The flu shot lasts for about 6 months: Here's when it starts working and why it may get less effective as time goes on

• The flu shot lasts for about six months after you receive it.
• However, it takes about two weeks for it to become effective after you've gotten it because your body is building up an immune response.  
• Also, according to recent research, the flu shot decreases in efficacy by about 16% every 28 days, so it may get less effective as time goes on.
• This article was medically reviewed by Jason R. McKnight, MD, MS, a family medicine physician and clinical assistant professor at Texas A&M College of Medicine. 
• This story is part of Insider's Flu Shot Guide. 

The flu shot lasts for about six months after you receive it. This should protect you through all of the flu season, from October through March. 

The flu vaccine is your first line of defense against getting sick from the influenza virus each year. By getting your shot at the start of flu season, your body can develop antibodies to help protect you from getting ill with the virus or minimize the severity of the infection.

There's never a wrong time to get the vaccine, since it will always offer some measure of protection, but getting it too late in the season can leave you vulnerable. Here's what you need to know about how long the flu shot can last and the best time to get it.  

How long the flu shot is effective

Once you get vaccinated, it will take about two weeks for your flu vaccine to become effective in your body. 

"That's because your body is building an immune response to adequately fight the flu," says Kellie Stecher, MD, a doctor at M Health Fairview Center for Women in Minnesota.

After it takes effect, both types of the flu vaccine — either the flu shot or the nasal spray — should offer protection for around six months. However, there may be a gradual decline in effectiveness over time. 

In fact, a 2018 study found that the flu vaccine decreases in efficacy around 16% every 28 days. This is because the influenza virus is constantly evolving, so the formula you got in October may not completely match the flu that is circulating in January. 

The best time to get a flu shot 

The best time to get a flu shot is in late September or early October. You should get a vaccine in the early fall because of the fact that it takes two weeks to build up antibodies and you want to be completely protected when the weather turns cold and flu season is in full swing. 

However, for some children, you may need to start the vaccination process earlier. That's because the CDC recommends that children between 6 months to 8 years receive two doses of the vaccine, if it's the first year they are getting vaccinated. 

Because the two vaccines should occur at least 4 weeks apart, you should start the immunization process as early in the season as possible to make sure they are covered when the peak of the season occurs. 

"The two vaccines allow their body to mount an adequate immune, antibody response," says Stecher. "When they haven't seen the vaccine or virus before, it is less effective if they receive the one dose. The first one typically readies the body for the response and the second helps efficiently make more antibodies." 

How often you should get the flu shot

In most cases, the CDC recommends getting a flu vaccine each year for people who are at least 6 months of age. 

But why not get a new flu shot at the beginning of spring, as soon as the other one wears off? "The virus survives better in this colder climate," says Stecher. The virus also mutates and changes during the winter, so getting vaccinated again with the same formula will likely offer little protection. In addition, the flu season has already started to wind down by the time the spring begins. 

Overall, the CDC recommends that you should ideally be vaccinated by the end of October. However, you can get vaccinated at any time during the flu season, and you shouldn't hesitate to get a vaccine if you are considering one later than October.  

The bottom line

The flu shot will help keep you covered for approximately 6 months. But keep in mind that it's not something that goes from 100% to 0% effective — the effectiveness decreases on a gradual basis. So, if you get the vaccine, you'll still have some immunity even after the six-month window when it's most effective.

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