You can get a flu shot once every year, and in fact, annual vaccination is recommended for everyone aged 6 months and older. Unlike vaccines that protect you for years at a time, the flu shot is designed to be a yearly event because the viruses it targets change from season to season.
Why You Need One Every Year
Flu viruses constantly shift their surface proteins through a process called antigenic drift. This means the strains circulating this winter may look different enough from last year’s strains that your old antibodies won’t recognize them well. Each year, scientists select three main virus components for the vaccine based on which strains are spreading, how far they’ve reached, and how well the previous season’s vaccine matched up. By the time the next flu season rolls around, you need updated protection.
Your immunity from the shot also fades over time. After your first-ever flu vaccination, antibody levels take roughly 32 months to drop by half. But if you’ve been vaccinated many years in a row, that timeline shortens. After a seventh consecutive annual dose, for example, antibody levels drop by half in about 9 months. That’s still long enough to cover a typical flu season, but it’s another reason the annual schedule works: you’re refreshing both the target and the strength of your immune response each fall.
Best Timing for Your Annual Shot
September and October are the ideal months for most people. Your body needs about two weeks to build full protection after the shot. Antibody production begins around day 6 and reaches meaningful levels by days 13 to 27, so getting vaccinated in early fall puts you in a good position before flu activity typically peaks in the winter months.
If you miss that window, getting vaccinated later is still worthwhile. Vaccination is recommended throughout the entire season as long as flu viruses are circulating, which can extend into spring. A late shot is better than no shot.
For most adults, especially those 65 and older, July and August vaccination is discouraged because protection may start to fade before the season ends. The exception is when there’s a real concern you won’t be able to get vaccinated later. Pregnant people in their third trimester can also reasonably get vaccinated in July or August, while those in their first or second trimester should wait until fall if possible.
Children Who Need Two Doses in One Season
Children between 6 months and 8 years old sometimes need two flu shots in a single season, spaced about four weeks apart. This applies to kids getting vaccinated for the first time, kids who have only ever received one dose of flu vaccine in their lives, or kids whose vaccination history is unknown. Their immune systems haven’t encountered the flu vaccine before and need that second exposure to build adequate protection. After that initial two-dose season, they move to the standard one-dose-per-year schedule like everyone else.
For these children, the first dose should go in as soon as vaccine becomes available, even in July or August, to leave enough time for the second dose before flu season ramps up.
Vaccines for Adults 65 and Older
The frequency doesn’t change for older adults: it’s still one shot per year. What does change is the type of vaccine recommended. Higher-dose and adjuvanted versions are preferentially recommended for people 65 and older because they appear to produce a stronger immune response in this age group. If those enhanced options aren’t available at the time of your appointment, a standard-dose vaccine is still recommended over skipping vaccination entirely.
Can You Get a Second Dose in the Same Season?
For adults, a second dose within the same flu season is not part of standard recommendations, but it’s an area of active interest. Modeling research published in the journal Vaccines found that a two-dose strategy in adults 65 and older could reduce flu cases in that group by anywhere from 1.6% to 18.7% and hospitalizations by 2.2% to 16.8%, depending on when the season peaked. The benefit was greatest when the flu season peaked later, giving the second dose more time to boost immunity before the virus hit hardest. However, the real-world effectiveness of a second dose given 90 days after the first hasn’t been confirmed in clinical trials, and this approach isn’t currently recommended.
Getting the Flu Shot After Having the Flu
Having the flu doesn’t exempt you from getting vaccinated that season. Multiple flu strains circulate each year, and infection with one doesn’t protect you against the others. Once you’ve recovered and are feeling well, you can get vaccinated. There’s no specific mandatory waiting period, though it makes sense to wait until your acute symptoms have resolved.
Traveling Between Hemispheres
If you travel between the northern and southern hemispheres, you may encounter two distinct flu seasons in a single calendar year. The southern hemisphere’s flu season runs roughly from April to September, while the northern hemisphere’s runs from October to May. Australian health authorities, for example, recommend that travelers who received their home country’s seasonal vaccine consider getting a second dose formulated for the opposite hemisphere before departure. This is one of the few scenarios where getting two flu shots in one year is a recognized practice, since the vaccine compositions differ between hemispheres to match the strains circulating in each region.