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This year's flu season did not wait for winter. Across the United States, influenza activity has been climbing since late August and early September — roughly two months ahead of the usual pattern — and by October it had reached levels that in recent years did not appear until December or January. Behind the early surge is a familiar virus with an unfamiliar twist: a mutated strain of H1N1 that may be partly slipping past the immunity people built up from earlier infections and vaccinations.
How early is early?
According to reports from USA Today, the US Centers for Disease Control and Prevention (CDC) says flu activity is now rising in 40 states — and declining in none. That is an unusually broad and uniform rise for this point in the year. In a typical season, activity builds slowly through the autumn and peaks somewhere between December and February. This time, October already looked like midwinter.
The laboratory numbers back up the picture. In the week ending 31 October, 9.1% of 51,351 clinical laboratory tests for flu came back positive — a high share for late October. And of the influenza A viruses that were subtyped, H1N1 accounted for 97.7%, meaning this season so far is overwhelmingly a single-strain story.
The human cost is already visible. The CDC estimates that as of 30 October, this season has caused around 1.7 million illnesses, 25,000 hospitalisations and 1,400 deaths in the United States. The season has also claimed its first reported paediatric death, according to Contemporary Pediatrics. These are estimates, not final counts, and they will rise as the season continues — but they underline that an early season is not a mild season.
The strain behind the surge
The dominant virus is influenza A(H1N1)pdm09 — a descendant of the 2009 pandemic strain that has circulated every year since. What is new is the subclade now in charge: D.3.1.1, which carries a mutation known as G155E. The mutation emerged in Japan around May 2026 and was first identified in the United States in Seattle in September. Since then it has spread fast enough to dominate the season.
Why does one mutation matter? The G155E change sits in a part of the virus that antibodies recognise, and early laboratory work suggests it may partly evade the antibody immunity built up from previous H1N1 infections and vaccinations. That is a plausible explanation for why this season took off so early: more people than usual may be susceptible to this particular version of the virus. But it is important to be precise about the evidence. The immune-evasion finding comes from a bioRxiv preprint — a study posted before peer review — not from a peer-reviewed paper. The result is a warning sign, not a settled fact, and it needs confirmation from further research.
What about the vaccine?
The obvious question is whether this year's flu vaccine still works against the mutated strain. The honest answer, for now, is that it is too early to say for certain — and the CDC says exactly that. What the early data does show is encouraging: 93–94% of the H1N1 viruses tested so far were well recognised by the antisera raised against the vaccine strain, which suggests the vaccine should still offer meaningful protection.
That distinction matters. A virus can spread earlier and faster without necessarily defeating the vaccine, and partial immune evasion is not the same as full escape. Public health officials are watching vaccine effectiveness data closely as the season develops, and the picture will sharpen over the coming weeks. In the meantime, the advice has not changed: vaccination remains the best available protection, especially for older adults, young children, pregnant women and people with chronic health conditions — the groups most likely to end up in hospital.
The early flu season is also a reminder of how quickly the picture can change. Two months ago, this strain was a curiosity spotted in Seattle; today it dominates the national numbers. The upheaval in America's pharmacy sector means the places many people get their flu shots are themselves in flux, and the new balance of power in Washington will shape how federal health agencies respond in the months ahead. For individuals, the practical conclusion is straightforward: if you have not had this season's flu shot, the early start to the season is a reason to get it sooner rather than later — and if you develop flu-like symptoms, take them seriously and seek care early, particularly if you are in a high-risk group. This article is general information only and is not medical advice.