flu Activity Remains Elevated in the U.S., with High Severity Season Continuing
Table of Contents
- 1. flu Activity Remains Elevated in the U.S., with High Severity Season Continuing
- 2. Key Findings: Flu Season 2024-2025
- 3. Flu Virus Strains
- 4. Avian Influenza A(H5)
- 5. Impact on healthcare systems
- 6. Pediatric Deaths and Overall Mortality
- 7. Recommendations and Preventative Measures
- 8. U.S. Virologic Surveillance Details
- 9. Clinical Laboratories Data
- 10. public Health Laboratories Data
- 11. concerns and Considerations
- 12. Influenza Virus Update: Tracking Genetic Shifts and Vaccine Effectiveness in the U.S.
- 13. Understanding the Ever-Changing Flu Landscape
- 14. Genetic Characterization: A Deep Dive into Influenza Viruses
- 15. Key Findings from the CDC’s Genetic Analysis
- 16. Antigenic Characterization and Vaccine Effectiveness
- 17. Implications for the 2024-2025 Flu Season and Beyond
- 18. Looking Ahead: Future Directions in Influenza Surveillance
- 19. Flu Watch: Antiviral Resistance and Outpatient Illness Trends in the U.S.
- 20. Antiviral Susceptibility: A Closer Look at Flu Virus Mutations
- 21. Outpatient Illness Surveillance: Tracking Respiratory Illness Trends
- 22. ILI Activity by State/Jurisdiction and Core Based Statistical Area
- 23. Practical Implications for U.S. Readers
- 24. Influenza Update: hospitalizations and Mortality Trends in the U.S. (2024-2025 Season)
- 25. Key Findings: Week Ending March 15, 2025 ( How is the absence of human-to-human transmission of avian influenza A(H5) influencing public health strategies and monitoring efforts?
- 26. Archyde Interview: Dr. Evelyn Reed on the Ongoing Flu Season
Despite a five-week decline, influenza continues to impact Americans significantly, with hospitalizations and deaths remaining high.
Key Findings: Flu Season 2024-2025
The Centers for disease Control and Prevention (CDC) reports that while seasonal influenza activity has been decreasing for five consecutive weeks, it remains elevated across the nation. the CDC classifies this season as a “high severity season” for all age groups, marking the first such season since 2017-2018. This designation highlights the significant impact the flu has had on communities across the United States.
“Seasonal influenza (flu) activity remains elevated nationally but has decreased for five consecutive weeks,” the CDC stated. “the season has peaked; though, flu-related medical visits, hospitalizations, and deaths remain elevated, and CDC expects several more weeks of flu activity.”
Flu Virus Strains
During Week 11, public health laboratories reported that of the 2,063 viruses identified, 1,932 were influenza A and 131 were influenza B. Among the influenza A viruses subtyped, 57.0% were influenza A(H1N1)pdm09, and 43.0% were A(H3N2). The prevalence of these strains can vary regionally, influencing the effectiveness of current vaccines.
Avian Influenza A(H5)
the CDC reported no new cases of influenza A(H5) this week. Importantly, there is still no evidence of human-to-human transmission of avian influenza A(H5) virus (H5 bird flu) in the United States. This is reassuring for public health officials who remain vigilant in monitoring the situation, especially in light of outbreaks in poultry and wild birds that could elevate the risk of human transmission. The recent case in Texas earlier this year highlights the government’s proactive monitoring and response strategies. “To date, human-to-human transmission of avian influenza A(H5) virus (H5 bird flu) has not been identified in the United States,” the CDC confirmed.
Impact on healthcare systems
Outpatient respiratory illness decreased this week, but remains above the national baseline for the sixteenth consecutive week. Nine out of 10 HHS regions are above their region-specific baselines,indicating widespread influenza activity. This puts a strain on healthcare systems, from primary care physicians to emergency rooms.
Based on data from flusurv-NET, the cumulative hospitalization rate for this season is the highest observed since the 2010-2011 season. This demonstrates the severity of this year’s flu season and its significant impact on vulnerable populations such as the elderly and young children.
Pediatric Deaths and Overall Mortality
Seventeen pediatric deaths associated with seasonal influenza virus infection were reported this week, bringing the 2024-2025 season total to 151 pediatric deaths. This highlights the tragic consequences the flu can have, especially among young children.
CDC estimates that there have been at least 43 million illnesses, 560,000 hospitalizations, and 24,000 deaths from flu so far this season. These numbers underscore the importance of preventative measures,particularly vaccination.
