Flu Season 2026: Why It Started Early and Which Strain

Did flu season start early in 2026?

Yes in the US: CDC reports early-season flu activity higher than usually seen at this time of year, with severity assessed as low. In England, UKHSA reports activity increasing and now above baseline, though still at low levels ^[1][2].

In its FluView report for the week ending 31 October 2026 (week 43), published on 6 November, CDC said multiple flu indicators were increasing in many parts of the country and that this "early-season influenza activity is currently higher than typically observed at this time of year" ^[1]. In clinical laboratories, 9.1% of respiratory specimens tested positive for flu that week ^[1].

In England, UKHSA's report of 12 November, covering 2 to 8 November (week 45), said flu activity increased and is circulating at low levels, with emergency department visits for flu-like illness above seasonally expected levels ^[2]. A briefing note was issued to tell the NHS that flu activity is now above baseline ^[2].

The journal Nature reported on 6 October that test positivity was unusually high for the time of year in the UK and on the US West Coast, and that flu outbreaks in Japan in August and early September forced some schools to close ^[3].

• US, week 43: 2.8% of outpatient visits were for flu-like illness, just below the national baseline of 2.9% ^[1].
• England, week 45: GP flu-like illness consultations rose to 7.3 per 100,000, from 6.2 the week before ^[2].

Why did flu season start early this year?

Neither CDC nor UKHSA has given a single cause for the early start; their reports describe what is circulating, not why it arrived sooner ^[1][2]. What they do say is about the viruses involved and what an early start does and does not mean.

• Changing H1N1 viruses: CDC says recent H1N1 viruses, in a group called subclade D.3.1.1, have split into distinct groups with different genetic changes, and "we are seeing more genetic diversity among co-circulating A(H1N1)pdm09 viruses than we have before" ^[1].
• A mutation scientists are watching: Nature's report said early Northern Hemisphere cases were dominated by a virus with a "concerning mutation" ^[3]. The full article is behind a paywall, so the details are not reproduced here.
• No prediction of a bigger season: CDC states that "an earlier increase in activity does not necessarily lead to a higher seasonal peak or greater overall severity" ^[1].

Explanations circulating online that blame a single factor are not supported by the official reports cited here. Chatbots can also mix up seasons and strains when asked about flu; why that happens is covered in AI Hallucinations Explained: Why Chatbots Make Things Up and How to Get Accurate Answers.

Which flu strain is going around in 2026?

Influenza A(H1N1)pdm09 is the dominant virus in both the US and England so far, with A(H3N2) subclade K and a small amount of influenza B also circulating ^[1][2].

United States

• Of 388 influenza A viruses subtyped by public health labs in week 43, 379 (97.7%) were H1N1, 7 (1.8%) were H3N2 and 2 (0.5%) were H5 bird flu ^[1].
• Of 486 H1N1 viruses genetically characterised since 24 May 2026, 99.8% were subclade D.3.1.1 ^[1].
• Of 106 H3N2 viruses characterised, 101 (95.3%) were subclade K and 5 were J.2.4.2 ^[1].
• Influenza B made up 1.4% of positive clinical lab tests in week 43 ^[1].

England

• Between late August and 8 November, UKHSA genetically characterised 397 flu viruses: 267 H1N1, 96 H3N2 and 34 influenza B ^[2].
• All H1N1 viruses belong to clade D.3.1.1, and all sequenced H3N2 viruses belong to subclade K ^[2].
• Influenza A positivity was highest in children aged 5 to 11, at 12.8% ^[2].

The H5 detections in the US relate to bird flu in dairy workers: CDC reported two new human H5 cases in Washington state in week 43, one of whom died, and said human-to-human spread has not been identified in the US ^[1].

Does the 2026-27 flu vaccine match the strains?

Early laboratory tests show good antibody recognition of most circulating viruses by the 2026-27 vaccine strains, but both agencies say it is too early to know how well the vaccine will protect against illness this season ^[1][2].

• H1N1 (CDC): 140 of 147 H1N1 viruses tested (95.2%) were well recognised by antibodies against the A/Missouri/11/2025-like vaccine reference virus ^[1].
• H3N2 (CDC): all 35 tested H3N2 viruses were well recognised by antibodies against the A/Darwin/1415/2025-like vaccine virus ^[1].
• Influenza B (CDC): all 33 B/Victoria viruses tested were well recognised ^[1].
• England (UKHSA): 11 of 13 H1N1 and all 6 H3N2 viruses tested were similar to the vaccine reference strains ^[2].

UKHSA adds a caution: observations from the WHO vaccine composition meeting in September 2026 suggest some D.3.1.1 H1N1 viruses react poorly with antibodies raised against the vaccine's H1N1 component ^[2]. It says it is not yet clear how this affects vaccine effectiveness, and that vaccines typically still protect against severe disease when drifted strains appear, with only a modest reduction in effectiveness ^[2].

CDC says annual flu vaccination is recommended, with vaccines available for people aged 6 months and older ^[1]. In England, uptake among people aged 65 and over was 63.1% by week 45, slightly below 65.1% at the same point last season ^[2].

What are the symptoms of flu in 2026?

The symptoms of flu this season are the usual ones: a sudden fever or chills, cough, sore throat, runny nose, body aches, headache and tiredness, according to CDC and WHO ^[4][5]. No official source cited here describes symptoms specific to the 2026 strains.

• Fever is not always present: CDC notes that not everyone with flu has a fever ^[4].
• Stomach symptoms: vomiting and diarrhoea can occur, more often in children than adults ^[4].
• Timing: WHO says symptoms begin 1 to 4 days after infection and usually last around a week, though the cough can last two weeks or more ^[5].
• Flu versus cold: CDC says flu symptoms usually start abruptly, while cold symptoms start gradually ^[4].

Who is at higher risk

CDC lists people aged 65 and older, people with chronic conditions such as asthma, diabetes or heart disease, pregnant women and children under 5, especially under 2, as being at higher risk of serious complications ^[4]. In US hospital surveillance, the highest flu hospitalisation rate so far is among adults aged 65 and older ^[1].

CDC lists emergency warning signs that need urgent medical care, including difficulty breathing, persistent chest pain or pressure, confusion, seizures and, in children, bluish lips or fast breathing ^[4]. CDC also says prescription antiviral drugs can treat flu and work best when started early; whether they are appropriate is a decision for a clinician ^[1].

This is not medical advice. Decisions about your own treatment belong with a qualified clinician who knows your history.

How bad will the 2026-27 flu season be?

It is too early to tell: CDC rates severity as low for all age groups so far, but says the assessment may change because activity is still increasing ^[1].

• Burden so far (US): at least 1.7 million illnesses, 25,000 hospitalisations and 1,400 deaths, CDC estimates ^[1].
• Children (US): three flu-associated child deaths had been reported for the 2026-27 season by week 43 ^[1].
• Forecast (US): CDC's FluSight forecasts predict a likely national increase in flu hospital admissions and emergency visits for the week ending 21 November ^[1].
• Hospital admissions (England): rose to 3.65 per 100,000 in week 45 from 3.00 the week before, still rated low ^[2].

Globally, WHO estimates seasonal flu causes 290,000 to 650,000 respiratory deaths a year ^[5]. UKHSA publishes its flu surveillance report weekly, and CDC updates FluView each week ^[1][2]. How AI tools are used in disease surveillance and clinical care is explained in AI in Healthcare: What It Can Do Today, Where It Fails and How It Is Regulated.