America's flu season has arrived weeks ahead of schedule, and the virus leading the charge is a mutated form of H1N1 that may partly slip past immunity built up from past infections and vaccinations. New data from the Centers for Disease Control and Prevention (CDC) show flu infections rising or likely rising in 44 states, test positivity far above this time last year, and hospital admissions climbing week after week — a pattern that has doctors urging people not to wait until winter to get vaccinated.
The early flu season is most advanced in the West, where about 10% of flu tests are coming back positive in Arizona, California, Hawaii and Nevada, according to the CDC. Flu season typically begins when test positivity reaches about 3%, epidemiologist Katelyn Jetelina, who consults for the CDC, has noted — which means parts of the West are already well past the starting line in late October, a time when many Americans have not yet thought about a flu shot.
The CDC's FluView report for the week ending October 17 paints a picture of a season starting early and gathering speed. Clinical laboratories tested 51,490 samples that week, and 5.7% came back positive for influenza — up from 5.0% the week before and 3.9% two weeks earlier. A year earlier, in the same window, positivity was under 1%. Outpatient visits for flu-like illness reached 2.2%, above the levels recorded in the previous two seasons at this point, though still below the national baseline of 2.9%.
Hospital admissions tell the same story of acceleration. Hospitals reported 4,811 lab-confirmed flu admissions in the week ending October 17, compared with 4,024 the previous week and 2,572 two weeks before that. The CDC's FluSurv-NET network estimated an adjusted hospitalisation rate of 1.5 to 2.0 per 100,000 people for that week — the highest rate for that week of the season since 2010-11 — while the observed rate was 1.1 per 100,000. Adults aged 65 and older had the highest observed rate, followed by adults aged 50 to 64 and children under 5. No flu-related child deaths had been reported this season, and flu accounted for 0.2% of death certificates.
The geographic spread is striking. Flu activity was growing or likely growing in 44 states as of October 21, according to CDC trend estimates, with only a handful of states not yet showing growth. Two states, Louisiana and Hawaii, had reached moderate activity, while the rest were low or minimal. A week earlier only Louisiana was moderate, and two weeks earlier every state was minimal or low. Cases increased in 16 states compared with the week before, including Alaska, North Dakota, Wisconsin, Vermont, Oregon, Wyoming, South Dakota, Ohio, Pennsylvania, New York, Utah, Arizona, New Mexico, Oklahoma, Hawaii and Florida.
Wastewater surveillance, which often picks up viral spread before clinics do, has been flashing an even earlier warning. By October 20, 38 states were seeing a high level of influenza A in wastewater, according to Wastewater SCAN, a dashboard developed by Stanford University, and the national wastewater concentration was nearly 19 times the level recorded on the same day in 2025. The 12 states still at a low wastewater level were clustered in the Midwest, including Kansas, Illinois, Indiana, Iowa, Michigan, Minnesota, Missouri, Ohio, Nebraska, North Dakota, South Dakota and Wisconsin.
In a typical year, flu activity builds in October and November and peaks between December and February, with the first rise usually appearing in the South before moving north and then west. This season has broken that pattern in two ways: it began climbing in late August and early September, and it moved west first. Washington state saw the share of emergency department visits diagnosed as flu rise from 0.25% in the week ending August 29 to 1.34% in the week ending October 3, based on CDC emergency department data.
Last winter's dominant strain was a fast-spreading H3N2 subclade, known as subclade K, which was associated with a high burden among children; federal tallies counted 195 paediatric deaths in the 2025-26 season. This season the picture has flipped. Of the influenza A viruses subtyped by public health laboratories in the week ending October 17, 99.2% were H1N1 and none were H3N2. H1N1 is the subtype behind the 2009 swine flu pandemic, which sickened an estimated 60 million Americans, hospitalised 240,000 and was linked to 12,400 deaths, according to figures cited in recent reporting — but it has circulated as an ordinary seasonal flu virus ever since.
What is new is the G155E mutation. Researchers first identified it in Seattle in September, in H1N1 subclade D.3.1.1. The mutation affects a part of the virus that matters for protective antibody responses. Michigan's weekly Flu Focus report puts it plainly: nearly all cases so far are influenza A (H1N1), most circulating H1N1 viruses carry G155E, and studies suggest antibodies from past infections or vaccinations may not recognise it, especially in teens and young adults. Researchers stress, however, that it is not yet clear whether the change will significantly affect how widely the virus spreads, how severe illness will be, or how well this season's vaccine performs. H1N1 is included in this year's flu shot, which experts say should still provide some protection.
