Early US Flu Season Surges as Mutated H1N1 Raises Vaccine Concerns

Flu activity in the US is climbing earlier than usual, with the West Coast already seeing the highest levels by early October and the A(H1)pdm09 strain dominating national cases.

Scientists are closely tracking an emerging G155E mutation in some H1N1 viruses that affects an antigenic region of the virus and may alter how antibodies recognise it, although overall activity across the country remains low.

US flu activity began increasing in late August and September 2026, according to the Centers for Disease Control and Prevention (CDC). By the week ending 10 October, the West Coast, including HHS regions 9 and 10, was recording the highest activity.

For the week ending 17 October, 2,918 of 51,490 respiratory specimens tested by clinical laboratories were positive for influenza, giving a positivity rate of 5.7 per cent. Influenza A accounted for 97.9 per cent of positive specimens.

Although activity is increasing, national levels remain low overall. An early start does not necessarily mean a more severe flu season.

H1N1 Dominates Early Flu Activity as G155E Spreads

The CDC reported that 239 of 241 influenza A viruses subtyped by public health laboratories during the week ending 17 October were A(H1N1)pdm09, representing 99.2 per cent of those analysed.

A(H1N1)pdm09 is an established influenza virus that emerged in 2009 and continues to circulate seasonally. Most A(H1N1)pdm09 viruses genetically characterised by the CDC this season belong to the D.3.1.1 subclade. Within this group, viruses carrying the G155E amino-acid substitution in the haemagglutinin protein have increased in recent weeks. The mutation occurs in an antigenic region of the virus and may affect how antibodies recognise some circulating viruses.

The World Health Organization (WHO) reported in October that viruses carrying G155E had increased in recent weeks. Human-serum studies cited by WHO found reduced antibody responses to some recently circulating D.3.1.1 viruses carrying G155E and other substitutions, raising questions about how effectively existing antibodies recognise some circulating variants.

However, the CDC's separate antigenic testing found that 82 of 88 characterised H1N1 viruses were well recognised by ferret antisera to the vaccine reference virus. This laboratory finding does not establish how effectively vaccination protects people against circulating variants in real-world conditions.

The CDC says it is too early to determine the impact of these genetic changes on vaccine effectiveness. There is also insufficient evidence that G155E caused the early rise in flu activity or makes the virus more severe.

Experts Call For Vaccination as Monitoring Continues

The CDC recommends annual flu vaccination for everyone aged six months and older, unless they have a relevant contraindication. Vaccination can reduce the risk of serious complications, hospitalisation and death, even though it does not prevent every infection.

Young children, adults aged 65 and older, pregnant people and those with certain underlying health conditions face a higher risk of severe flu complications. The CDC also recommends handwashing, covering coughs and sneezes, and staying home when sick. People at higher risk who develop flu symptoms should seek medical advice promptly, as antiviral treatment can be most beneficial when started early.

Early Flu Surge Sparks Debate Over Vaccination on Social Media

The early rise in flu activity has prompted debate on social media, with users sharing personal experiences and questioning the effectiveness of flu vaccines.

One user questioned the argument that vaccination may have reduced the severity of illness, pointing to cases in which vaccinated people still contracted flu.

Another user claimed to have gone a decade without receiving a flu shot or contracting flu.

Other users pushed back, with one expressing frustration over deaths among unvaccinated people, while another challenged the suggestion that fever or illness after vaccination proves that vaccines are ineffective.

As surveillance continues, researchers will need further data to determine whether G155E affects protection against circulating strains. The CDC continues to recommend annual vaccination to reduce the risk of serious flu complications.