Sonali Tiwary
Seasonal influenza typically arrives in force in late November or December. In October 2026, surveillance data from both the United States and Japan are showing a pattern that public health officials describe as earlier than usual — associated with a mutated H1N1 strain whose altered surface protein may help it partially evade immunity built up in prior seasons. The primary figures come directly from the CDC FluView Week 40 Surveillance Report (week ending October 10, 2026) and first-hand surveillance records from Japan’s Ministry of Health, Labour and Welfare (厚生労働省).
Why Is the 2026–27 Flu Season Starting Early in the US and Japan?
A child coming home with a fever in early October is not unusual. Clinical laboratory positivity climbing from 3.9 percent in Week 39 to 5.0 percent in Week 40, with higher-than-typical early-season activity concentrated on the West Coast, is.
Clinical laboratory positivity in the United States reached 5.0 percent during Week 40 (the week ending October 10, 2026), up from 3.9 percent in Week 39, according to the CDC FluView Week 40 Surveillance Report. Out of 45,331 respiratory specimens tested by clinical laboratories that week, 2,270 came back positive. Influenza A accounted for 97.9 percent of those clinical-laboratory positives. Separately, among influenza A viruses subtyped by public health laboratories, 96.4 percent — 242 of 251 specimens — were identified as A(H1N1)pdm09. The CDC reported that while indicators were rising, national influenza activity remained low and within expected seasonal variation, with the highest activity in West Coast HHS Regions 9 and 10.
The early surge is associated with an H1N1 variant carrying the G155E mutation on its hemagglutinin surface protein gene, which alters the virus’s antigenic properties and may help it partially evade some existing immunity. Professor Samuel Scarpino, executive director of the Institute for Experiential AI and professor of public health at Northeastern University, put it plainly:
“What we’re seeing in the U.S. right now is almost entirely H1N1, carrying a mutation that helps it evade some existing immunity.”
The same mutation was detected in Japan as early as May 2026. Japan’s Ministry of Health, Labour and Welfare (厚生労働省) — publishing weekly sentinel-clinic figures (定点当たり報告数) and tracking school and daycare closures (インフルエンザ様疾患発生報告) — recorded roughly 11.35 patients per clinic in the week ending October 4, 2026, compared with 1.56 patients per clinic in the same week of 2025, approximately 7.3 times as high. Japan announced the beginning of its 2026 influenza epidemic on August 28 — its second-earliest such announcement since 1999.
What Do CDC Week 40 Lab Data Show About H1N1 Subclade D.3.1.1 and Antivirals?
The CDC FluView Week 40 report provides the exact genetic and treatment breakdown for specimens collected across the United States since the surveillance season opened on May 24, 2026.
Genetic Subclade Characterization
Among 416 viruses genetically characterized by the CDC since May 24, 2026:
- 100 percent (252 of 252) of A(H1N1)pdm09 viruses belonged to hemagglutinin (HA) clade 5a.2a.1
- 99.6 percent (251 of 252) fell into the specific subclade D.3.1.1
- 93.6 percent (73 of 78) of A(H3N2) viruses belonged to clade 2a.3a.1, subclade K
Vaccine Antigenic Match
In CDC hemagglutination inhibition (HI) and neutralization assays using ferret antisera:
| Reference Virus | Specimens Well-Recognized | Match Rate |
|---|---|---|
| A/Missouri/11/2025-like (H1N1) | 70 of 74 | 94.6% |
| A/Darwin/1415/2025-like (H3N2) | 20 of 20 | 100% |
| B/Pennsylvania/14/2025-like (B/Victoria) | 30 of 30 | 100% |
Laboratory antigenic testing indicates the current season’s vaccine closely matches the dominant circulating strains, though ferret antisera recognition in the laboratory is not equivalent to measured real-world vaccine effectiveness.