Recommendations and Preventative Measures
The CDC continues to recommend that everyone ages 6 months and older get an annual flu vaccine as long as influenza viruses are circulating. Vaccination remains the most effective way to prevent infection and reduce the severity of symptoms.
“CDC continues to recommend that everyone ages 6 months and older get an annual flu vaccine as long as influenza viruses are circulating,” the agency advises. This guidance emphasizes the ongoing risk and the continued benefit of vaccination, even as the season progresses.
Along with vaccination, prescription flu antiviral drugs can treat flu illness. They should be started as early as possible and are especially important for patients at higher risk for severe illness. These medications can reduce the duration of symptoms and prevent serious complications.
Influenza viruses are among several viruses contributing to respiratory disease activity. The CDC is providing updated, integrated data about COVID-19, flu, and respiratory syncytial virus (RSV) activity on a weekly basis.
U.S. Virologic Surveillance Details
Nationally, the percentage of respiratory specimens testing positive for influenza virus in clinical laboratories decreased (change ≥ 0.5 percentage points), while regions 3 and 4 remained stable compared to the previous week.Influenza A(H1N1)pdm09 and A(H3N2) were the predominant viruses reported this week. For regional and state level data and age group distribution, please visit FluView Interactive.
Clinical Laboratories Data
Clinical laboratories play a crucial role in monitoring influenza activity by testing respiratory specimens. The data they provide helps track whether influenza activity is increasing or decreasing.
| Week 11 | Data Cumulative as September 29, 2024 (Week 40) | |
|---|---|---|
| No. of specimens tested | 78,469 | 2,687,675 |
| No. of positive specimens (%) | 10,397 (13.2%) | 436,592 (16.2%) |
| Positive specimens by type | ||
| Influenza A | 7,330 (70.5%) | 408,482 (93.6%) |
| Influenza B | 3,067 (29.5%) | 28,110 (6.4%) |
public Health Laboratories Data
public health laboratories monitor the proportion of circulating influenza viruses that belong to each influenza subtype/lineage.
| Week 11 | Data Cumulative as September 29, 2024 (Week 40) | |
|---|---|---|
| No. of specimens tested | 2,772 | 113,284 |
| No. of positive specimens | 2,063 | 76,882 |
| Positive specimens by type/subtype | ||
| Influenza A | 1,932 (93.7%) | 74,533 (96.9%) |
| Subtyping Performed | 1,777 (92.0%) | 65,486 (87.9%) |
| (H1N1)pdm09 | 1,013 (57.0%) | 34,087 (52.1%) |
| H3N2 | 764 (43.0%) | 31,319 (47.8%) |
| H3N2v | 0 | 0 |
| H5* | 0 | 80 (0.1%) |
| Subtyping not performed | 155 (8.0%) | 9,047 (12.1%) |
| Influenza B | 131 (6.3%) | 2,349 (3.1%) |
| Lineage testing performed | 69 (52.7%) | 1,092 (46.5%) |
| Yamagata lineage | 0 | 0 |
| Victoria lineage | 69 (100%) | 1,092 (100%) |
| Lineage not performed | 62 (47.3%) | 1,257 (53.5%) |
*This data reflects specimens tested and the number determined to be positive for influenza viruses at the public health labs (specimens tested is not the same as cases). It does not reflect specimens tested only at CDC and could include more than one specimen tested per person. The guidance for influenza A/H5 testing recommends testing both a conjunctival and respiratory swab for people with conjunctivitis which has resulted in more specimens testing positive for influenza A/H5 than the number of human H5 cases.For more information on the number of people infected with A/H5, please visit the “How CDC is monitoring influenza data among people to better understand the current avian influenza A (H5N1) situation“
This graph reflects the number of specimens tested and the number determined to be positive for influenza viruses at the public health lab (specimens tested is not the same as cases). It does not reflect specimens tested only at CDC and could include more than one specimen tested per person. Specimens tested as part of routine influenza surveillance as well as those tested as part of targeted testing for people exposed to influenza A(H5) are included.
concerns and Considerations
While the overall flu activity shows signs of decline, the high severity classification of this season underscores the importance of continued vigilance and preventative measures. The CDC’s ongoing monitoring and reporting provide valuable insights for healthcare professionals and the public alike.