The federal outlook for 2026-27 rates the season more likely to be moderate, but confidence in that forecast is limited, precisely because viral evolution — particularly changes to haemagglutinin, the protein targeted by vaccines — shapes both severity and vaccine performance. An early start, experts caution, does not guarantee a worse season: it guarantees only that the season has started early, and that the weeks ahead will show whether the wave builds, plateaus or fades before the usual winter peak.
The West is where the early flu season is most visible, and California is its epicentre. As of the week ending October 3, influenza-like illness accounted for 2.77% of outpatient and emergency department visits in California, compared with 1.85% nationally, according to CDC data. At Kaiser Permanente, flu positivity rates in mid-October resembled figures typically seen in early December. Dr Elizabeth Hudson, regional chief of infectious diseases at Kaiser Permanente Southern California, said the health system had been seeing December-like rates for two to three weeks, with the upward trajectory continuing. Former Food and Drug Administration commissioner Dr Scott Gottlieb said in a television interview that California has a full-on flu epidemic right now, adding that flu activity tends to be a regionalised epidemic and that if California's trend spreads across the country, that would indicate a potentially bad flu season.
Other western states show the same direction of travel. Hawaii, one of only two states at moderate activity, has seen sustained transmission alongside its tropical weather, and Arizona and Nevada are both in the roughly 10% positivity band for the region. In the Pacific Northwest, Dr Helen Chu, a respiratory infections specialist at the University of Washington in Seattle, said the region was having a flu surge very, very early, with an early rise also evident in California.
The East and Midwest are behind the West but catching up. Pennsylvania logged 806 flu cases in the week of October 4-10, up 385% from 166 in the same week last year, though state officials still describe activity as low but slightly increasing. In Virginia, about 0.6% of emergency department visits in the week ending October 10 were diagnosed with flu — more than five times the 0.11% recorded a year earlier — and statewide emergency and urgent care flu visits jumped 27% in a single week, from 839 to 1,069, with children aged 5 to 17 having the highest number of positive lab tests.
Michigan illustrates the national pattern in miniature. The CDC still showed flu-like activity as minimal in the state in the week ending October 17, the most recent data available, but the curve is trending upward, and the state's health department has flagged the mutated H1N1 strain as a likely driver of the national rise. About one in four Michiganders got a flu vaccine last season, roughly 2.7 million people — a drop of about 970,000 compared with the 2021-22 season — which leaves a large share of the population relying on immunity from past infection that the G155E mutation may partly evade.
Jetelina, writing in her Your Local Epidemiologist newsletter, summed up the western surge in a single line: "The West is clearly well on its way into the wave." She added: "The flu will slowly but surely move from the Pacific eastward in a sweeping motion. So, although rates are still relatively low in the Northeast, they will start increasing quickly soon." That eastward sweep is already visible in the state-by-state growth data and in the Virginia and Pennsylvania figures.
University of Washington virologist Alex Greninger, whose laboratory has been tracking the new strain, has described the current moment in stark terms, saying the country is basically seeing January levels of flu in the last week, and noting that as much as 98% of the flu samples arriving at his laboratory are now H1N1. He has also warned that the country could face two flu seasons this winter if the early H1N1 wave is followed by a second, later wave of a different strain — a scenario doctors say makes early vaccination more, not less, important.
Andrew Pekosz, a virologist at the Johns Hopkins Bloomberg School of Public Health, has urged a measured reading of the data. He notes that the first flu virus to arrive in a season is often the one that slips past existing immunity, and that cases are still relatively low, so the country is in the early phase of a surge. The West moving first, he has said, is an odd path for American flu, and the rest of the map is now catching the same H1N1 wave. His message, echoed by other experts, is that it is too soon to know whether the season will be severe — but not too soon to act.
