In the US, H1N1 accounted for 96% of early flu cases — Nature

In the United States and other Northern Hemisphere countries, seasonal influenza may begin several weeks earlier than usual. The share of positive tests is already unusually high for this time of year in the United Kingdom and on the US West Coast. In Japan, outbreaks in August and early September led to the closure of some schools after the summer holidays, Nature reports.

University of Pennsylvania virologist Scott Hensley noted that this would be an exceptionally early season for the United States if case numbers on the West Coast continued to rise at their current pace. At the same time, researchers caution that the data are based on a relatively small number of positive tests compared with the seasonal peak, so it is still too early to predict how the season will develop further. An early start does not necessarily mean a more severe season.

H1N1 dominates among early cases

In the United States, during the week ending September 26, the H1N1 subtype accounted for 96% of positive samples for which the virus subtype was determined. H3N2 accounted for nearly 4%, and influenza B was not detected. This differs from the previous season, when H3N2, including its K subclade, predominated.

More current news is available on the UA.News Telegram channel Telegram.

Seasonal influenza is usually caused by two influenza A subtypes — H1N1 and H3N2 — and influenza B. The dominant virus changes between seasons because viruses mutate genetically and people's immunity changes as a result of vaccination and previous infections.

Researchers track the G155E mutation

Specialists are concerned about the G155E mutation in H1N1. University of Washington virologist Alex Greninger said his laboratory detected it in 13 of 17 H1N1 samples with sufficiently complete genetic sequences. The samples came from Seattle hospitals and clinics over the past several months.

The mutation was also analyzed by a group led by Jesse Bloom, a virologist at the Fred Hutchinson Cancer Center. Their study was posted on the bioRxiv preprint server before peer review. The scientists tested how antibodies in blood samples from 260 people in Australia and the United States recognized and blocked infection by 148 influenza A viruses that had circulated through mid-2026. According to Bloom, some strains with G155E were less well recognized by existing antibodies, especially in samples from teenagers and young adults.