Influenza epidemiology and demographics

Jump to navigation Jump to search

Influenza Microchapters

Home

Patient Information

Overview

Historical Perspective

Classification

Pathophysiology

Causes

Differentiating Influenza from other Diseases

Epidemiology and Demographics

Risk Factors

Natural History, Complications and Prognosis

Diagnosis

History and Symptoms

Physical Examination

Laboratory Findings

Chest X Ray

Other Diagnostic Studies

Treatment

Medical Therapy

Primary Prevention

Cost-Effectiveness of Therapy

Future or Investigational Therapies

Case Studies

Case #1

Influenza epidemiology and demographics On the Web

Most recent articles

Most cited articles

Review articles

CME Programs

Powerpoint slides

Images

American Roentgen Ray Society Images of Influenza epidemiology and demographics

All Images
X-rays
Echo & Ultrasound
CT Images
MRI

Ongoing Trials at Clinical Trials.gov

US National Guidelines Clearinghouse

NICE Guidance

FDA on Influenza epidemiology and demographics

CDC on Influenza epidemiology and demographics

Influenza epidemiology and demographics in the news

Blogs on Influenza epidemiology and demographics

Directions to Hospitals Treating Influenza

Risk calculators and risk factors for Influenza epidemiology and demographics

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Mohammad Braizat, M.S. [2]

Epidemiology and Demographics

Overview

Seasonal influenza epidemics cause substantial morbidity and mortality worldwide. The World Health Organization (WHO) estimates annual epidemics result in **3–5 million cases of severe illness** and **290,000–650,000 respiratory deaths** globally.[1][2] In the United States, the CDC estimates that during the 2010–2020 decade, annual influenza-related burden ranged from 9–45 million symptomatic illnesses, 140,000–810,000 hospitalizations, and 12,000–61,000 deaths.[3][4] The 2024–2025 season was the first high-severity season since 2017–2018, with the highest measured influenza-associated hospitalization rates since the 2010–2011 season.[5]

Global Disease Burden

The WHO estimates that annual epidemics result in 3–5 million cases of severe illness and 290,000–650,000 respiratory deaths globally.[1][2] A 2021 Global Burden of Disease analysis found that the global age-standardized mortality rate from influenza-associated lower respiratory infections declined from 5.87 per 100,000 in 1990 to 1.30 per 100,000 in 2021, though absolute deaths increased by 0.85% annually from 1990 to 2019 due to population growth and aging.[6]

Among children younger than 5 years, an estimated 110 million influenza illnesses, 870,000 hospitalizations, and approximately 35,000 deaths from influenza-associated acute lower respiratory tract disease occurred worldwide in 2018, with most in-hospital deaths in low- and middle-income countries.[1] Among adults, more than 32 million cases and 5.7 million hospitalizations from influenza-associated lower respiratory tract disease occur annually, with the highest hospitalization rates in adults aged ≥65 years.[1]

United States Disease Burden

The CDC estimates that during the 2010–2020 decade, annual influenza-related burden in the United States ranged from:[3][4]

  • 9–45 million symptomatic illnesses
  • 4–21 million medical visits
  • 140,000–810,000 hospitalizations
  • 12,000–61,000 deaths

2024–2025 Season (High Severity)

The 2024–2025 season was the first high-severity season since 2017–2018 and had the highest measured influenza-associated hospitalization rates since the 2010–2011 season.[5][7][8] Preliminary estimates (later revised as surveillance data matured) include:[9]

  • 43–73 million estimated illnesses
  • 19–32 million medical visits
  • 560,000–1.1 million hospitalizations
  • 38,000–99,000 estimated deaths
  • Cumulative hospitalization rate: 127.1 per 100,000 (surpassing all end-of-season rates since 2010–2011)[5]
  • Rates were highest among persons aged ≥75 years (598.8 per 100,000)[5]
  • Influenza A(H1N1)pdm09 and A(H3N2) co-circulated in roughly equal proportions[10][8]
  • Pediatric deaths: 279–289 reported depending on reporting cutoff date[11][9][12] (final counts vary by surveillance endpoint); 89% of vaccine-eligible children who died were not fully vaccinated[11]

