United States flu season

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The United States flu season refers to the annual period of increased circulation of influenza viruses across the country, typically occurring during the fall and winter months. While influenza circulates year-round, activity in the United States most often rises in October, peaks between December and February, and can extend into May. [1] Centers for Disease Control and Prevention (CDC) estimates that flu on average has resulted in 9.3 million to 41 million illnesses, between 120,000 to 710,000 hospitalizations and 6,300 to 52,000 deaths annually between 2010 and 2024. [2]

Contents

Seasonal Timing in the United States

Seasonal influenza patterns in the United States broadly mirror trends observed across temperate regions of the Northern Hemisphere, where cooler temperatures and lower humidity support more efficient viral transmission. Independent epidemiological analyses, including those published in peer-reviewed journals and global health surveillance summaries, note that U.S. flu activity typically accelerates in early fall as indoor crowding increases and peaks during mid-winter when environmental conditions most favor viral stability and spread. These studies also highlight that year-to-year variation is driven by factors such as dominant viral subtypes, population immunity, and the degree of antigenic drift observed in circulating strains. [3]

Environmental and Epidemiological Drivers of Seasonality

Patterns of influenza spread in the United States are also influenced by demographic and geographic factors. Urban regions with dense populations often experience earlier and more intense waves of transmission, while rural areas may see slower but more longer activity. Travel patterns-both domestic and international-play a measurable role in seeding new outbreaks, as demonstrated in multiple phylogenetic studies that trace how influenza lineages move between states and across borders. These dynamics contribute to the patchwork nature of flu activity each year, with some states peaking weeks before others despite being part of the same national season. [4]

Viral Evolution and Year-to-Year Variability

The composition of circulating influenza strains in the United States shifts from year to year, with influenza A subtypes being most commonly H1N1 and H3N2 that are typically responsible for the majority of infections. Influenza B lineages, primarily Victoria and Yamagata, also contribute to seasonal activity, though often with different age-group impacts. Scientific literature notes that H3N2-dominant seasons tend to be associated with higher hospitalization rates and more severe outcomes, partly due to the virus's rapid antigenic evolution and reduced vaccine effectiveness against this subtype. These strain-specific patterns play a central role in shaping the overall severity and demographic distribution of each flu season. [5]

Notable occurrences

The 2014-2015 flu season in the United States was considered moderately severe, dominated by the H3N2 influenza A strain, leading to high hospitalization rates, particularly for older adults. Activity peaked in late December and lasting longer than average. As of May 23, 2015, the season saw about 30 million illnesses, 591,000 hospitalizations, and 51,000 deaths. [6]

The 2017–2018 flu season was particularly severe, and at the time, it was the worst flu season since the 2009–2010 flu season, which had occurred after the start of the 2009 swine flu pandemic. According to the director of the Centers for Disease Control and Prevention's influenza branch in January 2018, it was the first flu season where "we've had the entire continental US" show the same "widespread" flu activity, excluding the District of Columbia and Hawaii. [7] Twenty-six of those states were classified by the CDC as having "high" activity in January 2018. [8] This flu season was dominated by the H3N2 subtype, which is known for being the most severe of the 4 main seasonal flu subtypes. [9] [10] The Atlantic noted that the 2017 vaccine for the flu was only ten percent effective against H3N2 in Australia, indicating a severe mismatch for the vaccine component for the dominant flu strain that year. Similarly low effectiveness rates were reported against H3N2 across North America. [11] [12]

The 2018-2019 United States flu season was notable for being the longest flu season in a decade, lasting 21 weeks and characterized by moderate overall severity. [13] Influenza activity began rising in November 2018, peaked in mid-February 2019, and returned to below-baseline levels by mid-April 2019. According to final estimates, the 2018-2019 season resulted in 29 million illnesses, 13 million medical visits, 380,000 hospitalizations, and 28,000 deaths. [14]

The Centers for Disease Control and Prevention (CDC) estimates that, as of April 4, 2020, the 2019–2020 United States flu season had caused 39 million to 56 million flu illnesses, 410,000 to 740,000 hospitalizations and 24,000 to 62,000 deaths. [15] In January 2020, the Director of the National Institute of Allergies and Infectious Diseases, Dr. Anthony Fauci expected the 2019–2020 flu season to be one of the worst in several years, at least as severe as the 2017–2018 season. [16] By the third week in February the seasonal flu was near its peak with over 26 million people sickened, 250,000 hospitalized, and 14,000 who died. [17] Experts said that the flu came in two waves, with a hard impact on children. The season began in October, earlier than usual, with the expected wave of influenza B virus. The number of children who died, 105, was higher in late February than any season for the past ten years with about 67% associated with influenza B viruses. The second wave came with the influx of influenza A viruses, such as H1N1. [18]

From March 2020 through Fall 2021, there was no flu season for over a year, due to COVID restrictions. Due to the COVID measures, the Influenza B Yamagata lineage, one of the 4 main seasonal flu subtypes, ended up going extinct, with no detections since March 2020. [19] [20]