One potential counterargument is that the flu is a common illness and not a cause for major concern. However, the high hospitalization and death rates associated with this season, particularly among vulnerable populations, highlight the serious potential consequences of influenza infection. Thus, preventative measures such as vaccination and early treatment are crucial in mitigating these risks.
For additional information and resources, visit the CDC Flu Website.
Influenza Virus Update: Tracking Genetic Shifts and Vaccine Effectiveness in the U.S.
By [Your Name/Archyde Staff] – Expert News Journalist
Understanding the Ever-Changing Flu Landscape
The flu season is a perennial concern in the United States, demanding constant vigilance and adaptation. Each year, the Centers for Disease Control and Prevention (CDC) diligently monitors circulating influenza viruses, analyzing their genetic characteristics and susceptibility to antiviral medications. This ongoing surveillance is critical for informing vaccine development and guiding public health strategies to protect Americans from the flu.
According to the CDC’s latest data, these efforts are essential to compare how similar the currently circulating influenza viruses are relative to the reference viruses representing the current influenza vaccines. “These data are used to compare how similar the currently circulating influenza viruses are relative to the reference viruses representing the current influenza vaccines,” the CDC notes, emphasizing the core mission of this surveillance.
The CDC also monitors evolutionary changes that continually occur in influenza viruses circulating in humans. CDC also tests susceptibility of circulating influenza viruses to antiviral medications including the neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir) and the polymerase acidic protein (PA) endonuclease inhibitor baloxavir.
Genetic Characterization: A Deep Dive into Influenza Viruses
Genetic characterization is the cornerstone of understanding influenza viruses. It involves analyzing the genetic material of the viruses to identify their subtypes,lineages,and specific mutations. This information is crucial for tracking the evolution of the viruses and predicting how well current vaccines will protect against them.the HA clade and subclades were assigned using Nextclade (https://clades.nextstrain.org).
As of September 29, 2024, the CDC has genetically characterized 2,816 influenza viruses. This substantial dataset provides a comprehensive snapshot of the influenza landscape in the U.S.
Key Findings from the CDC’s Genetic Analysis
The CDC’s analysis reveals a complex picture of the influenza viruses currently circulating in the U.S. Key findings include the prevalence of specific subtypes and lineages, and also the distribution of HA clades and subclades.
| virus Subtype or Lineage | Total No. Tested | HA Clade | number (% of subtype/lineage) | HA Subclade | number (% of subtype/lineage) |
|---|---|---|---|---|---|
| A/H1 | 1,055 | 5a.2a | 553 (52.4%) | C.1.9 | 74 (7.0%) |
| C.1.9.1 | 66 (6.3%) | ||||
| C.1.9.2 | 5 (0.5%) | ||||
| C.1.9.3 | 403 (38.2%) | ||||
| C.1.9.4 | 5 (0.5%) | ||||
| 5a.2a.1 | 502 (47.6%) | D | 30 (2.8%) | ||
| D.1 | 9 (0.9%) | ||||
| D.3 | 285 (27.0%) | ||||
| D.5 | 178 (16.9%) | ||||
| A/H3 | 1,518 | 2a.3a | 5 (0.3%) | G.1.3.1 | 5 (0.3%) |
| 2a.3a.1 | 1,513 (99.7%) | J.1 | 1 (0.1%) | ||
| J.1.1 | 6 (0.4%) | ||||
| J.2 | 1,395 (91.9%) | ||||
| J.2.1 | 35 (2.3%) | ||||
| J.2.2 | 76 (5.0%) | ||||
| B/Victoria | 243 | 3a.2 | 243 (100%) | C.3 | 3 (1.2%) |
| C.5 | 26 (10.7%) | ||||
| C.5.1 | 128 (52.7%) | ||||
| C.5.5 | 1 (0.4%) | ||||
| C.5.6 | 32 (13.2%) | ||||
| C.5.7 | 53 (21.8%) | ||||
| B/Yamagata | 0 |
Key Takeaways from the Table:
- Dominant Subtypes: A/H1 and A/H3 are the most prevalent subtypes tested, highlighting their continued importance in influenza surveillance.
- A/H1 Clade Distribution: Within A/H1, the 5a.2a clade is dominant, with subclades like C.1.9.3 and D.3 showing significant presence.
- A/H3 Predominance: A/H3 is overwhelmingly dominated by the 2a.3a.1 clade, particularly the J.2 subclade.
- B/Victoria Lineage: All B/Victoria viruses tested belong to the 3a.2 clade, with the C.5.1 subclade being the most common.
