For the 2024-2025 flu season in the United States, the vaccine is a trivalent (three-strain) formula targeting two influenza A viruses and one influenza B virus. This is a change from recent years, when most flu shots were quadrivalent (four strains). The B/Yamagata lineage was dropped after it essentially stopped circulating worldwide.
The Three Strains in This Year’s Flu Shot
The World Health Organization finalized its strain recommendations in February 2024 after a four-day meeting where experts analyzed global surveillance data. Manufacturers in the U.S. then produced vaccines matching those picks. The exact virus names differ slightly depending on how the vaccine is made, but they target the same three flu lineages.
For egg-based flu vaccines (the most common type):
- A/Victoria/4897/2022 (H1N1)pdm09-like virus
- A/Thailand/8/2022 (H3N2)-like virus
- B/Austria/1359417/2021 (B/Victoria lineage)-like virus
Cell-based and recombinant vaccines use slightly different reference viruses for the H1N1 and H3N2 components because they don’t need to be adapted for growth in eggs. The B/Victoria component is the same across all vaccine types.
Why the Shot Went From Four Strains to Three
Until recently, flu vaccines included two B lineages: B/Victoria and B/Yamagata. The B/Yamagata lineage has not been detected in circulation since March 2020, likely driven to extinction during the pandemic. With no evidence of it coming back, health authorities removed it from the vaccine. That left three target strains instead of four.
How Well the Strains Match What’s Circulating
Early data from the 2024-2025 season shows influenza A dominates. During the week ending November 23, 2024, public health labs reported 394 influenza A specimens and just 15 influenza B specimens, meaning A viruses accounted for about 96% of circulating flu. The H1N1 and H3N2 subtypes are both in play, which is exactly what the vaccine targets.
A preview of how similar strains performed came from the 2024 Southern Hemisphere flu season in South America, where countries used a nearly identical vaccine formula. Across Argentina, Brazil, Chile, Paraguay, and Uruguay, the vaccine reduced flu-related hospitalizations by about 34.5% overall. Broken down by subtype, effectiveness was 37.1% against H1N1 and 36.5% against H3N2. Those numbers may sound modest, but hospitalization prevention is a high bar, and even partial protection significantly reduces severe illness and death, especially in older adults.
One concern flagged by surveillance: circulating H3N2 viruses in South America were overwhelmingly from a newer genetic subgroup (clade 2a.3a.1, subclade J.2) that had drifted somewhat from the vaccine strain. H3N2 is historically the hardest subtype to match because it mutates rapidly.
Vaccine Types Available This Season
All flu vaccines this season are trivalent, but they come in several forms. Standard-dose shots made from eggs remain the most widely available. Cell-based vaccines avoid egg adaptation, which can sometimes cause small changes in the virus that reduce effectiveness. Recombinant vaccines skip the virus-growing step entirely and instead manufacture the key protein directly.
For adults 65 and older, the Advisory Committee on Immunization Practices specifically recommends one of three enhanced options: a high-dose vaccine, a recombinant vaccine, or an adjuvanted vaccine (which contains an ingredient that boosts the immune response). If none of those are available at the time of your visit, any age-appropriate flu vaccine is still recommended over skipping vaccination.
Timing and Who Needs Two Doses
The CDC recommends getting vaccinated in September or October for most people. Vaccination should continue as long as flu viruses are circulating, which typically means through the winter months. For most adults, particularly those 65 and older, getting vaccinated in July or August is discouraged because protection can wane before the season peaks.
Children aged 6 months through 8 years who are getting a flu vaccine for the first time, or who have received fewer than two total doses in previous seasons, need two shots spaced at least four weeks apart. These children should get their first dose as soon as vaccine becomes available to allow time for the second dose before flu activity picks up.
What Changes for Next Season
The FDA has already announced the 2025-2026 strain selections, and both the H1N1 and H3N2 components are being updated. Egg-based vaccines will shift to an A/Croatia/10136RV/2023-like virus for H3N2, while cell-based and recombinant vaccines will use an A/District of Columbia/27/2023-like virus for that same subtype. The B/Victoria component stays the same. These updates reflect the ongoing drift in circulating viruses, particularly H3N2, and are why a new flu shot is needed each year.