US flu positivity was six times last year's level in the CDC's Oct. 9 report
US flu positivity reached 3.6% in the CDC's Oct. 9 FluView, six times the 0.6% recorded a year earlier. Scientific American reports it is too early to call the season severe, but the early H1N1 rise gives teams a reason to move vaccination dates up.
The Scientist · Science desk
Key figures: CDC Region 9, covering Arizona, California, Hawaii and Nevada, had 9.5% influenza positivity in the latest data. Nationwide positivity was 3.6%, against 0.6% at this time in 2025.
| Figure | Value | Note | Claim |
|---|---|---|---|
| CDC Region 9 flu positivity, latest data | 9.5% positive | 4 | |
| Nationwide, latest data | 3.6% positive | 2 | |
| Nationwide, this time in 2025 | 0.6% positive | 3 |
What happened
- According to the CDC report, flu activity has been climbing since early September.
- Region 9, which covers Arizona, California, Hawaii and Nevada, had 9.5% positivity.
- In the CDC's most recent weekly testing data, 98.4% of positive tests were H1N1 and 1.6% were H3N2.
- Johns Hopkins immunologist Andrew Pekosz said the circulating H1N1 differs somewhat from the vaccine strain and from earlier H1N1 strains, a change called antigenic drift.
Why it matters
- Who has to decide decision Vaccination dates are the one lever the evidence speaks to, and places already seeing spikes have the strongest case for moving them earlier than usual.
- What it limits constraint Severity cannot be forecast yet, so surge staffing can be timed to the early rise but not sized to expected harm.
- Who's exposed exposure With H1N1 dominant, the age groups hit hardest may skew younger than in H3N2 years, when older adults bore the most severe illness.
Percent positivity is the share of flu tests that come back positive, so it moves with how many people are tested and who they are. The comparison point is 0.6% at this time in 2025 [3], which puts the national rise from 3.6% [2] at 3.0 percentage points [15]. A sixfold ratio off a base that small is still a real signal. It rests on one comparison year.
Region 9 runs about 2.6 times the national rate [14]. "It's not unheard of to see things rising at this stage," said Andrew Pekosz, an immunologist at the Johns Hopkins Bloomberg School of Public Health. "But it's also something that we probably shouldn't be seeing until maybe November or December at the earliest." [5] The CDC's usual pattern is momentum in the fall and a peak between December and February [12].
H1N1 is dominant in the areas with the highest positivity [6]. The article does not say whether the CDC's weekly subtype split describes the nation or only those areas [7].
The thing the data don't tell you is severity. Scientific American reported that it is too early to predict whether these spikes signal a severe winter season [8]. Pekosz said the drift could be why the virus is emerging so early [16], and how it affects the current vaccine is not yet known [17].
Vaccine immunity is strongest for about four to six months, and flu usually peaks in December or January, Pekosz said. "So a September or October vaccination will get you through the worst part of the flu season." [10] Trish Perl, an infectious disease specialist at UT Southwestern Medical Center, said now is the best time for a flu shot [11].
We think the case for acting rests on timing. The evidence supports moving vaccination earlier where positivity is already high, and it does not support a severity forecast. That view holds only if the western rise spreads to other regions; one region at high positivity does not by itself make a national season.
What to watch
- Whether positivity in regions outside Region 9 climbs in coming FluView reports, which would separate a regional early start from a national one.
- Whether weekly CDC subtype data keep showing H1N1 at nearly all positive tests or H3N2 begins to appear.
- Early evidence on how well the current vaccine performs against the drifted H1N1, which Scientific American says is not yet known.
Clarity's read
What the record supports and how the coverage leans. The claims behind it follow.
Reality
- Evidence68
- Adoption
- Not scored
- Hype gap−10
- Incentives
- Not scored
- Confidence65
Claim ledger
Ranked by verification strength, evidence, and original report placement.
- [1]
According to the CDC's FluView report released October 9, flu activity has been climbing since early September.
- [2]
The CDC's latest data show a nationwide influenza percent positivity of 3.6 percent.
- [3]
The nationwide positivity rate was 0.6 percent at this time in 2025, so 3.6 percent is a sixfold increase.
- [4]
CDC Region 9, which includes Arizona, California, Hawaii and Nevada, had 9.5 percent influenza positivity in the latest data.
- [5]
"It's not unheard of to see things rising at this stage," said Andrew Pekosz, an immunologist at the Johns Hopkins Bloomberg School of Public Health. "But it's also something that we probably shouldn't be seeing until maybe November or December at the earliest."
- [6]
In the areas of the country with the highest positivity rates, H1N1, a subtype of influenza A, is dominant.
- [7]
According to the CDC, the most recent weekly testing data showed that 98.4 percent of positive tests indicated H1N1 and 1.6 percent indicated H3N2.
- [8]
It is too early to predict whether these spikes could signal a particularly severe winter flu season.
- [9]
H3N2-dominant seasons have historically been associated with particularly severe illness among older adults, while H1N1 has had a greater effect on younger populations; both can cause serious disease at any age.
- [10]
Pekosz said immunity from the flu vaccine is strongest for about four to six months and flu usually peaks in December or January, and said: "So a September or October vaccination will get you through the worst part of the flu season."
- [11]
Trish Perl, a professor of internal medicine and infectious disease specialist at UT Southwestern Medical Center, said right now is the best time to get a flu shot, and often tells people to get one at least before Thanksgiving.
- [12]
Flu season in the U.S. generally begins gaining momentum in the fall, with activity peaking between December and February.
- [13]
With certain areas already seeing spikes, Pekosz said getting the flu shot earlier than usual may be best.
- [14]
Region 9 positivity of 9.5 percent is about 2.6 times the national rate of 3.6 percent.
- [15]
The national positivity rate rose by 3.0 percentage points from 0.6 percent to 3.6 percent.
- [16]
Pekosz said the circulating H1N1 seems a little different from the vaccine strain and from H1N1 strains that have circulated before, which he called an antigenic drift, and that this could be why the virus is emerging so early.
- [17]
It is still too early to know how the antigenic drift might influence the effectiveness of the current flu vaccines.
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Sources
1 independent publisher whose own reporting we read for this story.
- scientificamerican.comEverything you need to know about flu season 2026
1 article · October 16, 2026
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