The flu shot is a vaccine that trains your immune system to fight influenza viruses before you get sick. It’s recommended annually for everyone 6 months of age and older, and it typically reduces your risk of getting sick with the flu by roughly 35% to 60%, depending on age, health status, and how well the vaccine matches circulating strains. The shot is reformulated every year because influenza viruses constantly evolve.
How the Flu Shot Works
Flu viruses have proteins on their surface that act like a fingerprint your immune system can learn to recognize. The vaccine introduces these proteins, either from inactivated (killed) viruses or manufactured through other methods, so your body can build antibodies against them without ever facing a real infection. Once those antibodies are in place, your immune system can identify and attack the actual virus much faster if you’re exposed later.
It takes about two weeks after vaccination for your body to develop full protection. That’s why public health agencies encourage getting the shot before flu season peaks, ideally in September or October. Protection doesn’t kick in overnight, so getting vaccinated while you’re already surrounded by sick coworkers won’t help much for those first couple of weeks.
Types of Flu Vaccines
There are three main categories of flu vaccines, and they differ primarily in how they’re made.
- Egg-based vaccines are the most common. The flu viruses are grown in fertilized chicken eggs, then inactivated and purified. Most flu shots you’d get at a pharmacy fall into this category.
- Cell-based vaccines use viruses grown in mammalian cell cultures instead of eggs. This option works for people with egg allergies and may offer a slight advantage in how closely the vaccine matches circulating strains, since growing viruses in eggs can sometimes cause small genetic changes.
- Recombinant vaccines skip the virus entirely. They use synthetic manufacturing to produce the key surface protein, which means no egg products and no live or killed virus in the final product.
There’s also a nasal spray version, which uses weakened but live influenza viruses rather than killed ones. It’s approved for people ages 2 through 49 and can be a good option for anyone who wants to avoid a needle.
What’s in the 2025-2026 Formula
Each year, the FDA selects which virus strains go into the vaccine based on global surveillance data. For the 2025-2026 U.S. flu season, the vaccine is trivalent, meaning it protects against three strains: two influenza A viruses (an H1N1 and an H3N2) and one influenza B virus from the Victoria lineage. The specific strains differ slightly between egg-based and cell-based versions because of how the viruses are grown, but both target the same circulating threats.
How Effective Is It?
Flu vaccine effectiveness varies from year to year and from person to person. Interim data from the 2024-2025 season gives a useful snapshot. Among children and teens, the vaccine reduced outpatient flu illness by 32% to 60% across different monitoring networks, and it cut flu-related hospitalizations by 63% to 78%. Those hospitalization numbers are significant: even when the vaccine doesn’t completely prevent illness, it often keeps people out of the hospital.
For adults aged 18 to 64, outpatient effectiveness ranged from 37% to 56%, with hospitalization protection between 48% and 51%. Adults 65 and older saw more variable results, with outpatient effectiveness between 18% and 51% and hospitalization protection between 38% and 57%. The wider range in older adults reflects the natural decline of immune function with age, which is why enhanced vaccines exist for this group.
These numbers might seem modest compared to, say, the measles vaccine. But even a 40% reduction in flu risk across a population prevents millions of illnesses and tens of thousands of hospitalizations each season. And because the flu can be fatal, particularly for older adults, young children, and people with chronic conditions, that partial protection carries real weight.
Enhanced Vaccines for Older Adults
People 65 and older are preferentially recommended to receive either a high-dose flu vaccine, a recombinant vaccine, or an adjuvanted vaccine rather than a standard-dose shot. These formulations are designed to provoke a stronger immune response in aging immune systems. An adjuvanted vaccine contains an added ingredient (in this case, a squalene-based oil-in-water mixture) that amplifies the body’s reaction to the vaccine proteins.
If none of these enhanced options are available, a standard flu shot is still recommended. Any vaccination is better than none for this age group, which accounts for the majority of flu-related deaths each year.
Vaccines for Children
Children can receive their first flu vaccine at 6 months of age. Kids between 6 months and 8 years old who are getting vaccinated for the first time (or whose vaccination history is unknown) need two doses spaced at least four weeks apart. This two-dose series gives their immune systems enough exposure to build a strong response. After that initial series, one dose per season is sufficient, just like adults.
Common Side Effects
Most people experience either no side effects or mild ones that resolve within a day or two. The injectable vaccine commonly causes soreness, redness, or swelling at the injection site. Some people also report headache, low-grade fever, muscle aches, nausea, or fatigue. These reactions are signs that your immune system is responding to the vaccine, not signs that you’re getting the flu. The vaccine cannot give you the flu because it contains no live virus (with the exception of the nasal spray, which uses a weakened form that can’t cause full-blown illness).
The nasal spray version has its own side effect profile. Children may experience a runny nose, wheezing, headache, vomiting, or low-grade fever. Adults using the nasal spray more commonly report a runny nose, headache, sore throat, or cough. These typically clear up quickly.
What’s Actually in the Vaccine
Beyond the flu virus proteins themselves, flu vaccines contain small amounts of other ingredients that serve specific purposes during manufacturing or storage. Stabilizers like sugars and gelatin keep the vaccine effective during transport and storage. Trace amounts of formaldehyde may remain from the process of inactivating the virus, though the quantities are far below levels that pose any health concern (your body actually produces more formaldehyde naturally through normal metabolism than what’s in a vaccine dose).
Egg-based vaccines contain residual egg protein, which matters for people with severe egg allergies. Cell-based and recombinant options avoid this entirely. Some multi-dose vials contain a preservative to prevent contamination when the vial is punctured multiple times, though single-dose options without preservatives are widely available. Trace amounts of antibiotics used during manufacturing may also be present, though the quantities are extremely small or undetectable in the final product.