complications.

  6. Centers for Disease and Control Prevention. How well flu vaccines work.

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