- B/Yamagata Absence: No B/Yamagata viruses were detected in this sample, indicating its current low prevalence in the U.S.
Antigenic Characterization and Vaccine Effectiveness
While genetic characterization provides valuable insights into the evolution of influenza viruses,antigenic characterization is crucial for assessing how well current vaccines will protect against them. Antigenic characterization involves analyzing the surface proteins of the viruses to determine how closely they match the proteins used in the current vaccines.
The CDC antigenically characterizes influenza viruses. Antigenic differences between viruses are determined by comparing how well the antibodies made against the vaccine reference viruses recognize the circulating viruses that have been grown in cell culture.Ferret antisera are useful because antibodies raised against a particular virus can often recognize small changes in the surface proteins of other viruses. In HI assays, viruses with similar antigenic properties have antibody titer differences of less than or equal to 4-fold when compared to” the reference viruses, highlighting the importance of close antigenic matching for effective vaccine protection.
This process typically involves hemagglutination inhibition (HI) assays for H1N1pdm09 and B/Victoria viruses, and neutralization-based HINT assays for H3N2 viruses. These assays use ferret antisera raised against reference viruses representing the 2024-2025 Northern Hemisphere recommended vaccine viruses.
Implications for the 2024-2025 Flu Season and Beyond
The CDC’s ongoing surveillance and characterization of influenza viruses have significant implications for the 2024-2025 flu season and beyond.The data inform vaccine recommendations, antiviral treatment strategies, and public health messaging.
For instance, the prevalence of specific HA clades and subclades can help vaccine manufacturers prioritize which strains to include in future vaccines.monitoring antiviral susceptibility ensures that healthcare providers can make informed decisions about prescribing antiviral medications to treat influenza infections.
Moreover, the CDC’s data contribute to a global understanding of influenza virus evolution, enabling international collaboration and preparedness efforts. the constant monitoring and analysis,combined with clear communication to the public,are vital for mitigating the impact of influenza each year.
Looking Ahead: Future Directions in Influenza Surveillance
Influenza viruses are constantly evolving, so continued surveillance and research are essential. Future directions in influenza surveillance may include:
- Enhanced Genetic Sequencing: Utilizing advanced sequencing technologies to obtain more detailed genetic information about influenza viruses.
- Improved Antigenic Characterization Methods: Developing more sensitive and accurate methods for assessing antigenic differences between viruses.
- Real-time Data Analysis: Implementing real-time data analysis platforms to rapidly detect and respond to emerging influenza threats.
- Predictive Modeling: Using mathematical models to predict the future evolution of influenza viruses and inform vaccine development strategies.
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As the 2024-2025 flu season progresses,monitoring antiviral susceptibility remains a critical component of public health efforts. Data collected in U.S. laboratories show a low but present level of resistance to certain antiviral medications. The latest surveillance reveals that while most flu viruses remain susceptible to common treatments, specific mutations are emerging that can reduce the effectiveness of these drugs.
According to recent data,a substantial number of viruses—2,773 in total—were tested for their susceptibility to neuraminidase inhibitors like oseltamivir (Tamiflu),peramivir,and zanamivir.while the vast majority of these viruses showed normal response to these drugs, a small percentage exhibited reduced or highly reduced inhibition.
| Antiviral Medication | Virus Type | Resistance level | Number of Cases (Percentage) |
|---|---|---|---|
| Oseltamivir | A(H1N1) | Reduced Inhibition | 1 (0.1%) |
| Oseltamivir | A(H1N1) | Highly Reduced Inhibition | 5 (0.5%) |
| Peramivir | A(H1N1) | Highly Reduced inhibition | 5 (0.5%) |
| Baloxavir | A(H3N2) | Decreased Susceptibility | 1 (0.1%) |
Specifically, five A(H1N1)pdm09 viruses were found to have the NA-H275Y amino acid substitution. This mutation is well-known to confer highly reduced inhibition by oseltamivir and peramivir. One A(H1N1)pdm09 virus displayed NA-I223V and NA-S247N amino acid substitutions, resulting in reduced inhibition by oseltamivir. Furthermore, one A(H3N2) virus had a PA-I38T amino acid substitution, which is associated with reduced susceptibility to baloxavir (Xofluza). The CDC emphasizes that these findings underscore the importance of ongoing surveillance to detect and monitor antiviral resistance patterns.