Age Distribution

Influenza viruses cause disease across all age groups, but the burden is distributed unevenly:[1][3]

  • Children: Infection rates are highest in children, particularly those aged 0–4 years. Hospitalization and in-hospital mortality rates are highest in infants younger than 6 months, a group that is too young for vaccination and depends on maternal immunization and cocooning strategies.[8] During the 2024–2025 season, hospitalization rates in children <1 year were 149.4/100,000 and rates in children 0–4 years were 100.8/100,000, exceeding 2023–2024 rates.[8][5]
  • Adults aged 50–64 years: Intermediate hospitalization rates; this group is recognized as higher risk by the CDC.
  • Adults aged ≥65 years: The highest mortality rates from influenza occur in this group. Epidemics caused by influenza A(H3N2) viruses are associated with particularly high morbidity and mortality in older adults. Elderly patients with comorbidities such as congestive heart failure, COPD, coronary artery disease, or late-stage chronic kidney disease have 3–7 times higher 30-day hospitalization rates compared with matched patients without influenza.[1][13]

Sex and Pregnancy

There is no significant sex-based difference in influenza infection rates. However, pregnant persons are at increased risk for severe illness, particularly in the third trimester through 2 weeks postpartum.[1][3] Among women of reproductive age hospitalized with influenza over nine U.S. seasons, nearly 28% were pregnant, and 62% were in their third trimester.[1] A meta-analysis found that pregnant persons were at higher risk of hospitalization but not ICU admission or death compared with non-pregnant persons.[1]

Racial and Ethnic Disparities

Significant racial and ethnic disparities exist in both influenza disease severity and vaccination coverage in the United States.[14][8] These disparities persist even after adjusting for insurance status, presence of a personal healthcare provider, and recent healthcare utilization, suggesting additional factors contribute to inequitable outcomes.[14]

Disease burden: From 2009–2010 through 2021–2022, age-adjusted influenza hospitalization rates were higher among Black (RR 1.8), American Indian/Alaska Native (RR 1.3), and Hispanic (RR 1.2) adults compared with White adults.[14] Among children ≤4 years, hospitalization rates were 2–3 times higher in Black, Hispanic, and American Indian/Alaska Native children compared with White children, and in-hospital death rates were 3–4 times higher in Black, Hispanic, and Asian/Pacific Islander children.[8]

Vaccination coverage: During the 2021–2022 season, influenza vaccination coverage was lower among Hispanic (37.9%), American Indian/Alaska Native (40.9%), and Black (42.0%) adults compared with White (53.9%) and Asian (54.2%) adults.[14] Among children, the coverage gap between non-Hispanic Asian and non-Hispanic Black children ranged from 15 to 33 percentage points depending on age group.[15]

Populations at Increased Risk for Complications

The CDC and ACIP identify the following groups as at increased risk for influenza complications:[3][16][17]

  • Children <5 years (especially <2 years)
  • Adults ≥65 years
  • Pregnant persons and those up to 2 weeks postpartum
  • Residents of long-term care facilities
  • Individuals with chronic medical conditions:
 * Pulmonary (asthma, COPD, cystic fibrosis)
 * Cardiovascular (excluding hypertension alone)
 * Renal
 * Hepatic
 * Neurologic/neurodevelopmental
 * Metabolic (diabetes mellitus)
 * Hematologic (sickle cell disease)
 * Immunosuppression (HIV, cancer, transplant, chronic steroids)
  • Individuals with obesity (BMI ≥40 kg/m²)
  • Children and adolescents receiving aspirin or salicylate-containing medications (risk of Reye syndrome)[16]
  • American Indian/Alaska Native persons[14]

Seasonality and Geographic Patterns

Influenza seasonality varies by latitude and climate:[1][3][18]

