This year’s flu shot protects against three strains of influenza: two type A viruses (H1N1 and H3N2) and one type B virus (B/Victoria lineage). That’s one fewer strain than previous years, marking a shift from the quadrivalent vaccines most people have been getting for the past decade. The change reflects a real-world development in how flu viruses are circulating globally.
Why the Vaccine Dropped to Three Strains
For years, flu shots included four components: two influenza A strains and two influenza B lineages (Victoria and Yamagata). In March 2024, the FDA’s advisory committee recommended that all U.S. flu vaccines for the 2024-2025 season switch to three-component (trivalent) formulas. The reason is straightforward: the B/Yamagata lineage hasn’t been detected in global surveillance since March 2020. It effectively disappeared during the COVID-19 pandemic, likely stamped out by the same masking and social distancing measures that suppressed other respiratory viruses. Including a virus that’s no longer circulating in the vaccine simply isn’t warranted.
The Specific Strains in This Year’s Shot
Not all flu vaccines are manufactured the same way, and the exact strain names differ slightly depending on the production method. If you’re getting a standard egg-based vaccine (which most people are), it contains an A/Victoria/4897/2022 (H1N1) strain, an A/Thailand/8/2022 (H3N2) strain, and a B/Austria/1359417/2021 (B/Victoria) strain.
Cell-based and recombinant vaccines use slightly different versions of the A strains: A/Wisconsin/67/2022 for H1N1 and A/Massachusetts/18/2022 for H3N2. The B/Victoria component is the same across all vaccine types. These differences exist because viruses can mutate slightly when grown in eggs, so cell-based production starts with strains better matched to what’s actually circulating in humans.
How These Strains Were Chosen
The World Health Organization runs a massive surveillance network called GISRS that tracks flu viruses year-round across the globe. Twice a year, WHO convenes a meeting of experts from collaborating centers and regulatory laboratories to analyze this surveillance data. For the northern hemisphere’s 2024-2025 season, that meeting took place in February 2024 over four days. The experts reviewed which virus variants were spreading, how they were evolving, and how well current antibodies neutralized them. The FDA then adapted those global recommendations for U.S. vaccines.
Vaccine Options by Age and Health Status
The CDC recommends flu vaccination for everyone 6 months and older. The type of vaccine you’ll be offered depends primarily on your age.
Most adults between 18 and 64 receive a standard-dose injectable vaccine. If you prefer to skip the needle, the nasal spray vaccine is available for healthy people ages 2 through 49. It uses a weakened live virus rather than an inactivated one, which means certain groups should avoid it: children under 2, adults 50 and older, pregnant women, people with weakened immune systems, and young children (ages 2 through 4) with asthma or recent wheezing. People with cochlear implants or those who’ve recently taken flu antiviral medications are also ineligible.
For adults 65 and older, the CDC preferentially recommends enhanced vaccines over standard-dose shots. These include high-dose formulas, recombinant vaccines, and adjuvanted vaccines, all designed to provoke a stronger immune response in older immune systems. Studies suggest these options are potentially more effective in this age group. If none of these enhanced vaccines are available, a standard-dose shot is still far better than skipping vaccination entirely.
Egg Allergies Are No Longer a Barrier
If you’ve avoided flu shots because of an egg allergy, the guidance has changed. Starting with the 2023-2024 season, the CDC dropped all additional safety precautions for egg-allergic individuals. You can now receive any flu vaccine, egg-based or not, that’s appropriate for your age and health. No special observation period, no requirement for a specific vaccine type, regardless of how severe your previous egg reaction was.
When to Get Vaccinated
The CDC recommends getting your flu shot ideally by the end of October. Flu season typically peaks between December and February, and your body needs about two weeks after vaccination to build full protection. That said, getting vaccinated later in the fall or even into winter still provides meaningful benefit. Flu activity can continue through March or April, so a late shot is better than none.
How Well This Year’s Vaccine Works
Flu vaccine effectiveness varies every season depending on how well the chosen strains match what actually circulates. Early data from the 2025 southern hemisphere season (which uses a similar vaccine composition and serves as a preview for the northern hemisphere) showed the vaccine reduced flu-related outpatient visits by about 50% and hospitalizations by roughly the same margin. Protection against the predominant H1N1 strain specifically was around 42% for preventing hospitalization.
Those numbers break down differently by group. Young children and people with chronic health conditions saw hospitalization protection above 50%, while adults 65 and older had somewhat lower effectiveness at about 38%. These figures are consistent with what flu vaccines typically deliver. A 50% reduction in your odds of needing medical care is substantial, particularly for a virus that hospitalizes hundreds of thousands of Americans each year. Even when the vaccine doesn’t prevent infection entirely, vaccinated people who do catch the flu tend to have milder and shorter illnesses.