Antiviral Susceptibility
Among viruses tested against antivirals since May 24, 2026:
- 8 of 259 tested A(H1N1)pdm09 viruses (3.1 percent) carry the NA-I223V and NA-S247N amino acid substitutions, conferring reduced inhibition to oseltamivir (Tamiflu)
- Zero viruses showed reduced inhibition to peramivir or zanamivir among 430 tested
- Zero viruses showed decreased susceptibility to baloxavir among 410 tested
The practical implication: oseltamivir remains effective for the large majority of cases, but the small percentage of reduced-inhibition viruses is a figure public health authorities are monitoring.
Historical Context: Immune Imprinting and Why This Mutation Matters
Human immune responses to influenza are shaped by the strain a person first encountered in childhood — a phenomenon epidemiologists call “immune imprinting.” After the 2009 H1N1 pandemic replaced earlier circulating H1N1 strains, the G155E mutation may help the virus partially evade immunity in some age groups. Preliminary testing suggests that teenagers and young adults may have less protection against the current strain, though this does not make the vaccine ineffective.
The mutation was first documented in Japan as early as May 2026, giving researchers and public health agencies several months of lead time before it became prominent in US surveillance data.
Practical Guidance: Vaccination, Antivirals, and When to Act
Get vaccinated in October. Public health experts including Professor Neil Maniar of Northeastern University and the CDC advise obtaining the seasonal flu vaccine in October, before Thanksgiving and holiday gathering season. Laboratory testing indicates that the 2026–27 vaccine antigens closely match the dominant circulating strains.
Start antivirals early for high-risk individuals. For older adults, young children, pregnant people, and those with chronic health conditions, starting prescription antivirals (oseltamivir or baloxavir) within 48 hours of symptom onset significantly reduces severity and duration. The CDC advises that antivirals should be started as early as possible and are especially important for people at higher risk of complications; high-risk or severely ill patients may benefit even when treatment begins later than 48 hours.
Monitor the West Coast spread. CDC surveillance shows the highest current activity in HHS Regions 9 (Pacific Southwest) and 10 (Pacific Northwest). Households with children in those regions, and travelers passing through, face the earliest exposure window.
Long-Term Perspective: Early Seasons and Global Spread Patterns
The 2026–27 season represents one of the earlier US influenza openings in recent years. Japan’s experience — where the epidemic threshold was crossed in late August 2026, the second-earliest such crossing since 1999 — suggests the G155E variant has been circulating and accumulating transmission advantage for several months before it registered in US clinical laboratories. The convergence of high H1N1 dominance (96.4 percent of subtyped public-health-lab specimens), near-complete subclade concentration (99.6 percent in D.3.1.1), and early geographic spread are consistent with this variant having competitive advantage in the current immune landscape.
Pre-Conclusion Summary
The data reviewed in this article covered the CDC FluView Week 40 Surveillance Report and first-hand influenza surveillance records from Japan’s Ministry of Health, Labour and Welfare. In the CDC Week 40 report, clinical laboratory positivity was recorded at 5.0 percent (up from 3.9 percent in Week 39), with Influenza A accounting for 97.9 percent of clinical-laboratory positives and public health laboratories identifying 96.4 percent of subtyped Influenza A as A(H1N1)pdm09. Among 416 genetically characterized viruses, 99.6 percent of A(H1N1)pdm09 specimens belonged to subclade D.3.1.1, and 3.1 percent of tested A(H1N1)pdm09 viruses exhibited reduced inhibition to oseltamivir. In Japan’s Ministry of Health reports, the week ending October 4 recorded 11.35 patients per clinic compared with 1.56 in the same week of 2025, and the G155E mutation was documented in Japan as early as May 2026.
Conclusion
In summary, the October 2026 CDC and Japan surveillance data documented higher-than-typical early-season influenza A(H1N1)pdm09 activity in the United States and Japan, with the G155E hemagglutinin mutation identified as a factor in the virus’s altered antigenic profile. Subclade D.3.1.1 accounted for 99.6 percent of genetically characterized H1N1 viruses in the United States, the 2026–27 vaccine antigens were well-matched in laboratory testing, and a small fraction (3.1 percent) of tested H1N1 viruses showed reduced inhibition to oseltamivir with no resistance detected to peramivir, zanamivir, or baloxavir.
