How Long Does the Flu Shot Last? Here's What Experts Say

Are you still covered from last year?

If you find yourself with those typical flu-like symptoms—fever, chills, nasal congestion, the works—you might start worrying that you've come down with the flu (yep, even in August).

But wait...didn't you get the flu shot last year? And if that's the case, how long does a flu shot last, exactly—and are you still covered? To help you figure things out, Health spoke with a few infectious disease experts (you know, doctors that actually specialize in diseases like the flu) to find out how long you're covered with a flu shot, and what that means for when you should get your next one.

All right, how long does the flu shot last?

The short answer: six months. But the long answer is a little bit more complicated, in part because every body is different. As a general rule “the flu shot is most effective in the first three months, [but] people still have protection after six months,” Vanessa Raabe, MD, pediatric infectious disease specialist at NYU Langone Health, tells Health.

Knowing how long the flu shot lasts plays a pretty important part in determining when to get your flu shot each year. That part requires a little bit of math (luckily, these doctors are here to do the dirty work for you). Flu season, in general, lasts from October to May, according to the Centers for Disease Control and Prevention (CDC). But flu season actually peaks sometime between December and March, Frank Esper, MD, pediatric infectious disease specialist at Cleveland Clinic tells Health.

That means you should definitely get the flu shot before flu season peaks. You should get it "at least two weeks—but preferably four [beforehand]—to have full immunity," says Dr. Esper. “You need a little preplanning time to get your shot, to have that shot develop its effectiveness,” he adds. That means you should plan on getting a new flu shot in September or October, Dr. Esper says. The CDC also recommends getting a flu shot by the end of October.

So, unfortunately, if you got your flu shot at the correct time last year, you're not currently covered (and yes, that can matter, since flu technically exists year round and only peaks during flu season). And if you got one later in the season last year, say, in March? Unfortunately, you're still not covered right now, even if you're within the six-month range. That's because the flu strains differ from year to year. The shot you got last year “was for last season’s strains, and the strains coming up this season may be different,” Dr. Esper says.

In fact, Dr. Raabe adds, the two A strains of flu (there are three types that affect humans: A, B, and C viruses), which are the most common, are expected to be different this flu season. For that reason, the flu shot from last year was “tweaked” to better protect people from those strains, Dr. Raabe says.

Basically, you need to get the flu shot every year—no exceptions, unless you're severely allergic to the vaccine or are six months of age or younger. And yes, while the flu shot isn't 100% effective, it can prevent the flu from becoming fatal. “The flu shot is not 100%, [but] it’s there to make sure you don’t die from the flu [or] get hospitalized,” Dr. Esper says.

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