With the first day of fall just around the corner, medical professionals are issuing a serious warning as this year’s flu season arrives amid the coronavirus pandemic: Get your flu shot.

“The flu shot is incredibly important because it reduces your risk of contracting the flu,” Michelle Lin, an emergency room doctor, and professor of emergency medicine at Mount Sinai in New York City, told Fox News. “It also reduces your risk for complications and passing it to other people, especially pregnant women, young children and the elderly,” who are more susceptible to the virus, she added.

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This year, flu season will coincide with the ongoing COVID-19 crisis in the U.S. and abroad, meaning it's doubly as important to get vaccinated.

"It's particularly important to get vaccinated this year because of the ongoing COVID pandemic: We want people to stay as healthy as possible," said Lin. "Since people are trying to stay home and out of the doctor's office [and/or] ER, there has been a push to make the vaccine available widely earlier."

Here's what you need to know about the 2020-21 flu season.

When is the best time to get a flu shot?

Attempt to get vaccinated by the end of October, suggested Lin.

"The best time to get the flu shot is whenever you have time and ideally by the end of October. Healthy adults, including pregnant women, should get the vaccine as early as possible (pregnant women are not only at particularly high risk of influenza complications, they can also transmit immunity to their fetuses)," she said.

"There is some evidence suggesting that immunity might wane over the course of flu season for adults age 65 or older and children, so there may be an advantage to waiting until October, but it is definitely better to get it early than not at all," she added.

How long does it take for the vaccine to take effect?

Roughly two weeks, Lin said. During this time, your body is developing antibodies to protect you against the virus.

Lin said she and other health professionals have recommended patients receive the vaccine before the end of October, as flu season typically peaks during the cold, dry weather between December and February.

But receiving the vaccine anytime during the season is “better late than never," Lin noted.

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How many strains of flu does the vaccine protect against?

Flu vaccines typically protect against three or four viruses, the Centers for Disease Control and Prevention (CDC) says.

"Nearly all versions of the flu vaccine are quadrivalent (cover 4 strains) this year. Most people can get vaccinated now at their local pharmacy or health care clinic," said Lin, who previously told Fox News that health professionals look at weather patterns and use statistics and other data to make an educated guess of which strains will be the most contagious during each flu season.

What is a common misconception about the flu vaccine?

While there are many misconceptions about the flu vaccine, “the most common one we hear is that it gives people the flu,” Lin said.

While reactions to the flu shot may include a low-grade fever or muscle aches, the vaccine cannot cause the flu virus. The shot is either made with a virus that has been “killed” or “inactivated" or made with “only a single gene from a flu virus (as opposed to the full virus) in order to produce an immune response without causing infection,” the CDC says.

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How severe will this year’s flu season be?

It's hard to predict. But the flu season in Australia, in particular, can give experts an idea of how severe the flu season in the U.S. might be — as the U.S. tends to echo Australia in both severity and strains.

That said, "It's too early to know what this year's season will be like, and obviously the southern hemisphere experience has been impacted by COVID," said Lin. "Bottom line: if you're six months or older and have no contraindication, definitely get your flu shot!"