  • Temperate regions: Epidemics occur during cooler months — October through March in the Northern Hemisphere and April through September in the Southern Hemisphere. Epidemic peaks typically occur from December to March (Northern) and May to August (Southern). Median epidemic duration is approximately 10 weeks above 30°N latitude.[18]
  • Tropical and subtropical regions: Influenza can circulate year-round, with one or more peaks during periods of higher absolute humidity or precipitation. Epidemic timing is highly heterogeneous, and duration ranges from 15 to 30 weeks.[1][18]
  • Post-pandemic patterns: Global influenza positivity rates rose from 3.0% in 2021 to 23.7% in 2024, indicating convergence toward pre-pandemic seasonal patterns, though changes in lineage ecology and epidemic duration persist.[18]

Circulating Virus Subtypes and Lineage Changes

Three types of influenza virus cause seasonal epidemics: influenza A(H1N1)pdm09, influenza A(H3N2), and influenza B (Victoria lineage). Key virological trends include:

  • Influenza A(H3N2) seasons are associated with higher morbidity and mortality, particularly in older adults. A(H3N2) dominated in 2021–2022 and 2022–2023.[1]
  • Influenza A(H1N1)pdm09 predominated in 2023–2024 and co-circulated with A(H3N2) in 2024–2025.[8][10]
  • Influenza B/Yamagata lineage has not been conclusively detected since March 2020 and is considered probably extinct. This was driven by reduced transmission from COVID-19 nonpharmaceutical interventions combined with a depleted susceptible population due to conserved antigenicity and the large 2017–2018 B/Yamagata outbreak.[19][20] The WHO recommended exclusion of B/Yamagata from influenza vaccines beginning with the 2024 Southern Hemisphere season, prompting a global transition from quadrivalent to trivalent vaccine formulations.[21]
  • Influenza B/Victoria is now the sole circulating influenza B lineage.[18]

Impact of the COVID-19 Pandemic on Influenza Epidemiology

The COVID-19 pandemic profoundly disrupted global influenza circulation:[3][22][23]

  • Nonpharmaceutical interventions (masking, social distancing, travel restrictions) caused a sharp decline in influenza activity during 2020–2021, with near-absence of seasonal influenza globally.[3][22]
  • Influenza A activity gradually recovered beginning in 2021–2022, while influenza B recovery has been slower and with reduced lineage diversity.[24]
  • Post-pandemic rebound has been notable: the 2022–2023 season featured early onset and high severity in children, and the 2024–2025 season was the most severe in over a decade.[25][5][8]
  • Working-age adults (15–64 years) showed the most pronounced post-pandemic surge in relative risk of influenza in some analyses, potentially reflecting immune waning during the period of low circulation.[23]
  • The B/Yamagata lineage was lost during this period (see above).[19][20]

Vaccination Coverage

Annual influenza vaccination is recommended by the CDC/ACIP for all persons aged ≥6 months without contraindications, with particular emphasis on healthcare personnel and household contacts of high-risk persons to reduce transmission to vulnerable populations.[16]

  • United States: Overall vaccination coverage peaked at approximately 46% in 2019–2020 and declined to approximately 40% in 2022–2023. Coverage is approximately 50–60% among children and adults combined. Declines have been most substantial among children, with coverage for children aged 6 months through 8 years falling by 14 percentage points from peak to the 2022–2023 season.[26][1]
  • Global: As of 2022, 128 of 194 WHO member states had a seasonal influenza vaccination policy. Median coverage rates were 37% for pregnant women, 55% for older adults, and 62% for health workers.[27]
  • Adults aged ≥65 years: Preferentially recommended to receive higher-dose or adjuvanted vaccines (HD-IIV, RIV4, or aIIV4).[16]

Zoonotic and Pandemic Influenza

Influenza A viruses circulate among animal populations and occasionally infect humans. The primary reservoirs are wild waterfowl, but influenza A viruses also circulate in domestic poultry, swine, and — since 2024 — dairy cattle.[3][28]

Highly pathogenic avian influenza A(H5N1):

