Tweet
SEPT. 26, 2025
WHAT TO KNOW
The Centers for Disease Control and Prevention (CDC) collects, compiles, and analyzes data on influenza viruses and associated morbidity and mortality in the United States. Influenza activity in the United States during the 2024–25 season (September 29, 2024 – August 30, 2025) was classified as high severity, marking the most severe influenza season since 2017–18. High severity was reflected across multiple indicators, including influenza-associated outpatient visits, hospitalizations, and deaths, underscoring the significant public health impact of the 2024–25 season. Influenza activity began increasing in mid-November and peaked in early February 2025. Influenza activity then steadily declined to interseasonal levels by May 2025 and has remained low through August 2025.
During the 2024–25 season, influenza A viruses were predominant, with A(H1N1)pdm09 and A(H3N2) viruses detected at approximately equal levels. Influenza B activity remained low throughout the season but increased slightly later in the season. No influenza B/Yamagata lineage viruses were detected for the fifth consecutive season.
This report summarizes influenza activity in the United States during the 2024–25 season as reported to CDC through its collaborative efforts with clinical and public health laboratories, outpatient providers, emergency departments, hospitals, vital statistics offices, and state and local health departments. It also includes the recommended composition of the Northern Hemisphere 2025–26 influenza vaccines and a brief update on influenza activity occurring between April and September of 2025 in the Southern Hemisphere.
Virus Surveillance
-snip-
Seasonal Severity Assessment
CDC classifies influenza season severity at the end of each season by comparing influenza virus activity data, including influenza-like illness, influenza hospitalization rates, and influenza deaths, from past seasons11. Severity is classified across all age groups and for children, adults, and older adults. The 2024–25 U.S. season was classified as a high severity season across all age groups combined and for children, adults, and older adults. This was the first high severity U.S. season since the 2017-18 season.
Discussion
The 2024–25 influenza season in the United States was a high severity season marked by elevated activity across multiple surveillance systems. National preliminary burden estimates suggest that influenza virus infections resulted in at least 43 million symptomatic illnesses, 19 million medical visits, 560,000 hospitalizations, and 38,000 deaths. These figures surpassed all recent influenza seasons, except for the number of deaths during the high severity 2017–18 season17.
Influenza A viruses predominated throughout the season, with nearly equal co-circulation of A(H1N1)pdm09 and A(H3N2) viruses overall, 53.1% and 46.9% of subtyped influenza A viruses, respectively. Until mid-January, A(H3N2) viruses were reported more frequently than A(H1N1)pdm09 viruses each week while the reverse was true from that point on. Influenza B viruses increased slightly later in the season but remained at lower overall levels compared to influenza A. Historically, A(H1N1) viruses have been the predominant virus in 8 of the past 26 seasons (1997-98 through 2023-24, excluding the 2020-21 season due to low influenza virus circulation, A(H3N2) in 16 seasons, and equal proportions of A(H1N1) and B Victoria in one season.
Genetic and antigenic characterization of circulating viruses showed that most A(H1N1)pdm09 and B/Victoria lineage viruses were similar to the 2024–25 Northern Hemisphere vaccine reference viruses. However, some A(H3N2) viruses showed antigenic drift due to hemagglutinin (HA) substitutions, potentially contributing to somewhat lower vaccine effectiveness (VE) against this subtype. Overall, VE was moderate to low, with higher protection against A(H1N1)pdm09 and B viruses18.
This season was marked by high severity; with levels of activity surpassing those observed in both recent and most pre-COVID-19 pandemic seasons. Clinical laboratories tested over 3.9 million respiratory specimens, with 12.3% testing positive for influenza viruses overall and peak weekly test positivity reaching 31.6%, the highest weekly percentage reported since the 2015–16 season. Influenza A viruses predominated, with the percent of specimens testing positive for influenza A peaking at 30.4% in late January, while influenza B viruses co-circulated at lower levels and later in the season with peak influenza B percent positivity (4.3%) occurring in late March. ILI activity peaked at 7.9% (4.9 percentage points above the baseline) and remained above baseline for 17 consecutive weeks. The number of weeks of elevated activity is similar to previous seasons while the peak percent of visits for ILI and the difference between the peak and baseline is the highest and third highest, respectively, since the 2009-2010 season.
Additionally, the timing of activity peaks for ILINet across the HHS regions ranged from late December through early February while peak percent of specimens testing positive for influenza occurred during the two weeks in late January and early February. This difference may be due to the circulation of other respiratory viruses causing ILI occurring at slightly different times across the country. The overall cumulative influenza-associated hospitalization rate reached recorded in FluSurv-Net reached the highest level since the 2010–11 season, and the weekly peak rate reported to NHSN HRD was the highest since reporting began in the 2020-21 season.
Mortality surveillance further illustrated the season's severity. The weekly percentage of all deaths attributed to influenza peaked at 2.8%, which is the highest peak since the 2015–16 season and substantially higher than the previous two seasons (1.4% in 2023–24 and 1.6% in 2022–23). Pediatric mortality was particularly notable, with 279 influenza-associated pediatric deaths reported. This is the highest number of deaths reported during a seasonal influenza epidemic since the system began in 2004. While over half of pediatric deaths occurred in
children and adolescents with underlying conditions, 44% occurred among previously healthy children. Among the deaths in children aged ≥6 months with known vaccination status approximately 90% were not fully vaccinated against influenza.
