Health experts are monitoring signs that the seasonal flu could arrive earlier than usual in parts of the Northern Hemisphere, while a newly identified mutation in the H1N1 influenza virus has raised concerns about how well existing immunity may protect against some strains.
The developments have prompted calls to consider getting vaccinated earlier than usual. Seasonal flu vaccination campaigns traditionally begin after mid-October, but unusually high numbers of positive flu tests have already been reported in parts of the United Kingdom and the United States, particularly along the U.S. West Coast.
Japan also recorded flu outbreaks in August and early September, forcing some schools to close temporarily soon after reopening from the summer holidays.
However, the picture in Europe is more reassuring for now. The European Centre for Disease Prevention and Control (ECDC) says influenza activity remains low across most of the continent, with no evidence of widespread or unusually early circulation.
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H1N1 dominates early U.S. samples
Seasonal influenza is caused mainly by two influenza A subtypes, H1N1 and H3N2, as well as influenza B. Which subtype dominates can change from one season to the next as the virus mutates and population immunity evolves through vaccination and previous infections.
The composition of flu vaccines is updated annually based on the strains that circulated during the preceding season. Because vaccine production requires time, the match between the vaccine and the viruses circulating in the following season can vary.
U.S. data show a marked shift from last year’s dominant H3N2 subtype to H1N1. As of September 26, 2026, H1N1 accounted for 96% of the positive samples that had undergone genetic sequencing, while H3N2 represented just 4%. No influenza B was detected in the samples analyzed.
The main concern, however, is not simply the change in the dominant subtype but the emergence of a mutation known as G155E, which may make some H1N1 strains harder for the immune system to recognize.
Mutation may help virus evade antibodies
Researchers from the University of Washington in Seattle identified the G155E mutation in 13 of 17 genetically sequenced H1N1 samples collected from hospitals and clinics in the area.
The mutation has also been investigated by virologists at the Fred Hutchinson Cancer Center in Seattle. Their findings were posted on the bioRxiv preprint platform last month and are awaiting publication in a scientific journal.
Professor Jesse Bloom and his colleagues examined how effectively antibodies found in blood samples from 260 people in Australia and the United States could recognize and block infection by 148 influenza A strains that had circulated through mid-2026.
The researchers found that some strains carrying the G155E mutation were less easily recognized by antibodies.
Antibodies from teenagers and young adults performed particularly poorly in recognizing the variant. The researchers suggested this could be because people in these age groups have had less exposure to H1N1 strains with similar mutated characteristics.
Bloom told Nature that the findings so far point to the possibility of a relatively severe flu season involving strains capable of evading some existing population immunity.
The results do not establish how the mutation will affect the coming season, but they have prompted further scrutiny of its potential significance.
ECDC reports low flu activity in Europe
Despite the concerns raised by U.S. findings, European surveillance data do not currently indicate an unusually early or widespread flu season.
Dr. Iva Bråberg, an ECDC virologist specializing in coronavirus and influenza, told To BHMA-Science that flu cases are beginning to increase in some European countries, but activity remains within normal seasonal levels in most.
“Overall, influenza activity in Europe is low at the moment, with a limited number of countries reporting detection of cases in both primary and secondary health care,” Bråberg said.
She noted a slight increase in influenza detections in hospitals over the past week compared with the previous week, although the numbers remain generally low.
Denmark and Spain are reporting low levels of respiratory virus activity in primary care, she added.
According to ECDC data, none of the 27 flu tests reported for Greece was positive. The number of tests was very small, however, and the result should be considered in that context.
Most of the influenza viruses detected in the European samples were influenza A. Hospital cases mainly involved adults aged 65 and older.
“At this stage, there is no evidence of widespread or unusually early influenza circulation in Europe,” Bråberg said.
G155E has already been detected in Europe
The G155E mutation is not new to Europe. According to Bråberg, it was detected during the 2025-2026 flu season, although it remained rare.
The ECDC analyzed influenza virus characteristics from that season using 6,636 genetic sequences of influenza A(H1) collected across 25 EU member states.
Of those sequences, 95% belonged to subclade D.3.1.1. The G155E mutation was detected in just 60 samples, representing less than 1% of the total.
Bråberg cautioned that it remains difficult to predict how influenza strains carrying the mutation will affect the current season.
While European surveillance has so far produced reassuring findings, the virus’s evolution makes it difficult to forecast how the season will unfold.
Vaccination and hygiene remain important
Health experts continue to emphasize the importance of preventive measures as the winter flu season approaches.
Regular hand hygiene and other basic precautions remain important, while timely vaccination is particularly recommended for people in vulnerable groups.
The emergence of a mutation that may reduce antibody recognition does not, by itself, establish how effective this season’s vaccines will be against circulating strains. The overall impact of G155E on the coming flu season remains uncertain.
With respiratory viruses capable of spreading internationally within hours, experts stress the importance of remaining alert and taking preventive measures before influenza activity rises significantly.