  • Since 1997, over 1,100 human cases of H5N1 have been reported globally. The currently dominant clade 2.3.4.4b has driven a global panzootic in birds and mammals, with spillover into dairy cattle in the United States since early 2024.[29][28]
  • As of mid-2025, 70 human H5N1 cases had been reported in the United States, mostly in adults with occupational exposure to dairy cattle or poultry.[8]
  • Adjusted case-fatality risk for clade 2.3.4.4b is estimated at 0.7% (95% CI: 0.02–3.9%), substantially lower than earlier clades (range 4.7–15.0%).[29]
  • Sustained human-to-human transmission has not been observed, but zoonotic transmission has increased with bovine-origin clade 2.3.4.4b.[29][28]
  • Clinicians should maintain awareness of H5N1 in patients with occupational exposure to poultry or dairy cattle presenting with severe respiratory illness.

Antiviral Treatment Gaps

Epidemiologic surveillance data from the 2024–2025 season revealed that only 40% of children who died from influenza received antiviral treatment, with antiviral receipt lowest among children aged 5–17 years (61.6%).[11][8] This represents a clinically actionable gap in care, particularly given that early antiviral treatment (within 48 hours of symptom onset) is associated with improved outcomes in high-risk patients.

High-Yield Clinical Pearls

  • Annual U.S. influenza burden varies widely (9–45 million illnesses, 12,000–61,000 deaths), driven largely by the predominant circulating subtype and antigenic match with the vaccine.[3][4]
  • A(H3N2)-predominant seasons tend to cause higher morbidity and mortality, especially in older adults.
  • Infants <6 months have the highest hospitalization rates among children but are too young for vaccination; they depend on maternal immunization during pregnancy and cocooning by vaccinated household contacts.[8]
  • Adults ≥65 years, pregnant persons, and individuals with chronic medical conditions are at highest risk for complications.
  • The 2024–2025 season was the most severe since 2017–2018, with pediatric deaths totaling 279–289 depending on the reporting cutoff.[11][9][12]
  • Influenza B/Yamagata is considered probably extinct; WHO has recommended removal from vaccines, with global transition to trivalent formulations.[20][21]
  • Significant racial/ethnic disparities persist in both influenza hospitalization rates and vaccination coverage, even after adjusting for healthcare access.[14]
  • COVID-19 pandemic-related nonpharmaceutical interventions disrupted influenza circulation and may have contributed to post-pandemic rebound and loss of B/Yamagata lineage.[22]
  • Vaccination coverage in the U.S. has declined since the pandemic, particularly among children.[26]
  • Among children who died from influenza during the 2024–2025 season, 44% had no underlying medical conditions, and only 11% of vaccine-eligible children who died were fully vaccinated.[11][8]
  • Only 40% of children who died received antiviral treatment, highlighting a critical gap in clinical care.[11]

