ScienceNot yet confirmed elsewhere1 publisher3 min readPublished
US flu test positivity climbs to about seven times last year's late-September rate
CDC data show 3.4% of respiratory samples at US clinical labs tested positive for flu in the week ending Sept. 26, about seven times last year's rate. The early rise, mostly in Western states, supports October flu shots but says nothing yet about how severe the season will be.
The Scientist · Science desk
What happened
- Vanderbilt infectious disease specialist William Schaffner said patients at clinics in his Tennessee area are already testing positive for flu.
- H1N1 made up 96% of the influenza A samples that public health labs subtyped in the week ending Sept. 26.
- Last season was dominated by H3N2 subclade K and was classed by the CDC as highly severe for children, with at least 195 pediatric flu deaths.
Compiled by The ScientistSomething wrong?How this is made
Why it matters
- decision Flu-shot drives timed to the usual November rise would open after flu is already spreading in the West, so employers and clinics have grounds to book October dates.
- constraint Moving campaigns earlier than October carries its own cost, because the CDC holds that shots given in July or August can lose protection before the season ends.
- exposure Children under 5, adults 65 and older, pregnant people and people with chronic conditions, the groups most prone to severe flu, carry the most risk from any gap between early spread and vaccination.
The sevenfold comparison rests on two small percentages. In the matching week of 2025, which ended Sept. 27, about 0.5% of samples tested positive [2]. This year's 3.4% [1] is 6.8 times that [24]. Live Science rounds that to about seven [3]. Measured the other way, the rate is 2.9 percentage points higher [25]. Each figure is a share of the samples clinical labs tested, and the reported data do not say how many samples that was in either year. Nationally, the CDC figures describe activity that is still low but rising [19].
Emergency department records give a second measure, and their denominator is visits. Washington's share of visits diagnosed as flu rose about 5.4-fold over six weeks [26]. At the latest reading, that was still only about one visit in 75 [27]. The emergency figures cited are for Washington alone. California and Arizona are also among the Western states where the rise is concentrated [5].
Schaffner, a professor of preventive medicine and infectious diseases at Vanderbilt Health, said the place where flu took off first is not a warning sign in itself [23]. "There's nothing unusual about flu starting in the West Coast and moving east," he said [7]. In the last few years, he added, flu has tended to get going first in the Southeast or the Midwest, but "it can start almost anywhere" [8].
The H1N1 lead comes from one week of subtyping and may not hold through the season [20]. Influenza A strains usually drive the largest early outbreaks, Schaffner said, while B strains "kind of smolder along" and account for most cases late in the season [21]. Paul Offit, director of the Vaccine Education Center at Children's Hospital of Philadelphia, said flu is "a moving target." The virus keeps changing hemagglutinin, the surface protein that vaccines mainly target [12]. When circulating strains differ from the vaccine, "that usually predicts a more difficult season," Offit said. It is too early to know whether this year's shots match [13].
The thing this doesn't tell you is how severe the season will be. An early start does not guarantee a severe one [22]. The CDC's 2026-2027 outlook projects a moderate season, and the agency holds that forecast with only low-to-moderate confidence [14].
The data are more useful on timing. In a typical year, US flu activity builds in October or November and peaks between December and February [4]. Protection after a shot takes up to about two weeks to reach full strength [17]. A dose given on Oct. 1 would therefore be fully effective by about mid-October [28]. I think this evidence supports October campaigns, with one condition: it rests on a few weeks of lab and emergency department data, much of it from the West [6]. "October is the ideal time to get vaccinated, and even more so right now," Schaffner said [16].
What to watch
- Whether national clinical-lab positivity keeps climbing through October, and whether emergency department flu shares start rising in the Southeast and Midwest.
- Early CDC assessments of how closely this season's vaccine matches the circulating H1N1 viruses.
- Whether H1N1 keeps its share of subtyped influenza A samples as H3N2 and influenza B show up later in the season.
Clarity's read
What the record supports and how the coverage leans. The claims behind it follow.
Reality
- Evidence68
- Adoption
- Insufficient
- Hype gap+12
- Incentives
- Insufficient
- Confidence64
Claim ledger
Ranked by verification strength, evidence, and original report placement.
- [1]
During the week ending Sept. 26, 3.4% of respiratory samples tested by clinical labs were positive for flu, according to CDC data.
- [2]
About 0.5% of samples tested positive for flu during the week ending Sept. 27, 2025.
- [3]
The 3.4% rate is about seven times the rate seen at the same point last year.
- [4]
In a typical year, U.S. flu activity starts building in late fall, typically in October or November, and peaks between December and February.
- [5]
The early uptick in flu is mostly unfolding in Western states, including Washington, California and Arizona.
- [6]
In Washington, the share of emergency department visits diagnosed as flu climbed from 0.25% in the week ending Aug. 29 to 1.34% in the week ending Oct. 3, according to CDC emergency department data.
- [7]
"There's nothing unusual about flu starting in the West Coast and moving east," William Schaffner said.
- [8]
Schaffner noted that recent seasons have more often begun in the Southeast or Midwest, but "it can start almost anywhere."
- [9]
William Schaffner, a professor of preventive medicine and infectious diseases at Vanderbilt Health in Tennessee, said patients at clinics in his area are already testing positive for flu.
- [10]
Of the influenza A samples that public health labs subtyped during the week ending Sept. 26, 96% were identified as H1N1, according to CDC data.
- [11]
Last season a fast-spreading version of H3N2 known as subclade K dominated; the CDC classified the 2025-2026 season as highly severe for children, as at least 195 children died of flu-related causes.
- [12]
Paul Offit, director of the Vaccine Education Center at Children's Hospital of Philadelphia, said flu is "a moving target"; the virus constantly mutates, especially hemagglutinin, the surface protein that is the main target of flu vaccines.
- [13]
When circulating strains' features differ from those captured in the vaccine, "that usually predicts a more difficult season," Offit said; it is too early to know if this year's flu shots are a good match.
- [14]
In its 2026-2027 season outlook, the CDC projected the season will likely be moderate, with only low-to-moderate confidence in that forecast.
- [15]
The CDC generally doesn't recommend flu vaccination in July or August for most adults because the shot's protection can wane over the course of a season.
- [16]
"October is the ideal time to get vaccinated, and even more so right now," Schaffner said.
- [17]
Protection begins building right after a flu shot but takes up to about two weeks to reach full strength.
- [18]
Influenza is especially dangerous for children under 5, adults 65 and older, people with chronic health conditions and pregnant people, as these groups are more prone to severe infections.
- [19]
Flu activity is currently low but rising nationwide.
- [20]
H1N1's early lead does not mean it will continue to dominate throughout the season; it gives a snapshot of what is happening now.
- [21]
Schaffner said influenza A strains usually cause the biggest outbreaks early in the season, while influenza B strains "kind of smolder along" and account for most cases toward the end.
- [22]
An early start to the flu season doesn't guarantee it will be a severe one.
- [23]
Schaffner noted that where the flu took off first isn't a warning sign in itself.
- [24]
This year's late-September positivity is 6.8 times last year's.
- [25]
This year's late-September positivity is 2.9 percentage points higher than last year's.
- [26]
Washington's share of emergency department visits diagnosed as flu rose about 5.4-fold over six weeks.
- [27]
At 1.34%, about one emergency department visit in 75 in Washington was diagnosed as flu.
- [28]
A flu shot given on Oct. 1 would reach full strength by about mid-October.
Sources
1 independent publisher whose own reporting we read for this story.
- livescience.com'October is the ideal time to get vaccinated': Flu season is kicking off early, experts say
1 article · October 9, 2026
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