Find out how long the flu shot lasts.

Every year, medical professionals and public health experts recommend people get the annual flu shot. The influenza vaccine reduces the risk of catching the flu or going to the hospital for the flu. But how long does that flu shot protection last?

How Much Protection Does the Flu Shot Provide?

The effectiveness of the flu shot varies from one flu season to the next. Overall, the flu shot lowers your chance of catching the flu by 40% to 60%, depending on the season. That means getting the vaccine usually cuts your risk of catching flu by about half.

The reason for different levels of protection has to do with what flu strains and types are circulating during the season. Most years, four main strains of flu viruses spread during flu season. These include:

  • Influenza A (H1N1)
  • Influenza A (H3N2)
  • Influenza B (Victoria)
  • Influenza B (Yamagata)

But each of these strains is a little different than it was in the previous flu season. Flu viruses collect mutations during the year that change how they look to the immune system. Scientists try to make the strains in the flu vaccine similar to the versions they predict will circulate in the next season.

Sometimes the scientists predict correctly, and the flu vaccine contains the same strains that spread most during flu season. During those seasons, the flu shot is very effective and prevents more people from getting the flu.

Some years, the flu virus changes more than what public health experts expect. Or some strains have changed since production of the flu shot.

During those flu seasons, the strains in the vaccine may not match the strains that are circulating very well. Flu shot protection against catching the flu is lower during those seasons.

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Can You Still Catch Flu after the Vaccine?

You can still catch flu after getting the flu shot because the vaccine is not 100% effective. The flu shot usually cuts your risk of catching the flu in half, but you can still get sick from flu.

Your risk of catching flu depends on what flu strains you come across. If you come across strains that match the ones in the shot you got, your risk of catching flu is lower. If you have exposure to flu viruses different from the ones in the vaccine, your risk of catching the flu is higher.

Even if you catch the flu, the flu vaccine lowers your risk of severe disease, hospitalization, or death.

Other Types of Flu Shot Protection

It is still important to get the flu shot even if you can still catch flu afterward. This is because the flu shot impacts how severe your infection will be if you do catch flu. That’s because getting vaccinated lowers your chance of needing the hospital or dying from flu.

For example, between 2009 and 2016, vaccinated adults were 40% less likely to be hospitalized for the flu than unvaccinated adults. Between 2012 and 2015, vaccinated adults were 82% less likely to need the ICU for an influenza infection than unvaccinated adults.

Children are also less likely to need to go to the hospital if they get the flu shot. Between 2010 and 2012, vaccinated children were 74% less likely to need the pediatric ICU for flu than unvaccinated children.

The flu shot also reduces your chances of dying from the flu if you do catch it. A study from 2021 showed that flu vaccination reduced the chances of dying from flu by 31%.

During the 2019-2020 flu season, the CDC estimates that flu vaccination prevented:

  • 7.5 million cases of the flu.
  • 3.7 million medical visits because of flu.
  • 105,000 hospitalizations due to flu.
  • 6,300 deaths from flu.

How Long Does the Flu Shot Protection Last?

The flu shot starts to offer you protection about two weeks after you get it. How long that protection lasts also changes from one season to the next. The more time that passes after you get your flu shot, the lower your protection is.

Some years, the flu shot’s protection wears off as soon as three months after vaccination. Most years, your flu shot protection lasts about five to six months. Flu season lasts about six months, beginning in October or November and peaking in February before dropping off in March and April.

Health experts recommend getting your flu shot by October or November to have protection for as much of flu season as possible. If you get it too early, it may wear off by early spring, when flu is still spreading. If you wait too long, you may catch flu in the early part of flu season.

Why Do I Get a Flu Shot Every Year?

You need a new flu shot each year for two main reasons. First, the flu shot’s protection against the flu wears off during the year.

Second, flu viruses change during each season. Each year, the flu shot teaches your immune system to recognize these new flu virus strains.

Even if you get sick from flu, your immunity from the infection gradually wears off over time. Also, the flu virus also mutates, or changes, from year to year. That means the immunity you developed from a previous infection may not work against a newer flu strain.

Why Does the Flu Virus Keep Changing?

The flu virus changes often because it reproduces very quickly. Each time the flu virus makes copies of itself in someone’s body, mistakes happen in the genetic code. These mistakes are mutations.

Mutations during the copying process mean the new viruses are not all identical to the original virus. Most of the new viruses with mutations will not survive. But some will survive.

The ones that survive go on to make more copies of themselves that are different from the original virus. Soon these slightly different versions of the virus will spread around. These are the viruses your body may not recognize.

This is why the flu shot from one year may not offer much protection against flu viruses in a different year. It’s also why your body may not recognize the flu even after fighting it in a past year. This is why experts recommend a new flu shot each year.

Editor's Note: This article was originally published on , and was last reviewed on .

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