As the United States enters the new year, seasonal influenza activity remains elevated and continues to increase nationwide, according to the latest influenza surveillance update from the Centers for Disease Control and Prevention (CDC). For Week 51 of the 2025–2026 season (ending December 20, 2025), multiple surveillance indicators showed sustained upward trends across outpatient, emergency department, and hospitalization data, while severity metrics remain relatively low to date. Influenza A(H3N2) viruses continue to predominate. Nationally, clinical laboratories reported that 25.6% of respiratory specimens tested positive for influenza, a sharp increase from the prior week. Percent positivity rose in all ten US Department of Health and Human Services (HHS) regions, ranging from 34.9% in Region 8 to 10.8% in Region 9. Influenza A viruses accounted for 94.7% of all positive clinical specimens reported during the week.
Outpatient respiratory illness increased in parallel. Data from the US Outpatient Influenza-like Illness Surveillance Network (ILINet) showed that 6.0% of outpatient visits were for influenza-like illness (ILI), exceeding the national baseline of 3.1% for the third consecutive week. ILI activity increased across all age groups and all HHS regions, reflecting widespread circulation of respiratory viruses, including influenza, SARS-CoV-2, and respiratory syncytial virus (RSV).
Hospitalizations continued to rise across multiple surveillance systems. The Influenza Hospitalization Surveillance Network (FluSurv-NET) reported a weekly influenza-associated hospitalization rate of 6.2 per 100,000 population, up from 4.4 per 100,000 the previous week. The cumulative hospitalization rate reached 18.2 per 100,000, representing the third highest cumulative rate at this point in the season since 2010–2011, following the 2022–2023 and 2023–2024 seasons.
Among hospitalized patients with influenza A subtype data available, 87.4% were infected with A(H3N2) and 12.6% with A(H1N1)pdm09. Hospitalization burden remained highest among adults aged 65 years and older (53.4 per 100,000), followed by children aged 0–4 years (21.5 per 100,000). Age-adjusted hospitalization rates were highest among non-Hispanic Black persons (35.9 per 100,000), followed by American Indian or Alaska Native persons (20.9), Hispanic persons (16.5), non-Hispanic White persons (14.4), and Asian and/or Pacific Islander persons (8.5).
Additional hospital data from the National Healthcare Safety Network (NHSN) showed 19,053 laboratory-confirmed influenza-associated hospital admissions nationally during Week 51, with admission rates increasing in all HHS regions. Long-term care facility surveillance indicated a hospitalization rate of 22.9 per 100,000 residents, with increasing trends observed in most regions.
Mortality indicators increased modestly. Preliminary data from the National Center for Health Statistics showed that .5% of deaths during Week 51 were attributed to influenza, up from the prior week. CDC also reported five influenza-associated pediatric deaths during the week, bringing the season total to eight pediatric deaths. All reported pediatric deaths were associated with influenza A viruses, predominantly A(H3N2).
Virologic surveillance from public health laboratories showed that 97.3% of reported viruses were influenza A, with 91.8% of subtyped viruses identified as A(H3N2). Genetic characterization indicated that 89.5% of sequenced A(H3N2) viruses belonged to subclade K, similar to prior weeks. CDC estimates that, to date this season, influenza has caused at least 7.5 million illnesses, 81,000 hospitalizations, and 3,100 deaths nationwide.
Antigenic and antiviral susceptibility testing showed no evidence of reduced susceptibility to neuraminidase inhibitors (oseltamivir, zanamivir, peramivir) or baloxavir among circulating viruses. High levels of resistance to adamantanes persist among influenza A viruses, and these agents remain not recommended for treatment or prevention. CDC reported no new human infections with avian influenza A(H5) during Week 51, and to date, no person-to-person transmission has been identified in the United States.
What You Need to Know
Week 51 showed sharp week-over-week increases across labs, outpatient visits, ED encounters, and hospitalizations, with influenza A—predominantly A(H3N2)—driving widespread transmission in all HHS regions.
Weekly and cumulative hospitalization rates increased substantially and are already among the highest seen at this point in a season since 2010–2011, with the greatest burden in adults ≥65 years, children 0–4 years, and non-Hispanic Black and American Indian/Alaska Native populations.
Mortality indicators are increasing modestly, antiviral susceptibility remains intact, and no H5 transmission has been detected; however, rapid spread amid suboptimal vaccination uptake suggests that boosting vaccination and early antiviral treatment are critical to limiting further hospitalization growth this season.
Week-Over-Week Acceleration From Week 50
Compared with Week 50 (ending December 13, 2025), influenza activity increased sharply across nearly all indicators. Clinical laboratory positivity rose from 14.8% to 25.6%, while the number of positive tests more than doubled (10,456 to 23,613), alongside increased testing volume. ILINet ILI visits increased from 4.1% to 6%, and the percentage of emergency department visits with an influenza discharge diagnosis increased from 2.8% to 5.4%. The number of jurisdictions reporting high or very high activity increased from 17 to 32.
Hospitalization measures showed similar week-over-week increases. FluSurv-NET’s weekly hospitalization rate rose from 3.5 to 6.2 per 100,000, while cumulative hospitalizations increased from 11 to 18.2 per 100,000. NHSN hospital admissions nearly doubled from 9,944 in Week 50 to 19,053 in Week 51, and long-term care hospitalization rates increased from 14.3 to 22.9 per 100,000 residents.
Context From Prior Coverage on Vaccination and H3N2
Based on our prior coverage, the rapid escalation observed in Weeks 50–51 is occurring in the context of a recent high-severity 2024–2025 influenza season, during which the cumulative hospitalization rate reached 127.1 per 100,000, the highest observed since 2010–2011. Despite that burden, only about one-third of hospitalized patients were vaccinated, underscoring persistent gaps in prevention that may be contributing to early-season transmission and hospitalization patterns this year.
That reporting also showed that A(H3N2) circulation is not new, and that vaccine effectiveness against H3N2 during the prior season was moderate but clinically meaningful, with hospitalization vaccine effectiveness around 55%. The continued predominance of A(H3N2) in the current season suggests that rising hospitalizations are being driven primarily by increasing incidence and suboptimal uptake, rather than major antigenic mismatch or increased virulence.
CDC continues to recommend annual influenza vaccination for everyone aged 6 months and older who has not yet been vaccinated this season. Approximately 130 million influenza vaccine doses have been distributed nationally. Antiviral treatment is advised as early as possible for patients with suspected or confirmed influenza who are hospitalized, have severe disease, or are at higher risk for complications.
References
1.Weekly US Influenza Surveillance Report: Key Updates for Week 51, ending December 20, 2025. CDC. December 30, 2025. Accessed December 31, 2025. https://www.cdc.gov/fluview/surveillance/2025-week-51.html
2.Weekly US Influenza Surveillance Report: Key Updates for Week 50, ending December 13, 2025. CDC. December 13, 2025. Accessed December 31, 2025. https://www.cdc.gov/fluview/surveillance/2025-week-50.html
3.O’Halloran A, Habeck JW, Gilmer M, et al. Influenza-Associated Hospitalizations During a High Severity Season — Influenza Hospitalization Surveillance Network, United States, 2024–25 Influenza Season. MMWR Morb Mortal Wkly Rep 2025;74:529–537. DOI: http://dx.doi.org/10.15585/mmwr.mm7434a1