References

  1. ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 Uyeki TM, Hui DS, Zambon M, Wentworth DE, Monto AS (2022). "Influenza". Lancet. 400 (10353): 693–706. doi:10.1016/S0140-6736(22)00982-5. PMID 36001333 Check |pmid= value (help).
  2. ↑ 2.0 2.1 Iuliano AD, Roguski KM, Chang HH; et al. (2018). "Estimates of global seasonal influenza-associated respiratory mortality: a modelling study". Lancet. 391 (10127): 1285–1300. doi:10.1016/S0140-6736(17)33293-2. PMID 29548687.
  3. ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 Molly Valleau and Christine M. Szablewski. Influenza. CDC Yellow Book.
  4. ↑ 4.0 4.1 4.2 Tokars JI, Olsen SJ, Reed C (2018). "Seasonal Incidence of Symptomatic Influenza in the United States". Clin Infect Dis. 68 (10): 1517–1524. doi:10.1093/cid/ciy793.
  5. ↑ 5.0 5.1 5.2 5.3 5.4 5.5 O'Halloran A, Habeck JW, Gilmer M; et al. (2025). "Influenza-Associated Hospitalizations During a High Severity Season - Influenza Hospitalization Surveillance Network, United States, 2024-25 Influenza Season". MMWR Morb Mortal Wkly Rep. 74 (34): 529–537. doi:10.15585/mmwr.mm7434a1.
  6. ↑ Shao C, Huang X, Zhang H; et al. (2026). "Global, regional, and national burden of influenza-associated lower respiratory infections, 1990-2021: a systematic analysis from the Global Burden of Disease Study 2021". BMC Infect Dis. 26 (1): 202. doi:10.1186/s12879-025-12282-7. PMID 39955400 Check |pmid= value (help).
  7. ↑ Sophie Zhu, PhD, Joshua Quint, PhD, Tomás M. León, PhD; et al. (2026). "Influenza Vaccine and Associated Infection and Death in California, 2024 to 2025". JAMA Netw Open. doi:10.1001/jamanetworkopen.2026.17684.
  8. ↑ 8.00 8.01 8.02 8.03 8.04 8.05 8.06 8.07 8.08 8.09 8.10 8.11 Committee on Infectious Diseases (2025). "Recommendations for Prevention and Control of Influenza in Children, 2025-2026: Technical Report". Pediatrics. doi:10.1542/peds.2025-073622.
  9. ↑ 9.0 9.1 9.2 Zambon M, Hayden FG (2025). "Influenza". JAMA. 334 (24). doi:10.1001/jama.2025.24831.
  10. ↑ 10.0 10.1 Villafuerte D, Fall A, Akin E; et al. (2026). "Genomic Evolution of Influenza a Virus During the 2024-2025 Season, the Johns Hopkins Health System: Antigenic Drift Reduces Serum Neutralization". J Infect Dis. 233 (6): e1386–e1395. doi:10.1093/infdis/jiag069.
  11. ↑ 11.0 11.1 11.2 11.3 11.4 11.5 Reinhart K, Huang S, Kniss K, Reed C, Budd A (2025). "Influenza-Associated Pediatric Deaths - United States, 2024-25 Influenza Season". MMWR Morb Mortal Wkly Rep. 74 (36): 565–569. doi:10.15585/mmwr.mm7436a2.
  12. ↑ 12.0 12.1 Leonard JS; et al. (2026). "Pediatric Influenza-Associated Deaths, United States, 2024-2025". Pediatrics. doi:10.1542/peds.2026-070000.
  13. ↑ Near AM, Tse J, Young-Xu Y, Hong DK, Reyes CM (2022). "Burden of influenza hospitalization among high-risk groups in the United States". BMC Health Serv Res. 22 (1): 1209. doi:10.1186/s12913-022-08586-y. PMID 36151503 Check |pmid= value (help).
  14. ↑ 14.0 14.1 14.2 14.3 14.4 14.5 Black CL, O'Halloran A, Hung MC; et al. (2022). "Vital Signs: Influenza Hospitalizations and Vaccination Coverage by Race and Ethnicity-United States, 2009-10 Through 2021-22 Influenza Seasons". MMWR Morb Mortal Wkly Rep. 71 (43): 1366–1373. doi:10.15585/mmwr.mm7143e1.
  15. ↑ Irving SA, Groom HC, Belongia EA; et al. (2025). "Differences in influenza vaccination coverage by race and ethnicity across age groups in the Vaccine Safety Datalink, 2017-18 through 2022-23 influenza seasons". Vaccine. 64: 127667. doi:10.1016/j.vaccine.2025.127667.
