Unusually early influenza activity in the UK, the US West Coast and Japan is dominated by H1N1 viruses carrying a mutation, G155E, that existing antibodies — especially in teenagers and young adults — appear to recognise less well. Scientists stress that an early season need not be a severe one.
Influenza usually gives public-health officials a predictable calendar: in temperate parts of the Northern Hemisphere, cases begin rising in November and peak around December and January. This year the calendar may be slipping. According to a news report published in Nature on 6 October, the proportion of positive influenza tests is unusually high for early autumn in places including the United Kingdom and the West Coast of the United States, while in Japan outbreaks in August and early September forced some schools to close just as they reopened after the summer break.
“It would be an incredibly early season” for the United States if cases on the West Coast keep rising at their current rate, Scott Hensley, a virologist at the University of Pennsylvania, told Nature.
A switch in the dominant virus
Seasonal flu is driven by a shifting mix of three viruses: two subtypes of influenza A, H3N2 and H1N1, and influenza B. Last season was dominated by a heavily mutated H3N2 variant, known as subclade K, which spread globally and triggered early seasons in several countries. This year the picture has flipped. US surveillance data for the week ending 26 September showed that H1N1 accounted for 96 per cent of positive specimens that were subtyped; H3N2 made up almost 4 per cent, and influenza B was not detected.
The concern centres on a specific change in the H1N1 haemagglutinin, the surface protein the virus uses to enter cells and the main target of protective antibodies. The mutation, G155E, was found by Alex Greninger’s laboratory at the University of Washington in 13 of 17 H1N1 samples from Seattle hospitals and clinics for which sequences were complete enough to analyse.
The same mutation had been flagged in a preprint posted on bioRxiv in September by a team led by Jesse Bloom of the Fred Hutchinson Cancer Center. The researchers tested how well antibodies in blood samples from 260 people in Australia and the United States recognised 148 influenza A viruses circulating by mid-2026. Several strains carrying G155E were, in some cases, less well recognised by pre-existing antibodies. Serum from teenagers and young adults performed especially poorly against the variant — probably, the researchers suggest, because these age groups have not previously been exposed to H1N1 viruses with similar features. The preprint has not yet been peer-reviewed.
Why one amino-acid change can matter
Antibodies recognise viruses by binding to particular patches on the haemagglutinin’s head, known as antigenic sites. A single amino-acid substitution in one of those patches — in this case a glycine replaced by glutamic acid at position 155 — can change the shape or charge of the binding surface enough to weaken recognition. Because each person’s antibody repertoire is shaped by the flu strains they met earliest in life, a change of this kind can affect age groups unequally, which is consistent with the weaker recognition Bloom’s team saw in younger people.
Timing compounds the problem. The World Health Organization recommends Northern Hemisphere vaccine strains in February, roughly eight months before vaccination campaigns begin, so manufacturers can grow and distribute hundreds of millions of doses. Any variant that rises to dominance after that decision may be less well matched by the vaccine. Last season, subclade K emerged too late to be included; WHO subsequently updated the H3N2 component for 2026–27. Whether this year’s H1N1 component adequately covers G155E viruses is one of the questions surveillance laboratories will now be trying to answer.
What it does — and does not — mean
The early data come from relatively few positive tests, far fewer than at the height of a typical season, and specialists caution against forecasting the season’s trajectory. “An early start doesn’t necessarily mean a more severe season,” James Hay, an epidemiologist at the University of Oxford, told Nature. Reduced antibody recognition in laboratory assays also does not translate directly into a known loss of vaccine protection; real-world vaccine effectiveness will only become clear as the season unfolds.
The practical question for many people is timing. Epidemiologist Caitlin Rivers of Johns Hopkins University, who normally waits until mid-to-late October to be vaccinated so that protection lasts through the winter peak, told Nature she is now considering getting the vaccine as soon as possible. Health authorities will be weighing the same trade-off between protection that wanes over months and a season that may be starting early.
The World Health Organization has scheduled a webinar for 8 October on preparing for respiratory virus circulation during the Northern Hemisphere winter, focusing on surveillance and vigilance.
Why Indian readers should pay attention
India’s influenza seasonality differs from that of temperate countries. Northern India typically sees a winter peak, while much of southern and coastal India — including Telangana and Andhra Pradesh — sees substantial activity during and after the monsoon. H1N1, which caused the 2009 pandemic and has circulated seasonally since, remains a significant cause of hospitalisation in India, particularly among older adults, pregnant women and people with chronic illnesses.
The emergence of a variant that evades some existing immunity is therefore directly relevant. Genomic surveillance through India’s laboratory networks will determine whether G155E-carrying viruses are circulating here and at what levels. For clinicians, the familiar advice still applies: early antiviral treatment for high-risk patients, vaccination for vulnerable groups, and attention to unusual clusters of severe respiratory illness. For policymakers, this season is a reminder that vaccine-strain decisions made months in advance can be overtaken by viral evolution, which is why faster platforms and broader vaccines remain research priorities.
– Rithvisha Kiran
Key facts
– Early influenza activity reported in the UK, US West Coast and Japan (Nature, 6 Oct 2026)
– US week ending 26 Sept: H1N1 96% of subtyped positives; H3N2 ~4%; influenza B not detected
– Mutation of concern: G155E in H1N1 haemagglutinin; detected in 13 of 17 Seattle sequences
– Bloom preprint: sera from 260 people, 148 viruses tested; reduced recognition, especially in teenagers and young adults (not yet peer-reviewed)
– Last season was dominated by H3N2 subclade K
