How Long Does a Flu Shot Last?

We are told we need to get flu shots every year. It seems strange, given that pretty much every other vaccine we get during our lifetime lasts much longer than a year. Some people might think that one should be enough and surely they last longer than one season. Unfortunately, that just isn't the case.

Typically, the effects of a flu vaccine last through one flu season and sometimes a little longer. So if you get your vaccine - starting two weeks after vaccination - you should be protected from the flu for the remainder of that flu season. Everyone's immune response to the vaccine is different though, so this isn't a guaranteed amount of time. Some people may be protected into the following year and others may not.

Why Vaccinate Every Year?

Yearly vaccination is needed because often the strains of influenza included each year are different from flu season to flu season.

Researchers work hard each year to determine what strains of influenza are likely to cause illness the following flu season and the top three to four possibilities (two strains of influenza A and one or two strains of influenza B) are included in the vaccine for the following flu season.

Although it doesn't typically change drastically, most years there are at least slight variations in the strains of influenza included in the vaccines. Influenza viruses mutate frequently, making these changes necessary to keep up with the mutations and ensure the vaccine is effective as possible.

How Effective Are Flu Vaccines?

The efficacy of flu vaccines varies from year to year. If the strains of influenza included in the vaccine are well matched to the strains causing illness in the community, the vaccine will be more effective than if it isn't.

Generally, when the strains are well matched, the vaccine reduces the chances that the general population will get the flu by 40-60%.

It is very important to remember though that the vaccine is not a guarantee that you won't get sick! Even if it works well, it won't protect you from every illness - only influenza. Many people decide flu vaccines don't work when they still get bad colds or a stomach virus during the season that they had a flu shot. Unfortunately, the vaccine won't protect you against those illnesses because they are not caused by the influenza virus.

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Article Sources

Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial policy to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Immunization Action Coalition. Ask the experts about influenza. Updated November 7, 2019.

  2. Centers for Disease Control and Prevention. Selecting viruses for the seasonal influenza vaccine. Updated September 4, 2018.

  3. Centers for Disease Control and Prevention. How well do the flu vaccines work? Updated January 3, 2020.

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