How Long Does a Flu Shot Last?

Given the potential complications of the flu and that some people are at increased risk for them, you may wonder just how long the flu shot lasts and if it will offer you the complete protection that you seek. The effects of the vaccine generally only last through one flu season, which is about six months long. That's part of the reason you need one every year.

Starting two weeks after you get a flu shot, you should be protected from certain influenza viruses for the remainder of that flu season. Given the delay, properly timing your flu shot is important.

When Should I Get Vaccinated?

In the United States, flu season typically:

• Starts in October
• Peaks sometime between December and February
• Tapers off by April or, at the latest, May

The Centers for Disease Control and Prevention (CDC) recommends getting vaccinated by the end of October. If you miss that date, though, don't consider it too late. The worst of the season is still ahead, so it's still worthwhile to get the shot in January or even later.

Because flu vaccines' effectiveness starts to wane after about six months, it's not a good idea to get it too early, such as in July or August. You want to make sure you're protected through the winter months and into spring.

Building Up Antibodies

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 isn't fully in effect for the first two weeks after you get it. That means you're most protected when you get the shot before people around you start coming down with the flu. Then, after about six months, the number of antibodies in your immune system starts to decline.

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.