Doctors in the United States are urging everyone aged 6 months and older to get a flu shot now, after an unusually early rise in influenza cases on the West Coast and the emergence of a mutated H1N1 strain — a new version of the virus behind the 2009 swine flu pandemic — that may hit children and young adults harder because their immune systems have not seen it before, Reuters reported on October 20, 2026.
Illustration: a healthcare worker giving a flu shot at a US clinic. This is an AI-generated illustration, not a photograph of a real clinic.
What Happened
Flu season in the United States is starting early and oddly. Positive flu tests reported to the Centers for Disease Control and Prevention by clinical laboratories hit a 10-year high for this time of year last week, reaching 5% nationwide, according to Reuters. On the West Coast, the figure was 10.7% — compared with just 0.5% at the same point a year earlier. At the peak of a typical flu season, positive test rates run around 30%. Activity has been rising since late August, and numbers are also elevated in eight Southern states, including Florida and South Carolina.
The dominant strain this year is H1N1 — the same subtype that caused the 2009 swine flu pandemic and has since become a standard part of the annual flu shot. But this year's version carries a new mutation called G155E that worries virologists. People who were previously exposed to H1N1, whether through infection or vaccination, would have only some residual protection from the older versions, scientists say.
The concern is concentrated on the young. "What people are a bit worried about is that the H1N1 strain this year hasn't been seen by a lot of younger folk, so it may be more severe in them," said Dr. Jennifer Duchon, a pediatric infectious disease expert at Mount Sinai Children's Hospital in New York, who added that her hospital has already started to see some flu cases. While most children recover, flu can be deadly — especially for the very youngest and those with underlying health conditions.
Supporting that worry, Jeanne Marrazzo, chief executive of the Infectious Diseases Society of America, told Reuters that studies of antibodies from people exposed to flu last year found they did not recognize the new mutation — and that this was especially true of antibodies from teenagers and young adults. Marrazzo cautioned that the studies were small: something to be aware of, but not cause for alarm.
Scott Hensley, a microbiologist at the University of Pennsylvania, said two of the three components of this year's vaccine appear to be very well matched to circulating flu strains. However, he added, the H1N1 mutation might affect antibodies in younger people — and with flu season picking up steam, "it is probably a good idea to get vaccinated early," he said via email. The 2026–2027 flu vaccines, approved by the FDA in August, target H1N1, H3N2 — the dominant strain last flu season — and one influenza B virus.
Background
This season is breaking the usual pattern in more ways than one. Typically, US flu cases start in Southern states and move north as winter sets in. This year, the surge is coming from the west — and epidemiologists are not sure why.
"There's no clear reason to me why this would be the case," said Caitlin Rivers, a Johns Hopkins epidemiologist who tracks pathogens in a weekly newsletter called Force of Infection. She offered two possible explanations. One is an "immunological gap": because last season was dominated by H3N2, population immunity to H1N1 is not as fresh. The other is the new mutation itself — G155E may simply be behaving differently enough to spread faster in a population with patchy defenses.
The backdrop is a country vaccinating less against flu than it used to. Some 58 million American adults received a flu shot last season, according to data firm IQVIA — down more than 20% from the nearly 75 million in the 2019–2020 season, the last season before the COVID-19 pandemic reshaped US vaccination patterns. Around half of US children got flu shots last year, according to CDC data, compared with more than 60% in 2019–2020.
Promotion of flu shots has shrunk at the federal level. Under US Health Secretary Robert F. Kennedy Jr. — a longtime anti-vaccine activist who has engineered an overhaul of long-standing childhood vaccine schedules — the government has cut back on promoting flu vaccination. The CDC dropped its recommendation for the shot for most children at the beginning of 2025, a move a court later put on hold, and pulled its ad campaign to promote flu vaccination in 2025, according to press reports cited by Reuters.
Paul Offit, a pediatric infectious disease expert at Children's Hospital of Philadelphia, said he believes immunization rates have been hurt by the health department's decision to cut back on promoting flu shots, as well as by statements from Kennedy himself. "He has done what he can to discourage flu vaccination," Offit said. When Reuters asked about public health messaging on flu shots this season, HHS spokesperson Emily Hilliard pointed to the CDC's "Respiratory Illness Season Toolkit" on the agency's website, which says vaccination is the most effective tool to prevent serious illness.
Supply is not the problem. Standard flu shots are made by GSK, Sanofi and CSL's Seqirus unit; AstraZeneca makes a nasal spray inoculation called FluMist; and a new mRNA vaccine from Moderna is also available. Vaccine manufacturers say demand has been consistent with last year — and two pharmacists told Reuters that vaccination rates have not picked up significantly despite the early surge.