  16. ↑ 16.0 16.1 16.2 16.3 Grohskopf LA, Blanton LH, Ferdinands JM; et al. (2022). "Prevention and Control of Seasonal Influenza With Vaccines: Recommendations of the Advisory Committee on Immunization Practices - United States, 2022-23 Influenza Season". MMWR Recomm Rep. 71 (1): 1–28. doi:10.15585/mmwr.rr7101a1.
  17. ↑ Committee on Infectious Diseases (2019). "Recommendations for Prevention and Control of Influenza in Children, 2019-2020". Pediatrics. 144 (4): e20192478. doi:10.1542/peds.2019-2478.
  18. ↑ 18.0 18.1 18.2 18.3 18.4 Del Riccio M, Caini S (2026). "Global influenza epidemiology after 2020: patterns of circulation, epidemic timing and duration, and implications for vaccination strategies". Euro Surveill. 31 (21). doi:10.2807/1560-7917.ES.2026.31.21.2500743.
  19. ↑ 19.0 19.1 Han W, Zeng J, Shi J; et al. (2025). "Unraveling the mechanism behind the probable extinction of the B/Yamagata lineage of influenza B viruses". Nat Commun. 16 (1): 10440. doi:10.1038/s41467-025-65396-6.
  20. ↑ 20.0 20.1 20.2 Caini S, Meijer A, Nunes MC; et al. (2024). "Probable extinction of influenza B/Yamagata and its public health implications: a systematic literature review and assessment of global surveillance databases". Lancet Microbe. 5 (8): 100851. doi:10.1016/S2666-5247(24)00066-1.
  21. ↑ 21.0 21.1 Fisman D, Pérez-Rubio A, Postma M, Smith DS, Mould-Quevedo J (2025). "Maintaining the value of influenza vaccination - the shift from quadrivalent to trivalent vaccines: an expert review". Expert Rev Vaccines. 24 (1): 499–508. doi:10.1080/14760584.2025.2515597.
  22. ↑ 22.0 22.1 22.2 Dhanasekaran V, Sullivan S, Edwards KM; et al. (2022). "Human seasonal influenza under COVID-19 and the potential consequences of influenza lineage elimination". Nat Commun. 13 (1): 1721. doi:10.1038/s41467-022-29402-5.
  23. ↑ 23.0 23.1 Gao X, Qin P, Qi X; et al. (2026). "Suppression and resurgence: the evolving epidemiology of seasonal influenza from 2015 to 2024 in a core urban district of Beijing, China". Front Public Health. 14: 1800701. doi:10.3389/fpubh.2026.1800701.
  24. ↑ Yang L, Riaz M, Rahman NU (2026). "Asymmetric post-pandemic recovery of influenza A and B since 2020 in Hong Kong: an interrupted time-series (ITS) analysis of weekly surveillance data with subtype and lineage characterization". BMC Infect Dis. doi:10.1186/s12879-026-13714-8.
  25. ↑ White EB, O'Halloran A, Sundaresan D; et al. (2023). "High Influenza Incidence and Disease Severity Among Children and Adolescents Aged <18 Years - United States, 2022-23 Season". MMWR Morb Mortal Wkly Rep. 72 (41): 1108–1114. doi:10.15585/mmwr.mm7241a2.
  26. ↑ 26.0 26.1 Irving SA, Groom HC, Belongia EA; et al. (2023). "Influenza vaccination coverage among persons ages six months and older in the Vaccine Safety Datalink in the 2017-18 through 2022-23 influenza seasons". Vaccine. 41 (48): 7138–7146. doi:10.1016/j.vaccine.2023.10.023.
  27. ↑ Goldin S, Brooks D, Jorgensen P; et al. (2024). "Seasonal influenza vaccination: A global review of national policies in 194 WHO Member States in 2022". Vaccine. 42 (26): 126274. doi:10.1016/j.vaccine.2024.126274.
  28. ↑ 28.0 28.1 28.2 Peacock TP, Moncla L, Dudas G; et al. (2025). "The global H5N1 influenza panzootic in mammals". Nature. 637 (8045): 304–313. doi:10.1038/s41586-024-08054-z.
  29. ↑ 29.0 29.1 29.2 Wang W, Xing J, Jiang H; et al. (2026). "Human infections with avian influenza A(H5) viruses with potential pandemic risk: 1997-2025". Natl Sci Rev. 13 (7): nwaf471. doi:10.1093/nsr/nwaf471.
Retrieved from "https://www.wikidoc.org/index.php?title=Influenza_epidemiology_and_demographics&oldid=1745229"