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Here's why doctors urge people to get the flu shot every year

When flu season rolls around, you need to get a new flu shot. No matter what.

With months to go in this year's flu season, the flu shot remains the best way to protect yourself from the potentially debilitating virus.

In fact, even if you've already been sick with the flu this season, it's still important to get a flu shot if you haven't yet done so. It could protect you from getting sick again.

Every major medical group recommends an annual flu shot. Here are five things doctors want you to know about the vaccine:

It's not perfect, but it may save your life

Infectious disease experts convene every February to determine which strains should be covered for the upcoming flu season, which runs from roughly October to April or May. The vaccine must be tweaked every year because influenza viruses are constantly changing and mutating. Multiple flu strains circulate in any given year, and the yearly vaccine aims to protect against three or four of the most likely strains.

This year's vaccine appears to be a good match for one of the strains currently circulating, called influenza A H1N1, said Dr. William Schaffner, a professor at Vanderbilt University and the medical director of the National Foundation for Infectious Diseases.

But it's not as good a match for another strain this year, the B/Victoria strain, which has been responsible for a high percentage of the illnesses so far this season, Schaffner said.

While there's never a guarantee the vaccine will prevent all flu cases, doctors say it can ease the severity of the flu if you do get sick, and help prevent complications, including pneumonia and having to be hospitalized.

And a 2018 study found people who got the vaccine each flu season were less likely to die or end up in the hospital with severe flu.

What's the best time to get the flu shot?

Infectious disease experts rarely if ever say it's "too late" to get a flu shot. But even Schaffner told NBC News this time of year — mid-January — is cutting it close.

"It is late," he said. "If you haven't been vaccinated yet, get it this afternoon."

There does appear to be a sweet spot when it comes to the best time of year that would likely offer the best protection: in the fall, preferably before Halloween. Indeed, the Center for Disease Control and Prevention's Advisory Committee on Immunization Practices recommends the flu vaccine be administered before the end of October.

It takes two weeks for the body to build immunity after getting a flu shot, so an October vaccination generally means a person would be as protected as possible before the season gets going, and before families get together for the holidays, which increases the likelihood of spreading germs.

The flu vaccine does not offer year-round protection

The antibodies your immune system produces after getting a flu shot decline over time. There's some evidence that immunity starts to wane within as little as six months, especially for people with less robust immune systems. A flu shot in October would therefore offer protection through the bulk of flu season.

And because flu strains evolve so rapidly, doctors say it's important to get a new flu shot at the start of each flu season, not just on an annual basis.

That means that even if you get a shot late in the season — in the early spring, for example - you'll still need a new one at the start of the next flu season — meaning several months later, in the fall.

I've already had the flu this season. Do I still need the shot?

If you haven't already received the shot, the answer is yes, according to physicians. Because multiple flu strains circulate simultaneously or pop up randomly, it's possible to become infected with different flu strains at different times in a single season.

And sometimes, there are two different waves of flu. The 2018-2019 flu season is a good example. It began with a relatively mild strain that appeared to be wrapping up around this time last year. But then in February a surprising second wave of a particularly virulent flu strain appeared. Flu season lasted into April, and ended up being one of the longest on record.

Who should get the flu vaccine?

Though the CDC recommends virtually everyone over age six months be vaccinated, some groups are particularly susceptible to flu's complications.

Those include children, pregnant women and adults over age 65. People with chronic diseases, such as asthma and diabetes, may also be at particular risk, as well as anyone with a compromised immune system.

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