Experts reveal everything you need to know about the 2019-2020 flu shot.

Vaccine injection. Syringe with medicine in the hands of a doctor in blue gloves. Prophylaxis and treatment of infectious and viral diseases. Inoculation against influenza. Vaccination against rabies.
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Flu season has begun, which means it's time to get the flu shot ASAP. But if you're not a fan of needles, you might be looking for more information, like how effective the flu shot is, and if it's even worth the trip to the doctor. (Spoiler: It is.)

First of all, if you're concerned that getting the flu shot will give you the flu, that's a total misconception. Flu shot side effects typically include soreness, tenderness, and swelling at the injection site. At worst, you might have some flu-like symptoms immediately after getting the shot, such as low-grade fever, muscle aches, tiredness, and headaches, Gustavo Ferrer, M.D., founder of the Cleveland Clinic Florida Cough Clinic, previously told us. (FluMist, the flu vaccine nasal spray, can have similar side effects.)

But considering the 2017-2018 flu season was one of the deadliest in decades—with over 80,000 deaths in total, according to the Centers for Disease Control and Prevention (CDC)—you're definitely better off getting vaccinated than not. (Related: Can a Healthy Person Die from the Flu?)

Plus, while last year's flu season wasn't quite as deadly, it was one of the longest on record: It started in October and continued through May, catching many health experts totally off-guard. On the bright side, by mid-season, statistics showed that the flu shot had reduced the risk of contracting the illness by 47 percent in vaccinated people, according to a report from the CDC. Compare that to the 2017-2018 flu season when the flu shot was 36 percent effective in vaccinated people, and it might sound like the vaccine is getting better each year, right?

Well, not exactly. Keep in mind, the flu shot's effectiveness is, in large part, a reflection of the dominant strain of flu, and how receptive it is to the vaccine.

So, how effective is the flu shot this year?

Flu season doesn't typically kick off until mid to late October, so it's too early to know for sure which strain(s) of the disease will be most prominent. Still, in order to have shots ready for the season, experts have to decide which strains to include in the vaccine months in advance. Strains H1N1, H3N2, and both strains of influenza B are anticipated to circulate this season, and the 2019-2020 vaccine has been updated to better match these strains, says Rina Shah, PharmD, pharmacy operations group vice president of Walgreens.

Still, the CDC says it's nearly impossible to determine exactly how effective the flu shot will be in any given year. It depends on a huge variety of factors, including the match between the vaccine virus and the circulating viruses, as well as the age and health history of the vaccinated person.

That said, experts predict this year's flu shot will be about 47 percent effective, says Niket Sonpal, M.D., an internist and gastroenterologist based in New York City. (Related: How to Fight the Flu with Exercise)

How effective is the flu shot in general?

If the flu vaccine isn't well-matched to the flu virus(es) circulating around you, there's a possibility that, even if you were vaccinated, you can still catch the flu, according to a CVS representative. However, if the vaccine is well-matched, research from the CDC suggests the flu shot is generally between 40 and 60 percent effective.

One thing's for sure, though: If you don't get a flu shot, you are 100 percent at risk of getting the flu.

The CDC recommends getting the flu shot early in the fall (aka now), as it can take up to two weeks after vaccination for the protective antibodies to develop in the body, explains Dr. Sonpal. You can get the flu shot later on in the season (it will still be beneficial), but given that flu season peaks between December and February—and, apparently, can last through May—your best bet to ward off the illness is to get the flu shot ASAP. Plus, there are plenty of places you can go to get the flu shot for free, so what are you waiting for?