H1N1 Rises Early After Last Winter’s H3N2 Wave

The 2026-2027 flu season started in August, with H1N1 carrying a G155E change after a winter run by H3N2. National activity is still low. The bill from last year’s strain mix is already on the calendar.

Andrew Pekosz, a virologist at the Johns Hopkins Bloomberg School of Public Health, said the first flu that arrives is often the one that slips past existing immunity. That is the pattern in the numbers, and it is showing up first in younger people.

Flu Is Rising in August, Not November

The U.S. Centers for Disease Control and Prevention said in its FluView report for the week ending Oct. 17 that early-season influenza activity is higher than typically observed at this time of year. Cases have been climbing since late August and early September. Two states or territories were at moderate outpatient intensity that week, and none were high or very high.

Clinical laboratories tested 51,490 samples and 5.7 percent of samples tested positive. A year earlier, in the same window, the CDC’s respiratory virus dashboard had positivity under 1 percent. Outpatient visits for flu-like illness sat at 2.2 percent, still under the 2.9 percent national baseline, and higher than the prior two seasons at this point.

WEEK 41 AT A GLANCE

• Test positivity: 5.7 percent of 51,490 clinical samples, versus under 1 percent a year earlier.
• Virus mix: 99.2 percent of subtyped influenza A was H1N1; none of those subtyped viruses was H3N2.
• Hospitals: 4,811 patients were admitted with influenza that week, with a cumulative rate of 2.1 per 100,000.
• Deaths: 0.2 percent of death certificates listed flu; no pediatric flu deaths had been reported this season.

Pekosz told interviewers that cases are still relatively low, so the country is in the early phase of a surge. The West moved first, which is an odd path for U.S. flu, and the rest of the map is now catching the same H1N1 wave.

Last Year’s H3N2 Wave Left an H1N1 Gap

Last winter was an H3N2 year. Subclade K, the variant many people called the super flu, dominated after picking up a cluster of changes on hemagglutinin. CDC modeling presented after that season put the toll at at least 32 million illnesses, 390,000 hospital admissions, and 24,000 deaths.

H1N1 barely had a speaking part. People who were infected or boosted against H3N2 last year did not refresh their H1N1 antibodies in the same way. Dr. Bernard Camins, an infectious disease physician at the Icahn School of Medicine at Mount Sinai, said even people who got the flu last year are vulnerable, because this virus is not last year’s virus.

Caitlin Rivers, an epidemiologist who directs the Center for Outbreak Response Innovation at Johns Hopkins, has described the same hole: an H3N2-heavy season can leave H1N1 immunity stale. Camins put it more simply. Maybe that is why people are getting sick sooner.

H1N1 here is the 2009 pandemic lineage, written as H1N1pdm09 in lab reports. It has been a seasonal virus for years, and it usually changes more slowly than H3N2. A slow virus can still find a gap when the last winter trained the country on something else.

The G155E Change Hits Younger Antibodies First

The H1N1 now spreading sits in subclade D.3.1.1 and carries one extra change, G155E, on a site that antibodies use. Pekosz said that position matters for protective antibody responses, so prior infection or vaccination may not see this virus as clearly.

A Fred Hutch team led by Caroline Kikawa and Jesse Bloom measured 52,268 titers, testing 148 viral strains against 355 human serum samples collected between April and August. Their preprint on bioRxiv found that D.3.1.1 strains with Sa-site changes such as G155E showed reduced neutralization by human sera, with a wide spread from person to person.

Bloom Lab put the age split in plain language on Sept. 17. For most samples, G155E only mildly reduced neutralization. Some samples did much worse. The people behind those samples tend to be younger.

Individuals who are strongly affected by G155E tend to be younger.

Bloom Lab, Fred Hutchinson Cancer Center, Sept. 17 post

But effect of G155E mutation in H1N1 is highly variable across people. For most sera, strains with G155E have only mildly reduced neutralization—but some sera neutralize G155E strains much more poorly. Individuals who are strongly affected by G155E tend to be younger. pic.twitter.com/NHzonHtZ0w

— Bloom Lab (@jbloom_lab) September 17, 2026

That is the part of the mutation story that a national average hides. Most adults still have antibodies that mostly work. The people whose antibodies miss G155E cluster younger, including teens and young adults who have had less contact with similar H1N1 shapes.

Amesh Adalja, an infectious disease physician at the Johns Hopkins Center for Health Security, said prior immunity neutralizes this virus less well, especially in those born after 2009, so it spreads sooner. Dr. Jennifer Duchon, a pediatric infectious disease specialist at Mount Sinai Children’s Hospital, said a lot of younger patients have not seen this H1N1, and it may hit them harder. Historically, H1N1 has been harder on young people than H3N2, which tends to weigh more on older adults.

Alex Greninger, a University of Washington virologist, said his Seattle lab found G155E in 13 of 17 H1N1 samples with sequence good enough to call individual changes. Those samples came from local hospitals and clinics over several months. A small city slice is not a national census, but it is the same mutation the serum work flagged, in the region that lit up first.

What the CDC Counted for Week 41

Public health laboratories reported 323 flu positives in the week ending Oct. 17. Of 241 influenza A viruses that were subtyped, 99.2% were H1N1pdm09. Two were H5. Zero were H3N2 that week. Influenza B was 1.5 percent of those public-health positives.

Since Oct. 4, the cumulative subtype mix is a little less one-sided: 780 of 798 subtyped influenza A viruses (97.7 percent) were H1N1, and 16 (2.0 percent) were H3N2. The H3N2 share is sitting in the season total, not in Week 41’s subtyped set.