Public health agencies have been consistent in their advice: get vaccinated now, rather than waiting for the traditional winter peak. Experts also reiterate that children receiving their first influenza vaccine often require two doses administered a month apart, making immediate scheduling critical before widespread winter peaks arrive. For families with young children, that timetable means a first dose given now may only be fully effective after the second dose, weeks later — another reason not to delay.
An early flu season matters for three reasons. First, timing changes risk. When flu arrives in October instead of December, it overlaps with the return to school, the start of indoor social life and the run-up to holiday travel — all of which give the virus more chances to spread before most people have been vaccinated. A wave that peaks early can also collide with later waves of COVID-19 and respiratory syncytial virus (RSV), stretching clinics and hospitals that are already seeing admissions rise.
Second, the mutation changes the immunity calculation. Most adults carry some protection against H1N1 from past infections or vaccinations, but if G155E helps the virus evade part of that protection — especially in teens and young adults, as Michigan's report suggests — then groups that usually fare better may be more vulnerable this year. That does not mean vaccines are useless: H1N1 is in this season's shot, and even partial protection reduces severe illness, hospitalisation and death. It does mean that "I had flu last year" is a weaker shield than usual.
Third, the geographic pattern changes preparation. Hospitals and pharmacies in the West are already dealing with December-like demand in October, while the East and Midwest still have a window — probably measured in weeks, not months — to vaccinate before the wave arrives in force. Michigan's falling vaccination coverage, down nearly a million doses from 2021-22, shows how narrow that window can be if uptake stays low. The CDC's new local trend tool, which can identify outbreak peaks two to three weeks earlier at state level and up to a week and a half earlier at community level, is designed to give local health departments exactly that kind of early warning — but only if people act on it.
The next CDC FluView report, covering the week ending October 24, will show whether the 5.7% positivity rate keeps climbing and whether more states join Louisiana and Hawaii at moderate activity. If the eastward sweep Jetelina describes continues, the Northeast — still relatively low — should start rising quickly, and the Midwest's low wastewater states may flip to high. Hospital admissions, which have nearly doubled in two weeks, are the number to watch for severity: a continued climb would signal a season that is not just early but heavy.
Researchers will also be watching the virus itself. Further sequencing will show whether G155E continues to dominate, whether new branches of the H1N1 subgroup emerge, and — critically — how well this season's vaccine matches the circulating strain in real-world effectiveness studies. Those studies take time, so for now the guidance rests on laboratory findings and on the principle that some protection beats none.
For individuals, the next steps are practical: get a flu shot now, especially if you are over 65, under 5, pregnant, living with a chronic condition, or caring for someone in those groups; keep children home when ill; and seek care promptly for trouble breathing, persistent fever, or symptoms that improve and then return worse — the warning signs doctors associate with flu complications. US News Pulse will continue to follow the early flu season as new CDC data arrive.
Is flu season really starting early this year? Yes, according to CDC data. Flu infections began climbing in late August and early September, test positivity reached 5.7% in the week ending October 17 (versus under 1% a year earlier), and infections are rising or likely rising in 44 states — weeks ahead of the usual October-November build-up.
What is the G155E mutation? It is a change in the H1N1 virus, first identified in Seattle in September, that affects a part of the virus involved in antibody recognition. Studies suggest antibodies from past infection or vaccination may recognise the mutated strain less effectively, especially in teens and young adults, but researchers do not yet know how much it will affect severity or vaccine performance.
Does this year's flu shot still help? Experts say yes. H1N1 is included in this season's vaccine, and vaccination should still provide some protection, even if it is partial against the mutated strain. Health officials recommend getting vaccinated now rather than waiting for the winter peak.
Which states are hit hardest so far? The West is running hottest, with about 10% test positivity in Arizona, California, Hawaii and Nevada. Louisiana and Hawaii are the only states at moderate activity so far, but cases are rising in 44 states and the wave is moving eastward.
Should children get vaccinated now? Yes, and urgently if it is their first flu vaccine: children receiving their first influenza vaccine often need two doses a month apart, so starting now matters before winter peaks arrive. Children aged 5 to 17 already account for the highest number of positive lab tests in states such as Virginia.
Related coverage on US News Pulse: FDA warns over two lots of Darwin's cat food after Salmonella find, Haifa smoked sprats recalled over botulism risk in 3 states and Petition seeks FDA botulism testing rule for infant formula after 52 babies sickened.
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