Globally, influenza activity during the Southern Hemisphere season showed regional variability in virus subtype predominance and timing. Early and intense influenza activity in some countries further underscores the unpredictable nature of seasonal influenza; however, activity in the Southern hemisphere cannot be used to definitively predict the timing or intensity of influenza activity in the United States this fall and winter.
Though influenza activity has remained low in the summer, maintaining influenza surveillance year-round is important. Sporadic human infections with novel influenza A virus subtypes distinct from currently circulating seasonal strains were reported during the 2024–25 season. These included 56 confirmed avian influenza A(H5) virus infections, most linked to exposure to infected dairy cattle or poultry, and one swine origin variant influenza A(H1N2v) virus infection. While the majority of cases were mild and no human-to-human transmission was identified, these detections underscore the continued threat posed by zoonotic influenza viruses. As outbreaks of highly pathogenic avian influenza A(H5N1) persist among animals, including poultry and dairy cows, healthcare providers and individuals exposed to potentially infected livestock and other animals should remain alert to symptoms such as shortness of breath, fever, cough, or conjunctivitis19. Prompt identification of suspected novel influenza A virus infections, patient isolation, specimen collection for public health testing, and immediate initiation of antiviral treatment remain critical to preventing further spread and mitigating pandemic risk2021.
The 2024–25 influenza season was among the most severe in recent years, characterized by widespread co-circulation of multiple influenza A virus subtypes and substantial morbidity and mortality. These findings reinforce the critical importance of annual influenza vaccination, timely antiviral treatment, and robust surveillance systems to mitigate the public health impact of influenza, particularly for high-risk populations. CDC continues to recommend that everyone aged 6 months and older receive a seasonal influenza vaccine each year, as annual vaccination remains the most effective way to protect against influenza and its potentially serious complications2223. CDC also recommends initiating influenza antiviral drug treatment as soon as possible for patients with confirmed or suspected influenza virus infection who have severe, complicated, or progressive illness; who require hospitalization; or who are at increased risk for influenza-associated complications24.
Four influenza antiviral drugs approved by the Food and Drug Administration are currently recommended for use in the United States25.
WHAT TO KNOW
- This report summarizes influenza activity in the United States during the 2024–25 season.
- It also includes the recommended composition of the Northern Hemisphere 2025–26 influenza vaccines and a brief update on influenza activity occurring between April and September of 2025 in the Southern Hemisphere.
The Centers for Disease Control and Prevention (CDC) collects, compiles, and analyzes data on influenza viruses and associated morbidity and mortality in the United States. Influenza activity in the United States during the 2024–25 season (September 29, 2024 – August 30, 2025) was classified as high severity, marking the most severe influenza season since 2017–18. High severity was reflected across multiple indicators, including influenza-associated outpatient visits, hospitalizations, and deaths, underscoring the significant public health impact of the 2024–25 season. Influenza activity began increasing in mid-November and peaked in early February 2025. Influenza activity then steadily declined to interseasonal levels by May 2025 and has remained low through August 2025.
During the 2024–25 season, influenza A viruses were predominant, with A(H1N1)pdm09 and A(H3N2) viruses detected at approximately equal levels. Influenza B activity remained low throughout the season but increased slightly later in the season. No influenza B/Yamagata lineage viruses were detected for the fifth consecutive season.
This report summarizes influenza activity in the United States during the 2024–25 season as reported to CDC through its collaborative efforts with clinical and public health laboratories, outpatient providers, emergency departments, hospitals, vital statistics offices, and state and local health departments. It also includes the recommended composition of the Northern Hemisphere 2025–26 influenza vaccines and a brief update on influenza activity occurring between April and September of 2025 in the Southern Hemisphere.
Virus Surveillance
-snip-
Seasonal Severity Assessment
CDC classifies influenza season severity at the end of each season by comparing influenza virus activity data, including influenza-like illness, influenza hospitalization rates, and influenza deaths, from past seasons11. Severity is classified across all age groups and for children, adults, and older adults. The 2024–25 U.S. season was classified as a high severity season across all age groups combined and for children, adults, and older adults. This was the first high severity U.S. season since the 2017-18 season.
Discussion
The 2024–25 influenza season in the United States was a high severity season marked by elevated activity across multiple surveillance systems. National preliminary burden estimates suggest that influenza virus infections resulted in at least 43 million symptomatic illnesses, 19 million medical visits, 560,000 hospitalizations, and 38,000 deaths. These figures surpassed all recent influenza seasons, except for the number of deaths during the high severity 2017–18 season17.
