# Early Northern Hemisphere influenza surge and G155E mutation > Live situation record from CLSTR: https://clstr.news/situations/early-northern-hemisphere-influenza-surge > Updated: 2026-10-23T05:57:00.000Z. Sources: 35. Developments: 6. Public health officials in the Northern Hemisphere are reporting an unusually early onset of influenza activity. In the United States, the surge is notably driven by the West Coast rather than the South. Surveillance data from the United Kingdom and Japan also indicate higher-than-expected positive test rates. In Japan, outbreaks in August and early September resulted in the closure of some schools. Scientific concern has centered on the H1N1 subtype, which has accounted for approximately 96 percent of subtyped cases in some regions. Virologists at the University of Washington have identified an emerging G155E mutation, detected in 13 out of 17 H1N1 samples tested. This has raised concerns regarding the effectiveness of existing immunity. While the H3N2 strain was predominant last year, H1N1 is currently the primary subtype responsible for most cases in the United States. Recent laboratory findings suggest the G155E mutation may lead to reduced neutralization by antibodies, particularly in individuals aged 15 to 25 who may have limited prior exposure to this specific strain. Dr. Jennifer Duchon of Mount Sinai Hospital noted that the circulating H1N1 subtype could cause more severe illness in younger people due to this lack of immunity. While the mutation raises questions regarding antibody recognition, experts emphasize that current seasonal vaccines remain important for preventing complications. In La Réunion, the epidemic has persisted for five consecutive weeks, with 75 percent of emergency room visits for flu-like symptoms involving children and adolescents under 15. In the United States, the season began approximately two and a half months earlier than typical years. In Washington state, positivity rates have reached levels typically seen in January. ## Timeline ### 2026-10-23: Influenza A G155E mutation poses risks for younger populations The G155E mutation in Influenza A (H1N1) may reduce antibody effectiveness in younger populations, prompting health experts to urge timely vaccination amid an early flu season in the US. 2 sources. https://clstr.news/cluster/influenza-a-g155e-mutation-poses-risks-for-younger-populations ### 2026-10-22: United States experiences unusually early flu season A mutated virus has triggered an unusually early flu season in the US, hitting West Coast states months ahead of schedule. 7 sources. https://clstr.news/cluster/united-states-experiences-unusually-early-flu-season ### 2026-10-13: Flu epidemic rises globally amid concerns over H1N1 mutation Health officials report an early flu season in the Northern Hemisphere, with concerns rising over a new H1N1 mutation (G155E) and high emergency room visits among youth in La Réunion. 7 sources. https://clstr.news/cluster/flu-epidemic-rises-globally-amid-concerns-over-h1n1-mutation ### 2026-10-10: Flu activity rises early on the US West Coast Flu activity is rising earlier than usual in the United States, with significant surges reported along the West Coast, deviating from the typical seasonal pattern. 2 sources. https://clstr.news/cluster/flu-activity-rises-early-on-the-us-west-coast ### 2026-10-08: Influenza cases rise early in Northern Hemisphere amid H1N1 mutation concerns Influenza cases are appearing earlier than usual in the Northern Hemisphere, with concerns rising over a new H1N1 mutation (G155E) that may evade existing immunity in younger populations. 2 sources. https://clstr.news/cluster/influenza-cases-rise-early-in-northern-hemisphere-amid-h1n1-mutation-concerns ### 2026-10-06: Flu season begins early with H1N1 dominance and West Coast surge Health officials report an unusually early flu season across the Northern Hemisphere, driven by H1N1 strains and rising activity on the US West Coast, prompting urgent calls for vaccination. 15 sources. https://clstr.news/cluster/influenza-season-shows-unusual-early-surge-across-northern-hemisphere --- Cite as: Early Northern Hemisphere influenza surge and G155E mutation. CLSTR, https://clstr.news/situations/early-northern-hemisphere-influenza-surge