An influenza virus can make you ill any time of year. What we think of as the seasonal flu in the U.S., however, usually spreads throughout the fall and winter, peaking between December and February, according to the Centers for Disease Control and Prevention (CDC). Even though vaccines can stave off severe infection, hospitalization, and death, their protection declines over time. That said, you may be wondering, When is the best time to get my annual flu shot?

September and October are ideal flu vaccination periods for most people, who need only a single dose each season, the CDC says. Your state or local health department may have chartered a “vaccine before Halloween” or “flu before boo” campaign to encourage people in your community to get immunized ahead of November. The CDC recommends an annual flu shot for everyone ages 6 months and older; high-dose and adjuvanted vaccines are available to adults 65 and older.

“For older adults and for folks who’ve got chronic medical problems, I usually recommend getting the flu vaccine around anytime from the middle of September through early October,” Dr. Robert Hopkins Jr., medical director of the National Foundation for Infectious Diseases, tells Fortune. “Because in those of us with immune systems that aren’t quite as robust, we want to make sure that we vaccinate at a time that’s likely to give us immune protection throughout the flu season.”

By getting yourself vaccinated against the flu, you’re also helping limit infection spread to the most vulnerable members of your community, says Dr. Michael Ben-Aderet, associate medical director of hospital epidemiology at Cedars-Sinai Medical Center in Los Angeles.

“Every year in this country we have thousands and thousands of patients who are hospitalized and die with influenza, and they are particularly older people or people with medical illness,” Ben-Aderet tells Fortune. “It is beneficial to those people and everyone else that we get the flu vaccine.”

Whenever you choose to get your updated flu vaccine, keep in mind that it takes as much as two weeks for your body to build up immunity to the virus.

Is it ever too late in the year to get a flu shot?

From 1982–83 through 2021–22, seasonal flu activity peaked 17 times in February, followed by seven in December and six in January, CDC records show. But it also peaked six times in March and once in April; though less common, early spring flu peaks do happen. In addition, the CDC notes that COVID has rendered the timing and duration of flu activity less predictable.

Flu vaccination before November is ideal, but any time you’re able to get a dose throughout the season is better than no protection at all, Hopkins says.

“Get it as soon as you can, and it’s not too late,” Hopkins says. “If you’ve got virus circulating in your community, get the vaccine, because that may help reduce the severity of your infection, even if it doesn’t necessarily prevent infection because you got it a little late.”

No, the flu vaccine doesn’t give you the flu

Some people feel sick after getting vaccinated, enough to perpetuate the myth that immunizations such as the flu and COVID vaccines give you those respective viruses. But regardless of when in the season you get your flu shot, getting it annually should minimize side effects over time, explains Dr. Priya Sampathkumar, an infectious disease specialist at the Mayo Clinic.

“If you’ve not had the flu vaccine in a while, or if you’re getting the high-dose flu vaccine, you do get a sore arm, you can get generalized muscle aches and pains, you can get a low-grade fever, you can get a headache,” Sampathkumar tells Fortune. “These are all signs that your body is responding to the vaccine and not really the flu. The more frequently you get flu vaccines—if you get them year after year—you generally don’t have any of these side effects at all.”

Some children, pregnant people can get flu shot in summer

The CDC considers July and August too early for the general population to get an annual flu shot. Pregnant people in their third trimester, though, are encouraged to get immunized during these summer months; vaccine-induced protection extends to their infants, who won’t be eligible for a shot of their own until they reach 6 months of age.

Children under 5, particularly those under 2, are at high risk of severe influenza infection and may get vaccinated as soon as doses are available—even before September. Children 8 and under with fewer than two lifetime flu vaccine doses require two doses of the new vaccine, four weeks apart. The CDC recommends these young children get their first dose in the summer. Your child’s pediatrician can help you determine an ideal dosing schedule.

Pre-September vaccination is too early for adults 65 and older, as well as pregnant people in their first or second trimesters, the CDC says. However, people who would otherwise be unable to get vaccinated in September or October—those traveling abroad, for instance—may consider summer immunization.

For more on vaccinations:

• Public health experts are warning of a ‘quad-demic’ this winter. Here’s where flu, COVID, RSV, and norovirus are spreading
• Yes, you can get the COVID, flu, and RSV vaccines at once. Here are the pros and cons
• Older adults should get their flu shots now. Here’s why they also need stronger vaccines
• RSV can be deadly, especially for older adults. What to know about symptoms and the new vaccine
• New COVID vaccines are here. What to know about the latest shots as summer surge continues