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

A flu shot doesn't start to work right away, say experts.

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 that research indicates will be most common during the upcoming season, according to the Centers for Disease Control (CDC).

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 "flu shots" 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 a week in flu agony.

Why 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. According to the CDC, millions of people get flu every year, hundreds of thousands of people are hospitalized and thousands to tens of thousands of people die from flu-related causes. But it's important to remember that each flu season is different, and flu can affect people differently during different flu seasons.

Also, if you're wondering if you really need the flu shot, the answer is 100% yes. The CDC recommends that just about everyone over the age of six months gets vaccinated.

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

Vaccination to prevent flu and its potentially serious complications is particularly important for people who are at higher risk of developing serious flu complications. The CDC says being over age 65, or certain health factors such as chronic lung disease, asthma, and kidney disease, are known to increase a person's risk of getting serious complications from flu.

When To Get the Flu Shot

To keep yourself safe during the flu season, it's best to get your shot as soon as you can. But there is one thing the flu shot 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.

That's because the flu shot causes antibodies to develop in the body about two weeks after vaccination. These antibodies provide protection against flu illness, says the CDC.

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 to Ward Off the Flu

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 Is the Flu Shot?

According to the CDC, 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 having to go to a healthcare provider with flu by 40% to 60%, says the CDC.

So, to stay healthy during flu season, don't delay getting your flu shot. Flu vaccines can prevent hospitalizations, severity of illness, and keep you from getting sick with flu.

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