Influenza activity is beginning to rise across the United States as the 2026–27 flu season gets underway, but national levels remain low, according to the latest surveillance report from the Centers for Disease Control and Prevention (CDC).
In its October 16, 2026, FluView report, covering the week ending October 10, the CDC said 5.0% of respiratory specimens tested by clinical laboratories were positive for influenza, up from 3.9% the previous week.
The increase comes as updated seasonal influenza vaccines are available nationwide. Early laboratory findings suggest that many circulating viruses remain antigenically similar to reference viruses used for the new vaccines. However, those findings do not establish how effectively the vaccines will prevent illness or hospitalization this season.
What the Latest CDC Flu Data Show
The CDC reports that multiple indicators of influenza activity have been increasing since late August or early September 2026. Although activity remains low nationally, the agency says early-season levels are higher than typically observed at this point in the year, while still falling within expected seasonal variation.
During the week ending October 10, clinical laboratories tested 45,331 respiratory specimens, of which 2,270 were positive for influenza. The resulting 5.0% positivity rate measures the proportion of submitted specimens testing positive, not the percentage of Americans infected.
Outpatient surveillance showed that 2.1% of patient visits involved influenza-like illness, below the national baseline of 2.9%. Because other respiratory infections can produce similar symptoms, this indicator does not represent laboratory-confirmed influenza alone.
Hospital surveillance recorded 4,024 laboratory-confirmed influenza-associated admissions nationwide during the reporting week. Separately, the CDC's FluSurv-NET surveillance network recorded a weekly hospitalization rate of 0.7 per 100,000 people across participating surveillance areas.
These figures come from different reporting systems and should not be treated as interchangeable measures.
Influenza activity has been particularly noticeable in western regions of the country. Nevertheless, no state-level jurisdiction was classified as having high or very high influenza-like illness activity in the latest CDC report.
Influenza A(H1N1)pdm09 accounted for 242 of the 251 influenza A viruses subtyped by public health laboratories during the reporting week, making it the predominant subtype in that surveillance sample.
What's in the 2026–27 Flu Vaccine?
The 2026–27 seasonal influenza vaccines were formulated to protect against three categories of influenza viruses: A(H1N1)pdm09, A(H3N2), and influenza B viruses belonging to the Victoria lineage.
These three-component formulations are known as trivalent vaccines.
On March 12, 2026, the Food and Drug Administration's Vaccines and Related Biological Products Advisory Committee reviewed global surveillance findings, circulating virus characteristics, immune-response information, and candidate vaccine strains.
According to the FDA's vaccine composition recommendations, the selected strains differ slightly depending on the production method.
For egg-based vaccines, the recommended components include A/Missouri/11/2025-like H1N1, A/Darwin/1454/2025-like H3N2, and B/Tokyo/EIS13-175/2025-like Victoria-lineage viruses.
Cell-based and recombinant formulations use the same recommended H1N1 reference strain but different representative H3N2 and B/Victoria strains.
These selections reflect efforts to anticipate which influenza viruses will circulate during the Northern Hemisphere season. They do not guarantee that circulating viruses will remain unchanged or that vaccination will prevent every infection.
Does a Good Vaccine Match Mean Better Protection?
Influenza viruses change continually, making vaccine matching an important part of seasonal vaccine development.
Scientists compare circulating influenza viruses with reference viruses representing vaccine components through genetic sequencing and laboratory tests that examine how antibodies recognize viral proteins.
In the CDC's October 16 surveillance report, 70 of 74 tested A(H1N1)pdm09 viruses were well recognized by antibodies generated against the corresponding cell-grown vaccine reference virus.
All 20 tested A(H3N2) viruses and all 30 tested B/Victoria viruses were also well recognized by antibodies against their respective reference viruses.
These findings are encouraging indicators of antigenic similarity, but they require careful interpretation. The tested viruses represent selected laboratory samples rather than every influenza virus circulating nationwide.
Additionally, the tests use antibodies produced in laboratory animals, not direct measurements of protection among vaccinated people.
A favorable antigenic match can support expectations about vaccine performance, but actual effectiveness depends on additional factors, including individual immune responses, age, prior immunity, and how circulating viruses evolve during the season.
Consequently, the laboratory findings cannot be translated directly into a percentage of people protected against influenza.
What Previous Flu Vaccine Studies Tell Us
While real-world effectiveness estimates for the 2026–27 U.S. influenza season are not yet established, previous studies provide useful context.
In a March 12, 2026, CDC report, researchers presented interim estimates of influenza vaccine effectiveness during the 2025–26 season using data from three U.S. vaccine effectiveness networks.
Among children and adolescents, estimated vaccine effectiveness against influenza-associated outpatient visits ranged from 38% to 41%, while effectiveness against influenza-associated hospitalization was estimated at 41%.
Among adults aged 18 and older, effectiveness against influenza-associated outpatient visits ranged from 22% to 34%, while effectiveness against hospitalization was estimated at 30%.
These figures describe estimated relative reductions in specified influenza-associated outcomes among vaccinated people compared with the study's unvaccinated comparison groups, after statistical adjustment.
They do not mean that only those percentages of vaccinated people developed immunity, nor do they indicate that everyone else remained unprotected.
The estimates also reflect the viruses, populations, study periods, and vaccine formulations associated with the 2025–26 season. They should not be presented as predictions for 2026–27.
What Current Vaccination Guidance Says
The CDC continues to recommend annual influenza vaccination for eligible people aged six months and older who have no contraindications.
Its September 1, 2026, interim clinical considerations state that recommendations from the July 2025 immunization schedule remain in effect for the 2026–27 influenza season because of legal uncertainties and related inquiries.
For most people, the CDC identifies September and October as the preferred period for seasonal influenza vaccination. Vaccination can continue later in the season while influenza viruses are circulating.
Influenza can cause complications requiring hospitalization, particularly among older adults, young children, pregnant people, and individuals with certain chronic medical conditions.
Vaccination is intended to reduce the risk of influenza and its complications, although the degree of protection varies from season to season.
Vaccination does not eliminate every possibility of infection, and medical decisions involving contraindications, specific vaccine products, or individual risk factors require appropriate clinical assessment.
The CDC also notes that prescription antiviral medicines are available to treat influenza. Early treatment is particularly important for people at higher risk of serious complications, with treatment decisions guided by healthcare professionals.
What Scientists Will Watch Next
Several developments will help determine how the 2026–27 influenza season unfolds.
First, subsequent CDC surveillance reports will show whether increases in test positivity, influenza-like illness, emergency department visits, and hospitalizations continue, stabilize, or decline.
Second, ongoing genetic and antigenic characterization will help scientists determine whether circulating viruses remain similar to the strains represented in this season's vaccines.
Third, vaccine effectiveness studies will provide direct evidence about protection against influenza-associated medical visits and hospitalization in the populations studied.
Those findings will be more informative than vaccine-strain matching alone when assessing how well the updated vaccines perform in everyday clinical settings.
For now, the available evidence supports a measured conclusion: influenza activity is rising in the United States, but remains low nationally, and preliminary laboratory findings cannot yet establish the real-world effectiveness of the 2026–27 vaccines.
Continued surveillance will be essential for understanding both the season's severity and the protection provided by vaccination.
0 Comments