CDC FLUVIEW, WEEK ENDING OCT. 17

The same FluView week also logged a confirmed H5N1 infection in a Washington dairy worker 18 or older who had eye irritation and upper-airway symptoms. CDC said human-to-human spread of H5 has not been identified in the United States. That case is a separate virus from the H1N1 filling clinics.

CDC ferret tests, which check circulating viruses against antibodies raised to the vaccine reference strain, still look friendly. Of 88 H1N1 viruses collected since May 24 and run in those assays, 82 (93.2 percent) were well recognized by ferret antisera to cell-grown A/Missouri/11/2025-like viruses. That is a different test from Bloom’s human serum panel. Ferrets say the shot’s H1N1 component still resembles most circulating viruses. Human blood says a subset of people, often younger, neutralize G155E strains poorly.

Most Circulating H1N1 Still Fits the Shot

The Food and Drug Administration’s vaccine committee met on March 12 and updated all three components from the 2025-2026 formula, including an H3N2 change aimed at last year’s subclade K. Egg-based shots for this season use an A/Missouri/11/2025 (H1N1)pdm09-like virus, an A/Darwin/1454/2025 (H3N2)-like virus, and a B/Tokyo/EIS13-175/2025 (B/Victoria lineage)-like virus. Cell and recombinant shots use the same H1N1, an A/Darwin/1415/2025 H3N2, and a B/Pennsylvania/14/2025 Victoria virus.

G155E was not on that March menu. Strain picks happen months before fall, which is why a late-summer change can land after the recipe is locked. Pekosz still expects the shot to help.

A mismatch is not a black-or-white situation. We may have less immunity, not no immunity.

Andrew Pekosz, virologist, Johns Hopkins Bloomberg School of Public Health

CDC said it is too early to know which viruses will predominate, or to say what that means for vaccine effectiveness this season. The H3N2 and influenza B legs of the shot still sit there for whatever arrives after H1N1. Manufacturers told CDC they can make as many as 135 million doses for the U.S. market. The whole projected supply is trivalent. Twenty-seven percent is non-egg. Ninety-nine percent is made without thimerosal as a preservative.

SHOTS ON OFFER THIS SEASON

• Standard inactivated shots: For most people 6 months and older, matched to the three updated strains.
• High-dose and adjuvanted shots: Built for adults 65 and older, who still need a dose even in an H1N1-leaning year.
• Nasal mist: A live option for people who qualify, sitting beside the injected shots.
• New mRNA flu shot: A first-season option this year, on top of the egg, cell, and recombinant lineup.

Camins noted that a flu shot is never 100 percent, and vaccinated people can still get infected. They are much less likely to get very sick or land in a hospital. Pharmacies have not seen demand jump just because positivity moved early, which fits a market where most doses were ordered months ago. An early curve changes timing more than it changes how many vials were already sitting in refrigerators.

Get the Dose Before Immunity Lags

CDC says flu shots should ideally go in during September or October for most people, and that July and August doses are a poor default because protection can fade before spring. H1N1 did not wait for that calendar. When it started rising in late August, the updated shots had only just begun to roll out.

Pekosz said you need at least two weeks after the shot for antibody levels to reach a protective range. A dose on Oct. 24 lines up around Nov. 7. Halloween is Oct. 31. The usual “before the end of October” advice still holds, and a later dose still helps if H3N2 or influenza B moves in after this H1N1 wave.

HOW THE SEASON GOT AHEAD

1. March 12, 2026: The FDA vaccine committee locks in the three 2026-2027 strains, including A/Missouri/11/2025 for H1N1.
2. Late August 2026: U.S. flu indicators start rising, weeks ahead of the usual fall build.
3. September 2026: The Fred Hutch serum study and Seattle sequencing flag G155E on D.3.1.1 H1N1.
4. Week ending Oct. 17, 2026: Clinical positivity hits 5.7 percent; 99.2 percent of subtyped influenza A is H1N1.
5. End of October 2026: The preferred shot window closes for people who still want two weeks of buildup before a typical winter peak.

If the test is flu, antivirals can shorten the illness, and they matter most for people 65 and older, children under 5, and anyone with a weak immune system. They are prescription-only. Pekosz said the prescription has to come fast, within the first 48 hours after symptoms start, or the drugs do not work as well. Camins said flu often hits suddenly, with fever, chills, fatigue, body aches, a runny nose, and cough, and that COVID can look the same, which is why a test is worth getting before you guess.

H1N1 May Peak Before a Second Wave

Camins said it is too soon to know how bad this season will be. He thinks it is just starting earlier. An early H1N1 wave does not lock in a mild winter or a brutal one. Last year’s H3N2 season was harsh. This year’s national ILI is still below baseline, pediatric flu deaths are still at zero, and ferret tests still read most H1N1 samples as close to the shot.

Greninger has warned that H1N1 could peak on its own timetable, with another flu still able to arrive in January, February, or March. Different strains often take turns. The updated H3N2 and influenza B components are there for that second act, which is one reason a shot given for today’s H1N1 is not wasted if the mix changes.

The mutation is real, the early curve is real, and the people who look worst on paper are younger patients whose antibodies miss G155E. The rest of the country still has a shot that covers three viruses, antivirals that work if they are started in 48 hours, and a week-41 hospital count of 4,811 that is rising from a low base, not a winter peak.

Disclaimer: This article is news reporting and analysis of CDC surveillance, FDA strain selections, and published lab work. It is for information only and is not medical advice, a diagnosis, or a recommendation of any specific flu shot, antiviral, or test for any person. Readers should talk with a physician, pediatrician, pharmacist, or other qualified clinician about vaccination, timing, and treatment, especially if they are over 65, under 5, pregnant, or immunocompromised. Figures and activity levels reflect the CDC FluView week ending Oct. 17, 2026, and related documents cited here, and they can change as later weeks are reported.