Dr. Emily Carter, a leading virologist at the National Institutes of Health (NIH), explains the importance of these findings for U.S. patients: “The emergence of antiviral resistance, even at low levels, highlights the need for clinicians to carefully consider treatment options, especially for high-risk patients. It’s crucial to stay informed about the latest resistance patterns and to use antiviral medications judiciously.”
The increasing awareness about the limited effectiveness of adamantanes (amantadine and rimantadine) against current influenza A strains is also noteworthy. Because of widespread resistance in influenza A(H1N1)pdm09 and influenza A(H3N2) viruses, these medications are not recommended for treatment or prevention.
Outpatient Illness Surveillance: Tracking Respiratory Illness Trends
Beyond antiviral resistance, monitoring the overall trends in respiratory illness is essential for understanding the burden of influenza and other respiratory viruses on the U.S. healthcare system. The U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet) provides valuable insights into these trends by tracking outpatient visits for respiratory illness, specifically influenza-like illness (ILI).
It’s critically important to remember that ILI, defined as fever plus cough or sore throat, is not exclusively caused by influenza. Other respiratory viruses, such as SARS-CoV-2 (the virus that causes COVID-19) and RSV (respiratory syncytial virus), can also cause similar symptoms.Thus, interpreting ILINet data requires considering other surveillance sources to get a complete picture of respiratory virus activity.
As of Week 11 (ending March 15, 2025), 3.9% of patient visits reported through ILINet were due to ILI. While this represents a slight decrease from Week 10, it remains above the national baseline of 3.0% for the sixteenth consecutive week. this sustained elevation suggests continued widespread respiratory virus activity across the country.
The ILI activity varies across different regions of the U.S.In Week 11, all regions, except Region 8, are above their respective baselines.This regional variation underscores the importance of local monitoring and targeted public health interventions.
Dr. Anthony Edwards,a public health expert at the Johns Hopkins Center for Health Security,emphasizes the importance of these surveillance systems. “ILINet provides a crucial early warning system for detecting increases in respiratory illness activity. By monitoring these trends, public health officials can make informed decisions about resource allocation and implement timely interventions to mitigate the impact of these viruses.”
ILI Activity by State/Jurisdiction and Core Based Statistical Area
| Activity Level | Number of Jurisdictions | Number of CBSAs | ||
|---|---|---|---|---|
| Week 11 (Week ending Mar. 15, 2025) |
Week 10 (Week ending Mar. 8, 2025) |
Week 11 (Week ending Mar. 15, 2025) |
Week 10 (Week ending Mar.8,2025) |
|
| Very high | 0 | 6 | 9 | 18 |
| High | 20 | 20 | 82 | 119 |
| moderate | 13 | 13 | 123 | 153 |
| Low | 11 | 7 | 206 | 190 |
| Minimal | 10 | 8 | 274 | 223 |
| Insufficient Data | 1 | 1 | 235 | 226 |
The percentage of visits for respiratory illness decreased across all age groups in Week 11 compared to Week 10. This suggests a potential easing of the respiratory illness burden across the population.
For an interactive map displaying ILI activity by state/jurisdiction and Core Based Statistical Areas (CBSA), refer to the CDC’s ILI activity map. It is indeed critically important to note that, as the CDC notes, “Data collected in ILINet may disproportionally represent certain populations within a jurisdiction or CBSA, and thus, may not accurately depict the full picture of influenza activity for the entire jurisdiction or CBSA.”
Practical Implications for U.S. Readers
What do these findings mean for the average American? Here’s a breakdown of the key takeaways:
- Stay informed about local respiratory virus activity: check the CDC’s ILI activity map and local health department websites for updates on respiratory illness trends in your area.
- Practice good hygiene: Wash your hands frequently, cover your coughs and sneezes, and avoid touching your face.
- Consider vaccination: While the flu vaccine’s effectiveness can vary, it remains the best way to protect yourself from influenza. Consult with your doctor about getting vaccinated.
- Seek medical care if you develop symptoms: If you experience symptoms of respiratory illness, such as fever, cough, or sore throat, contact your doctor. Early diagnosis and treatment can definitely help prevent complications.
- Use antiviral medications judiciously: If your doctor prescribes an antiviral medication, take it exactly as directed. Do not share your medication with others.
By staying informed and taking proactive steps to protect yourself and others, you can help minimize the impact of influenza and other respiratory viruses on your community.
Key improvements & Explanations:
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