Influenza A viruses predominated throughout the season, with nearly equal co-circulation of A(H1N1)pdm09 and A(H3N2) viruses overall, 53.1% and 46.9% of subtyped influenza A viruses, respectively. Until mid-January, A(H3N2) viruses were reported more frequently than A(H1N1)pdm09 viruses each week while the reverse was true from that point on. Influenza B viruses increased slightly later in the season but remained at lower overall levels compared to influenza A. Historically, A(H1N1) viruses have been the predominant virus in 8 of the past 26 seasons (1997-98 through 2023-24, excluding the 2020-21 season due to low influenza virus circulation, A(H3N2) in 16 seasons, and equal proportions of A(H1N1) and B Victoria in one season.
Genetic and antigenic characterization of circulating viruses showed that most A(H1N1)pdm09 and B/Victoria lineage viruses were similar to the 2024–25 Northern Hemisphere vaccine reference viruses. However, some A(H3N2) viruses showed antigenic drift due to hemagglutinin (HA) substitutions, potentially contributing to somewhat lower vaccine effectiveness (VE) against this subtype. Overall, VE was moderate to low, with higher protection against A(H1N1)pdm09 and B viruses18.
This season was marked by high severity; with levels of activity surpassing those observed in both recent and most pre-COVID-19 pandemic seasons. Clinical laboratories tested over 3.9 million respiratory specimens, with 12.3% testing positive for influenza viruses overall and peak weekly test positivity reaching 31.6%, the highest weekly percentage reported since the 2015–16 season. Influenza A viruses predominated, with the percent of specimens testing positive for influenza A peaking at 30.4% in late January, while influenza B viruses co-circulated at lower levels and later in the season with peak influenza B percent positivity (4.3%) occurring in late March. ILI activity peaked at 7.9% (4.9 percentage points above the baseline) and remained above baseline for 17 consecutive weeks. The number of weeks of elevated activity is similar to previous seasons while the peak percent of visits for ILI and the difference between the peak and baseline is the highest and third highest, respectively, since the 2009-2010 season.
Additionally, the timing of activity peaks for ILINet across the HHS regions ranged from late December through early February while peak percent of specimens testing positive for influenza occurred during the two weeks in late January and early February. This difference may be due to the circulation of other respiratory viruses causing ILI occurring at slightly different times across the country. The overall cumulative influenza-associated hospitalization rate reached recorded in FluSurv-Net reached the highest level since the 2010–11 season, and the weekly peak rate reported to NHSN HRD was the highest since reporting began in the 2020-21 season.
Mortality surveillance further illustrated the season's severity. The weekly percentage of all deaths attributed to influenza peaked at 2.8%, which is the highest peak since the 2015–16 season and substantially higher than the previous two seasons (1.4% in 2023–24 and 1.6% in 2022–23). Pediatric mortality was particularly notable, with 279 influenza-associated pediatric deaths reported. This is the highest number of deaths reported during a seasonal influenza epidemic since the system began in 2004. While over half of pediatric deaths occurred in
children and adolescents with underlying conditions, 44% occurred among previously healthy children. Among the deaths in children aged ≥6 months with known vaccination status approximately 90% were not fully vaccinated against influenza.
Globally, influenza activity during the Southern Hemisphere season showed regional variability in virus subtype predominance and timing. Early and intense influenza activity in some countries further underscores the unpredictable nature of seasonal influenza; however, activity in the Southern hemisphere cannot be used to definitively predict the timing or intensity of influenza activity in the United States this fall and winter.
Though influenza activity has remained low in the summer, maintaining influenza surveillance year-round is important. Sporadic human infections with novel influenza A virus subtypes distinct from currently circulating seasonal strains were reported during the 2024–25 season. These included 56 confirmed avian influenza A(H5) virus infections, most linked to exposure to infected dairy cattle or poultry, and one swine origin variant influenza A(H1N2v) virus infection. While the majority of cases were mild and no human-to-human transmission was identified, these detections underscore the continued threat posed by zoonotic influenza viruses. As outbreaks of highly pathogenic avian influenza A(H5N1) persist among animals, including poultry and dairy cows, healthcare providers and individuals exposed to potentially infected livestock and other animals should remain alert to symptoms such as shortness of breath, fever, cough, or conjunctivitis19. Prompt identification of suspected novel influenza A virus infections, patient isolation, specimen collection for public health testing, and immediate initiation of antiviral treatment remain critical to preventing further spread and mitigating pandemic risk2021.
The 2024–25 influenza season was among the most severe in recent years, characterized by widespread co-circulation of multiple influenza A virus subtypes and substantial morbidity and mortality. These findings reinforce the critical importance of annual influenza vaccination, timely antiviral treatment, and robust surveillance systems to mitigate the public health impact of influenza, particularly for high-risk populations. CDC continues to recommend that everyone aged 6 months and older receive a seasonal influenza vaccine each year, as annual vaccination remains the most effective way to protect against influenza and its potentially serious complications2223. CDC also recommends initiating influenza antiviral drug treatment as soon as possible for patients with confirmed or suspected influenza virus infection who have severe, complicated, or progressive illness; who require hospitalization; or who are at increased risk for influenza-associated complications24.
Four influenza antiviral drugs approved by the Food and Drug Administration are currently recommended for use in the United States25.