Illustration: virologists studying a mutated H1N1 influenza strain in a laboratory. This is an AI-generated illustration, not a photo of a real laboratory.
Why It Matters
Last year's toll is a reminder of what is at stake. Over the past decade, the flu vaccine has been between 30% and 60% effective at reducing flu hospitalizations and deaths, depending on how well it was matched — months in advance — to the strains that actually circulated. Last year, an estimated 32 million Americans had the flu, 390,000 were hospitalized and 24,000 died.
The G155E mutation adds a wildcard to this year's math. Because the mutation may blunt antibody recognition — particularly in teens and young adults who lack older H1N1 exposure — even a well-matched vaccine could face a season in which the virus behaves differently from the version scientists planned for. That is why Hensley and others are advising early vaccination: get protection in place before the virus has weeks to spread.
The early geographic pattern also matters. A season that begins in the West in October, rather than building in the South and marching north in December and January, could mean a longer, less predictable wave — and the 10.7% positive-test rate on the West Coast versus 0.5% a year ago suggests transmission is already firmly established in some communities.
There is also a policy dimension. The American Medical Association still recommends the shot for all people 6 months and older — but the federal government's retreat from promoting it, from the dropped child recommendation to the pulled ad campaign, has coincided with a steady slide in uptake. If this season proves severe, especially among the young, the question of how official messaging shaped behavior will be asked again.
Illustration: adults and seniors getting flu shots at a pharmacy vaccination station in autumn. This is an AI-generated illustration, not a photograph of a real pharmacy.
What Happens Next
The immediate advice from clinicians is simple: get vaccinated now, not later. Mount Sinai's Duchon and Penn's Hensley both point to the same logic — with the season starting early and the mutation potentially eroding protection in younger people, earlier shots mean earlier protection. The standard shots from GSK, Sanofi and CSL's Seqirus, AstraZeneca's FluMist nasal spray and Moderna's new mRNA vaccine are all available.
Virologists will be watching the G155E mutation closely in the coming weeks. Small antibody studies flagged the recognition gap; larger surveillance data on how the strain spreads — and who it hospitalizes — will show whether the early concern about severity in younger people is borne out or overblown. The CDC's weekly flu data will also reveal whether the Western surge spreads nationwide or remains regional.
The legal and political picture around vaccination policy remains in flux. The CDC's dropped recommendation for most children is on hold after a court intervened, and the agency's website still points to vaccination as the most effective tool against serious illness. Whether federal promotion of flu shots is restored — and whether uptake recovers from its post-pandemic slide — will shape how this early, unusual season ends.
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Frequently Asked Questions
Why are doctors urging flu shots earlier than usual this year?
Flu season is starting unusually early — positive tests hit a 10-year high for this time of year at 5% nationwide, and 10.7% on the West Coast versus 0.5% a year ago. On top of that, this year's dominant H1N1 strain carries a new G155E mutation that may be less recognized by existing antibodies, so clinicians like UPenn's Scott Hensley say "it is probably a good idea to get vaccinated early."
What is the G155E mutation?
It is a new mutation in this year's dominant H1N1 influenza strain — the same subtype behind the 2009 swine flu pandemic. Small studies found that antibodies from people exposed to last year's flu did not recognize the new mutation, especially antibodies from teens and young adults. The Infectious Diseases Society of America's Jeanne Marrazzo said the findings are worth watching but not cause for alarm.
Who is most at risk this season?
Children and young adults may face higher risk, because the mutated H1N1 strain "hasn't been seen by a lot of younger folk," in the words of Mount Sinai pediatric infectious disease expert Dr. Jennifer Duchon, so it may be more severe in them. The very youngest children and people with underlying health conditions remain the most vulnerable overall. The AMA recommends the shot for everyone 6 months and older.
How well do this year's flu vaccines match the circulating strains?
According to microbiologist Scott Hensley of the University of Pennsylvania, two of the three components of this year's vaccine appear to be very well matched to circulating strains. But the H1N1 mutation might affect antibodies in younger people. Over the past 10 years, flu vaccines have been 30% to 60% effective at reducing hospitalizations and deaths — and last year, when 32 million Americans had the flu, 390,000 were hospitalized and 24,000 died.
Why are fewer Americans getting flu shots?
Adult uptake fell more than 20% from about 75 million in 2019–2020 to 58 million last season, per IQVIA, and about half of US children got shots last year versus over 60% in 2019–2020. Federal promotion has also shrunk: under Health Secretary Robert F. Kennedy Jr., the CDC dropped its shot recommendation for most children in early 2025 (a court put that on hold) and pulled its flu vaccination ad campaign in 2025, according to press reports.
Sources
- Reuters — "Early US flu season, current strain may signal risk for younger people; doctors urge shots" (October 20, 2026)
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