The 2024-2025 flu season was dominated by influenza A, which accounted for roughly 89% of all positive tests in the United States. Two subtypes split the season nearly down the middle: H1N1 made up 53% of subtyped influenza A cases and H3N2 made up 47%. A small share of cases, about 11%, were influenza B, and every single one that was lineage-tested belonged to the B/Victoria lineage.
The Two Influenza A Subtypes
H1N1 and H3N2 are both influenza A viruses, but they behave differently and can affect people in different ways. H1N1, sometimes still called “swine flu” from its 2009 pandemic origin, tends to hit younger adults and children harder. H3N2 historically causes more severe illness in older adults and is associated with higher hospitalization rates in that group. This season, neither subtype clearly dominated the other, which is somewhat unusual. Many flu seasons see one subtype account for 70% or more of cases.
The World Health Organization noted that a new subclade of H3N2 viruses emerged this season, marking a notable evolutionary shift. Despite that genetic change, surveillance data from multiple countries showed no increase in disease severity compared to previous H3N2 seasons.
A Historically Severe Season
The 2024-2025 season was not a mild one. The cumulative hospitalization rate reached 127.1 per 100,000 people, surpassing every season on record going back to 2010-2011. For context, the median end-of-season hospitalization rate over that period was 62.0 per 100,000. The previous high was 102.9, set during the 2017-2018 season. This season exceeded that by nearly 25%.
The reasons for a severe season are complex. When two influenza A subtypes circulate in roughly equal proportions, both younger and older populations can be hit hard simultaneously. That broad impact across age groups likely contributed to the elevated hospitalization numbers.
What Happened to Influenza B/Yamagata
If you’ve gotten a flu shot in past years, it probably protected against four strains: two influenza A subtypes and two influenza B lineages (Victoria and Yamagata). That changed this season. All U.S. flu vaccines for 2024-2025 were trivalent, meaning they contained only three components, because the B/Yamagata lineage was dropped entirely.
The reason is straightforward: B/Yamagata hasn’t been detected anywhere in the world since March 2020. Scientists believe the combination of pandemic-era masking, distancing, and reduced travel drove it to extinction. The FDA formally recommended removing it from vaccines, concluding that B/Yamagata “no longer poses a public health threat.” This season’s surveillance data confirmed that decision was correct. Every influenza B virus identified belonged to the Victoria lineage.
What Was in the Vaccine
The 2024-2025 flu vaccine included three virus components: an H1N1 strain, an H3N2 strain, and a B/Victoria strain. The exact laboratory strains varied slightly depending on how the vaccine was manufactured. Egg-based vaccines used virus strains grown in chicken eggs, while cell-based and recombinant vaccines used different reference viruses that more closely match what circulates in humans.
Both versions targeted the same three circulating virus types. The B/Victoria component was updated from the previous season, using a strain called B/Austria/1359417/2021 as the reference virus.
How Well the Vaccine Worked
Flu vaccine effectiveness varies every year depending on how well the vaccine strains match what’s actually circulating. For the follow-up 2025-2026 season (using updated strains), interim CDC estimates showed that vaccination reduced the risk of flu-related outpatient visits by 24% to 36% across all age groups, and reduced the risk of hospitalization by 31%.
Children fared somewhat better. Among those under 18, the vaccine reduced outpatient visits by 38% to 41% and hospitalizations by 41%. For adults 18 and older, effectiveness against outpatient visits ranged from 22% to 34%, with a 30% reduction in hospitalizations. These numbers may sound modest, but even a 30% reduction in hospitalization risk translates to tens of thousands of prevented hospital stays during a severe season.
Antiviral Treatments Still Work
One piece of reassuring news: the flu viruses circulating this season remained highly susceptible to currently available antiviral medications. Out of more than 3,800 viruses tested, only 10 showed any reduced response to the most commonly prescribed antiviral (oseltamivir, sold as Tamiflu), and just one showed reduced response to the newer antiviral baloxavir (sold as Xofluza). That translates to a resistance rate well below 1%.
An older class of antivirals called adamantanes remains completely ineffective against all current flu strains, which is why doctors stopped prescribing them years ago. If you’re prescribed an antiviral for the flu today, it will almost certainly be oseltamivir or baloxavir, and both remain effective against virtually all circulating viruses.
Who Was Hit Hardest
The season’s severity was felt across age groups, but children were not spared. Pediatric flu deaths are tracked separately by the CDC because every case must be individually reported. The toll this season reinforced the importance of vaccination for children, who are eligible for flu shots starting at six months of age.
Older adults, as in most seasons, bore the heaviest hospitalization burden. The near-equal split between H1N1 and H3N2 meant that neither end of the age spectrum was protected by the other subtype’s absence. In seasons dominated purely by H1N1, older adults sometimes see lower hospitalization rates. This season offered no such reprieve.