How Long Does a Flu Shot Last?

The annual flu shot protects against influenza for up to six months. The Centers for Disease Control and Prevention (CDC) recommends a yearly influenza vaccine for everyone 6 months and older, preferably in September or October.

Properly timing your flu shot can help to ensure the vaccine is effective during the peak of flu season. It takes about two weeks to build your immunity after getting the flu shot.

This article explains how long a flu shot lasts and why. It also discusses the best time to get your flu shot and why you need a new one each year.

COVID vaccine syringe.

Cavan Images / Getty Images

When Should I Get a Flu Shot?

The CDC currently recommends most adults get the flu shot in September or October. This helps ensure that you're vaccinated before peak flu season starts and that the protection will last through spring.

Getting vaccinated earlier in the season is no longer recommended for adults. The influenza vaccine's effectiveness starts to wane after five or six months. A July or August flu shot may not keep you protected through the end of the winter.

If you miss the end of October deadline, though, it's not too late. The worst of flu season is still ahead, so it's still worthwhile to get the shot in January or even later.

When Should Children Get a Flu Shot?

Children between 6 months and 8 years old should get the influenza vaccine annually in September or October. However, children who get the flu vaccine for the first time (or who only had one dose of the vaccine in total from previous years) will need two doses at least a month apart. The CDC recommends children receive the first dose in July or August.

When Is Flu Season?

While you can get the flu any time of year, you are more likely to get it during flu season. In the United States, flu season was traditionally considered to begin in October, peak between December and February, and taper off by April or May at the latest.

However, in 2022, the CDC updated its recommendations to note that the timing of flu activity is difficult to predict and can vary in different parts of the country and from season to season.

Flu activity can ramp up earlier or later in different parts of the country. For example, in Tennessee, the 2022 flu season began in late September, with high pediatric hospitalizations reported in November. 

Influenza Season Timeline
New flu vaccine becomes available July or August
Get vaccinated September or October
Vaccine in full effect Two weeks after you get it
Flu season peaks December through February
Flu season wanes April or May
Flu vaccine wanes Six months after you get it

How Long Does the Flu Shot Take to Work?

Flu vaccines work by getting your immune system to produce antibodies to specific strains of the influenza virus. Then, if the virus finds its way into your body, your immune system already has the tools to fight it.

It takes time for your body to create antibodies, though, and they don't last forever.

The flu vaccine takes about two weeks to fully take effect. The number of antibodies in your immune system starts to decline after about five or six months.

Changing Flu Strains

Aside from the fact that the flu vaccine is only effective for several months, yearly flu vaccination is necessary because the strains of influenza included usually differ from flu season to flu season.

The most common types of seasonal flu, which are also the most serious, mutate quickly.

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

Although it doesn't typically change drastically, the vaccine usually varies slightly from one year to the next to account for new strains.

How Effective Are Flu Vaccines?

The effectiveness of flu vaccines, like their composition, 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 they aren't.

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

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 get a bad cold or a stomach virus after getting a flu shot. The vaccine only protects against respiratory illness caused by the influenza virus.

Summary

The flu shot protects against the influenza virus for about six months. The CDC recommends adults get the annual flu shot in September or October. Children under age 8 who need two doses of the vaccine should get the first in the series in July or August. 

It takes about two weeks after getting the flu shot for it to be fully effective. Getting vaccinated by the end of October will help to ensure you are protected before the holidays when flu activity typically ramps up. But if you get your flu shot too early, its protection may not last through the full season.

Traditionally, U.S. flu season starts in October, peaks from December through February, and tapers off by April or May. However, the timing varies in different parts of the country and can change from year to year. 

The influenza vaccine's effectiveness also varies from year to year. The flu shot contains three or four different strains of influenza, which change based on predictions for the upcoming season. If the predictions are right, the vaccine reduces flu activity by 40% to 60%.

6 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. National Center for Immunization and Respiratory Diseases. Ask the Experts: Influenza

  2. Centers for Disease Control and Prevention. Frequently asked influenza (flu) questions: 2022-2023 season.

  3. Centers for Disease Control and Prevention. Who needs a flu vaccine?

  4. Thomas CM, White EB, Kojima N, et al. Early and Increased Influenza Activity Among Children — Tennessee, 2022–23 Influenza Season. MMWR Morb Mortal Wkly Rep. 2023;72:49–54. doi:10.15585/mmwr.mm7203a1

  5. Centers for Disease Control and Prevention. Selecting viruses for the seasonal influenza vaccine.

  6. Centers for Disease Control and Prevention. Vaccine Effectiveness: How Well Do the Flu Vaccines Work?

Additional Reading

By Kristina Herndon, RN
Kristina Herndon, BSN, RN, CPN, has been working in healthcare since 2002. She specializes in pediatrics and disease and infection prevention.