You need a flu vaccine once every year. The CDC recommends annual vaccination for everyone 6 months and older, with rare exceptions. Unlike childhood vaccines that protect for years or decades, the flu shot is designed for a single season of protection.
Why the Flu Vaccine Is Annual
Two factors make yearly vaccination necessary: the virus changes, and your protection fades. Influenza viruses mutate constantly through a process called antigenic drift, where small changes accumulate in the proteins on the virus’s surface. These are the same proteins your immune system learned to recognize from last year’s vaccine. Over time, the mutations add up enough that your existing antibodies bind poorly, or not at all, to the newer version of the virus. This is also why you can catch the flu more than once in your lifetime.
Because of this drift, scientists review and update the vaccine composition every year for both the Northern and Southern Hemispheres. Even if the dominant strains haven’t changed much from one season to the next, your immunity from the previous shot is declining. Studies tracking vaccine effectiveness over the course of a single season found it dropped by roughly 2 to 5 percentage points per month, depending on the strain and how actively the virus was spreading. By the time the next flu season rolls around, last year’s shot offers little reliable protection.
Best Time to Get Vaccinated
Flu activity in the United States typically peaks between December and February. The CDC recommends getting vaccinated by the end of October, since your body needs about two weeks after the shot to build a full antibody response. September and October are the sweet spot for most people.
If October passes and you still haven’t gotten your shot, it’s not too late. Vaccination is recommended as long as flu viruses are still circulating in your area, even into January or later. A late vaccine still provides protection during the tail end of the season and any late-season surges. The only real mistake is skipping it entirely because you think you’ve missed the window.
Children May Need Two Doses in One Season
Most people need just one flu shot per season, but some young children are the exception. Kids between 6 months and 8 years old need two doses, spaced at least four weeks apart, if any of the following apply:
- First-time vaccination: They’ve never received a flu vaccine before.
- Only one prior dose ever: They’ve only had one flu shot total in their lives.
- Unknown history: There’s no record of their previous flu vaccinations.
After that initial two-dose series, these children switch to one dose per season going forward, just like everyone else. If your child falls into this age range, getting that first dose in early September gives enough time to complete both doses before flu activity picks up.
What Changes for Adults 65 and Older
The frequency stays the same (once per year), but the type of vaccine matters more as you age. The immune system weakens with age, so standard flu shots produce a weaker response in older adults. For people 65 and older, the CDC preferentially recommends enhanced versions of the vaccine: high-dose, recombinant, or adjuvanted formulations. These are designed to provoke a stronger immune response.
If none of those options are available when you go for your shot, a standard-dose vaccine is still far better than none. People in this age group should not receive the nasal spray version.
Immunocompromised and High-Risk Groups
People with weakened immune systems, whether from medications, chemotherapy, organ transplants, or conditions like HIV, still follow the once-a-year schedule. They don’t get vaccinated more frequently, but there are restrictions on which type of vaccine they can receive. The nasal spray vaccine contains a live, weakened virus and is not recommended for immunocompromised individuals. Injectable vaccines are the standard choice.
Pregnant women should also get a flu shot during any trimester, again avoiding the nasal spray. And if you’re a caregiver or close contact of someone with a severely weakened immune system, your own vaccination matters too. If you do opt for the nasal spray, you should avoid contact with severely immunosuppressed individuals for seven days afterward.
Could the Annual Requirement Ever Change?
Possibly. As of early 2026, 46 next-generation influenza vaccines are in clinical development, using a range of new technology platforms. The World Health Organization has outlined goals for these vaccines: broader protection that covers more strains and lasts beyond a single season. WHO modeling estimates that if these improved vaccines become widely available between 2025 and 2050, they could prevent up to 18 billion flu cases and save 6.2 million lives globally. But none have reached the market yet, so for now, the annual shot remains the standard.