Early December means the height of flu season is almost here. Every year, millions of Americans get the flu. Last season, 310,000 were hospitalized with flu-related illnesses.

Yet just more than 40 percent of adults have gotten their flu shots so far this year, according to new data from the Centers for Disease Control and Prevention—roughly the same percentage as in the past few years. If you’ve been putting off your shot, now is the time to get it: It can take anywhere from two to four weeks after being vaccinated to develop antibodies to fight off the virus, and flu season usually peaks between December and March.

Right now, only about 1.8 percent of people in the U.S. who visited the doctor are reporting flu-like symptoms, according the CDC, which is slightly less than normal. But it’s still too early to tell whether this year’s flu season will be mild or severe, says influenza researcher Andrew Pekosz, Ph.D., a professor in the department of molecular biology and immunology at Johns Hopkins University in Baltimore. And when the flu strikes, it can spread quickly.

Here’s why you shouldn’t delay getting your shot:

The Flu Shot Is Effective

Every year experts predict the flu strains that are most likely to be in circulation and put those into the vaccine. Last year’s flu season was mild—in part because those predictions were accurate. And while it’s too early to be sure about this year’s vaccine, so far, "it looks like the vaccine and the circulating strains are matched pretty well," Pekosz says.

The CDC’s new report also reveals that almost 46 percent of people in the U.S. were immunized.

If flu vaccination rates last year had been just 5 percentage points higher, the CDC says, an additional 500,000 cases of the flu and 6,000 hospitalizations could have been avoided.

It’s true that some people who get the shot may still contract the flu. An unexpected strain can always emerge—one that the shot does not protect you against. But in the past five years, with the exception of the 2014 to 2015 vaccine (which was only 19 percent effective), people who got a seasonal flu vaccine were roughly 50 percent less likely to get the virus than people who skipped the shot.

Your Flu Shot Protects Others

If you’re a healthy adult, you may not be all that worried about getting the flu, because beyond making you feel lousy for a week, chances are it won't cause you serious harm.  

But children younger than 5, the elderly, pregnant women, and those with chronic health problems are more likely to suffer complications from the flu. The flu vaccine also tends to be less effective in some of these people.

That’s one reason the CDC recommends that healthy adults get the flu vaccine: If enough people are protected from the flu with a vaccine, those most at risk for complications can be protected as well—a phenomenon known as herd immunity. That’s also why it’s especially important to get your shot if you live or work with young children, older adults, or people with certain medical conditions.

The CDC has estimated that between 2005 and 2014, the flu vaccine averted roughly 40,000 flu-related deaths.

The Flu Shot Is Affordable and Safe

Currently, insurance plans are required to cover your flu shot with no co-pay and no co-insurance. If you’re uninsured, your local health department may provide resources on where to get a flu shot at little or no cost. And if your doctor’s office or employer is not offering flu shots, you can find options near you at vaccinefinder.org.

The vaccine is safe for almost everyone. People with life-threatening allergies or a history of Guillain-Barré syndrome should talk with their doctor before they get a flu shot. Severe allergic reactions are extremely rare.

Some people think you can actually get the flu by having a flu shot. But this is a myth: The flu vaccine is made with inactivated or dead viruses, so it can’t cause the flu. It can, however, can cause minor side effects such as soreness at the site of the injection, headache, body aches, and fever.

This year, the CDC no longer recommends the FluMist Nasal Spray Vaccine, because it is less effective than the standard shot. So be sure you get either the trivalent or quadrivalent vaccine (which protect against either three or four strains). An egg-free version of the trivalent shot is available for adults with allergies, and a high-dose version is available for people 65 and older.