The 2022-2023 flu season was notable for its early onset, high pediatric impact, and its overlap with widespread circulation of RSV and COVID-19, a cluster of viruses circulating together called the "tripledemic." Seasonal influenza activity began earlier than usual, with cases starting to rise in October 2022, and reached high nationwide levels by December 2022. [21] During the 2022-2023 flu season, there were an estimated 31 million flu-related illness, 14 million flu-related medical visits, 360,000 flu-related hospitalizations, and 21,000 flu-related deaths. [22] The 2022-2023 flu season was overall considered moderate in severity for the general population but severe for children. [23]

The 2023-2024 flu season began in late 2023, with activity rising steadily through the fall. Influenza activity reached its main peak in December 2023, followed by a smaller secondary rise in February 2024, giving the season two distinct peaks. Activity declined afterward, and the season ended in early 2024 as influenza circulation returned to low levels. [24]

The 2024-2025 flu season was considered to be the worst in over a decade, with the highest hospitalization rates recorded in the US since the 2009 swine flu pandemic, and infection levels far outstripping any flu season since the COVID lockdowns in early 2020. [25] In the US, the H1N1 and H3N2 subtypes co-dominated that flu season. [26]

Based on the most recent data (as of mid-January 2026), the 2025-2026 influenza season is characterized by a rapid, high-intensity surge driven by a new, mutated variant of H3N2. [27] Classified as a "moderately severe" to severe season in the Northern Hemisphere, with rapid surges occurring around the holidays in late December 2025. [28] As of January 30, 2026, the CDC estimates that there have been at least 20 million illnesses, 270,000 hospitalizations, and 11,000 deaths from the flu in the United States during the 2025-2026 season. [29]

See also

References

  1. ↑ Centers for Disease Control and Prevention. "The Flu Season." https://www.cdc.gov/flu/about/season.html
  2. ↑ https://www.cdc.gov/flu-burden/php/about/index.html
  3. ↑ Skowronski, D. M., et al. “A Perfect Storm: Impact of Genomic Drift and Immunity on Influenza Vaccine Effectiveness.” Journal of Virology. https://journals.asm.org/doi/full/10.1128/jvi.03544-13
  4. ↑ Princeton Environmental Institute. “Urban Population, Transportation Patterns Affect How Flu Epidemics Play Out.” https://environment.princeton.edu/news/urban-population-transportation-patterns-affect-how-flu-epidemics-play-out/
  5. ↑ Petrova, V. N., & Russell, C. A. “The Evolution of Seasonal Influenza Viruses.” https://pmc.ncbi.nlm.nih.gov/articles/PMC6149781/
  6. ↑ https://pubmed.ncbi.nlm.nih.gov/26042650/
  7. ↑ https://www.theatlantic.com/health/archive/2018/01/the-perfect-storm-behind-this-years-nasty-flu-season/550469/
  8. ↑ https://www.cnn.com/2018/01/09/health/california-flu-season-deadly-bn/index.html
  9. ↑ https://www.theatlantic.com/health/archive/2018/01/the-perfect-storm-behind-this-years-nasty-flu-season/550469/
  10. ↑ https://www.cnn.com/2018/01/09/health/california-flu-season-deadly-bn/index.html
  11. ↑ https://www.theatlantic.com/health/archive/2018/01/the-perfect-storm-behind-this-years-nasty-flu-season/550469/
  12. ↑ https://www.cdc.gov/flu-vaccines-work/php/effectiveness-studies/index.html
  13. ↑ https://time.com/5610878/2018-2019-flu-season/
  14. ↑ https://archive.cdc.gov/www_cdc_gov/flu/about/burden/2018-2019.html
  15. ↑ https://www.cdc.gov/flu-burden/php/data-vis/past-seasons.html
  16. ↑ https://time.com/5758953/flu-season-2019-2020/
  17. ↑ https://www.usnews.com/news/health-news/articles/2020-02-21/flu-season-thats-sickened-26-million-may-be-at-its-peak
  18. ↑ https://www.usnews.com/news/health-news/articles/2020-02-21/flu-season-thats-sickened-26-million-may-be-at-its-peak
  19. ↑ https://www.nytimes.com/interactive/2021/04/22/science/flu-season-coronavirus-pandemic.html
  20. ↑ https://www.livescience.com/health/flu/a-branch-of-the-flu-family-tree-has-died-and-wont-be-included-in-future-us-vaccines
  21. ↑ https://familiesfightingflu.org/flu-season-2022-2023-in-review/
  22. ↑ https://www.cdc.gov/flu-burden/php/data-vis/2022-2023.html
  23. ↑ https://www.ajmc.com/view/severe-influenza-incidence-strikes-us-children-and-adolescents-in-2022-23-season
  24. ↑ https://pmc.ncbi.nlm.nih.gov/articles/PMC11542548/
  25. ↑ https://www.scientificamerican.com/article/why-this-years-flu-season-is-the-worst-in-more-than-a-decade/
  26. ↑ https://www.nbcnews.com/health/health-news/why-flu-bad-2025-severity-vaccines-immunity-rcna194250
  27. ↑ https://ksphealth.org/current-influenza-trends-united-states-2026/
  28. ↑ https://www.cdc.gov/fluview/surveillance/2025-week-52.html
  29. ↑ https://www.cdc.gov/fluview/surveillance/2026-week-03.html
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