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Flu activity in the United States started climbing in late August 2025, months ahead of the usual pattern, and the sharpest rise showed up on the West Coast. Positive tests reported to the Centers for Disease Control and Prevention by clinical labs reached 10.7% there in mid-October 2025, compared with 0.5% at the same point a year earlier. Nationally the figure was 5%, a ten-year high for that week. The dominant strain is H1N1, carrying a mutation called G155E that scientists had not tracked in earlier seasons. Doctors across several institutions responded the same way: get the shot, and get it early.
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Older adults may be better protected than teenagers
Flu severity usually tracks with age in one direction, with the oldest at highest risk. The concern this season partly inverts that. H1N1 circulated widely during the 2009 pandemic and in seasons after it, so adults who were alive and infected then carry antibodies shaped by those encounters. Younger people have less of that history to draw on. Dr. Jennifer Duchon, a pediatric infectious disease specialist at Mount Sinai Children’s Hospital in New York, said the worry is that this year’s H1N1 has not been seen by many younger people and could hit them harder. Her hospital had already recorded flu cases by October 2025.
How one letter change in a protein matters
G155E describes a single amino acid swap in hemagglutinin, the surface protein the flu virus uses to attach to cells in the airway. That protein is also the main target antibodies latch onto. Change its shape slightly and antibodies built against the older version may bind less tightly. Jeanne Marrazzo, chief executive of the Infectious Diseases Society of America, said studies of antibodies from people exposed to flu during the previous season found those antibodies did not recognize the new mutation. The effect was strongest in samples from teenagers and young adults. Marrazzo described the studies as small, worth noting rather than alarming.
What 10.7% positivity actually represents
The percentages in flu surveillance are not infection rates. They describe how many specimens sent to clinical laboratories test positive for influenza, so they rise when flu makes up a larger share of the respiratory illness being tested. At the peak of a typical season, that figure runs near 30%. A 10.7% reading on the West Coast in October 2025 sits well below peak, but it arrived roughly two months early and more than twenty times above the same week in 2024. Eight Southern states, including Florida and South Carolina, also showed elevated numbers, according to Johns Hopkins epidemiologist Caitlin Rivers.
The spread reversed its usual direction
American flu seasons tend to follow a recognizable geography. Cases build first across Southern states, then move north and east over several weeks. This season ran the other way. Rivers, who tracks pathogen data in a weekly newsletter called Force of Infection, said activity was coming from the west, and said she saw no clear reason why. She offered two possibilities without settling on either. Because H3N2 dominated the previous season, population immunity to H1N1 may simply be less recent, leaving what she called an immunological gap. The alternative explanation is the G155E mutation itself.
This season’s shot targets three viruses
The Food and Drug Administration approved the 2026-2027 formulations in August 2025. Each targets three viruses: H1N1, H3N2, and one influenza B lineage. Scott Hensley, a microbiologist at the University of Pennsylvania, said two of the three components appear very well matched to what is circulating. He noted the H1N1 mutation might affect antibody response in younger people, and wrote by email that with flu season picking up steam, early vaccination is probably a good idea. Standard shots come from GSK, Sanofi, and CSL’s Seqirus unit. AstraZeneca makes the FluMist nasal spray, and Moderna has an mRNA version available.
Matching happens months before anyone gets sick
A common assumption is that flu shots are formulated against whatever is spreading now. They cannot be. Manufacturing runs on a schedule that requires strain selection months ahead, which means each year’s formula is a forecast of what will circulate, made before the season starts. That gap explains why effectiveness varies rather than holding steady. Over the ten years to 2025, flu vaccination reduced flu hospitalizations and deaths by between 30% and 60%, depending on the season and how closely the forecast matched reality. A mutation appearing after strain selection, as G155E did, is exactly the scenario that narrows the match.
Adult flu shots fell more than 20% in five years
About 58 million American adults received a flu shot during the 2024-2025 season, according to data firm IQVIA. That is down more than 20% from the nearly 75 million recorded in 2019-2020, the last season before COVID-19 reshaped US vaccination habits. CDC data show roughly half of American children were vaccinated against flu last season, compared with more than 60% in 2019-2020. Vaccine manufacturers told Reuters that demand has tracked with the previous year. Two pharmacists said they had not seen rates pick up significantly. The decline spans both adults and children, across five seasons.
Federal messaging changed and doctors disagree about it
At the start of 2025, the CDC dropped its flu vaccination recommendation for most children, a decision a court later put on hold. Press reports said the agency also pulled its flu vaccination advertising campaign that year. Health Secretary Robert F. Kennedy Jr., a longtime vaccine critic, has overseen changes to childhood immunization schedules. Paul Offit, a pediatric infectious disease specialist at Children’s Hospital of Philadelphia, said he believes those decisions and Kennedy’s own statements have hurt immunization rates. HHS spokesperson Emily Hilliard pointed to the CDC’s Respiratory Illness Season Toolkit, which states that vaccination is the most effective tool against serious illness.
Last season put 390,000 Americans in the hospital
The scale is easy to lose behind percentages. An estimated 32 million Americans had influenza during the 2024-2025 season. Of those, 390,000 were hospitalized and 24,000 died. That season was dominated by H3N2, not the H1N1 strain now circulating, and it occurred with vaccination coverage already well below pre-2020 levels. Most children who catch flu recover without complications, but the virus remains dangerous for infants and for people with underlying health conditions. Current guidance from the American Medical Association covers everyone aged 6 months and older. Anyone weighing timing or eligibility should raise it with a clinician or pharmacist.