H1N1 Vaccine in America 2025
The H1N1 vaccine continues to serve as a critical component of seasonal influenza protection across the United States, particularly following the unprecedented severity of the 2024-2025 flu season. The A(H1N1)pdm09 virus, which first emerged during the 2009 pandemic, has remained a persistent seasonal threat that circulates alongside other influenza strains each year. During the most recent flu season, this particular strain accounted for more than half of all identified influenza A cases, contributing substantially to one of the most severe respiratory illness seasons documented since the 2017-2018 period. The vaccine formulation specifically targets this strain through trivalent vaccines that protect against H1N1, H3N2, and B/Victoria lineage viruses, demonstrating the ongoing importance of annual vaccination efforts.
The American healthcare system faced extraordinary challenges during the 2024-2025 influenza season, with H1N1pdm09 playing a dominant role in driving hospitalizations and severe outcomes across multiple age groups. Despite the availability of effective vaccines, coverage rates fell below optimal levels, with only 49.2% of children and 46.7% of adults receiving their flu vaccination by late April 2025. This represents a concerning decline from previous seasons and coincided with record-breaking hospitalization rates, emergency department visits, and pediatric mortality figures. The convergence of reduced vaccination coverage, waning population immunity from pandemic-era precautions, and the co-circulation of multiple respiratory viruses created perfect conditions for widespread influenza transmission, underscoring the urgent need for improved vaccine uptake and public health interventions.
Interesting Facts About H1N1 Vaccine in the US 2025
| Fact Category | Statistical Finding | Significance |
|---|---|---|
| Dominant Strain Status | 53.1% of all influenza A cases were H1N1pdm09 | H1N1 was the predominant subtype during 2024-25 season |
| Vaccine Distribution | 147.6 million doses distributed by March 2025 | Largest vaccine supply deployment in recent seasons |
| Pediatric Vaccination Gap | Only 49.2% of children vaccinated | 4.2 percentage point decline from previous season |
| Adult Coverage Rate | 46.7% of adults received flu vaccine | Vaccination rates remained stagnant compared to 2023-24 |
| Senior Coverage | 71% of adults aged 65+ vaccinated | Highest coverage among all age demographics |
| Vaccine Effectiveness – Children | 32-60% effective in outpatient settings | Three different networks showed varying effectiveness |
| Vaccine Effectiveness – Hospitalization | 63-78% prevention rate for pediatric hospitalizations | Strong protection against severe outcomes in children |
| Adult VE – Outpatient | 36-54% effectiveness range | Moderate protection across two surveillance networks |
| Adult VE – Hospitalization | 41-55% effectiveness against admission | Significant reduction in severe disease requiring hospitalization |
| Prevented Illnesses | 9.4-16 million symptomatic cases prevented | Vaccination impact despite moderate effectiveness |
| Prevented Hospitalizations | 170,000-360,000 admissions averted | Substantial reduction in healthcare system burden |
| Prevented Deaths | 12,000-39,000 lives saved | Critical mortality prevention through vaccination |
| H1N1 Clinical Testing | 44,733 H1N1pdm09 viruses identified | From 84,260 total subtyped influenza A specimens |
| Trivalent Vaccine Formula | 3 virus components in 2024-25 vaccines | First season using exclusively trivalent formulations |
| Vaccine Timing Peak | Activity peaked in early February 2025 | Latest peak compared to previous three seasons |
Data source: Centers for Disease Control and Prevention (CDC) FluView, MMWR Reports, Weekly Flu Vaccination Dashboard, 2024-2025 Influenza Season Summary
The 2024-2025 influenza season revealed critical patterns in H1N1pdm09 circulation and vaccine performance across the United States. With H1N1 accounting for 53.1% of all subtyped influenza A viruses, this strain dominated the landscape and drove much of the severe disease burden observed throughout the season. Public health laboratories tested 155,297 specimens and reported 100,015 influenza-positive results, with the vast majority being influenza A. Among the 84,260 seasonal influenza A viruses that underwent subtyping procedures, 44,733 were confirmed as H1N1pdm09, demonstrating the widespread prevalence of this particular variant. The United States distributed 147.6 million vaccine doses by March 2025, representing manufacturers’ efforts to meet demand, yet vaccination coverage fell short of public health goals with only 49.2% of eligible children and 46.7% of adults completing their immunization.
Vaccine effectiveness data from four major surveillance networks provided valuable insights into how well the 2024-2025 formulation performed against circulating strains. Among children and adolescents, effectiveness ranged from 32% to 60% in preventing medically attended influenza in outpatient settings, while demonstrating substantially higher protection against severe outcomes with 63% to 78% effectiveness in preventing influenza-associated hospitalizations. Adult populations experienced similar patterns, with outpatient vaccine effectiveness ranging from 36% to 54% and hospitalization prevention rates of 41% to 55%. Despite these moderate effectiveness figures, the real-world impact of vaccination proved substantial, with preliminary CDC estimates suggesting that flu vaccines prevented between 9.4 and 16 million symptomatic illnesses, 170,000 to 360,000 hospitalizations, and 12,000 to 39,000 deaths during the 2024-2025 season. These prevention numbers highlight the critical importance of vaccination even when effectiveness percentages appear moderate, as the population-level benefits translate to significant reductions in disease burden, healthcare utilization, and mortality.
H1N1 Virus Circulation Patterns in the US 2025
| Circulation Metric | H1N1pdm09 Data | H3N2 Comparison | Total Influenza A |
|---|---|---|---|
| Subtyped Specimens | 44,733 (53.1%) | 39,527 (46.9%) | 84,260 (100%) |
| Peak Activity Week | Late January 2025 | Late January 2025 | Week ending Feb 1, 2025 |
| Test Positivity Peak | 30.4% (late January) | Co-circulated equally | 31.6% (highest in 9 years) |
| Age 0-4 Distribution | 50.2% of subtyped cases | 49.8% of subtyped cases | Nearly equal distribution |
| Age 25-64 Distribution | 55.5% of subtyped cases | 44.5% of subtyped cases | H1N1 predominant in adults |
| Age 65+ Distribution | 59.9% of subtyped cases | 40.1% of subtyped cases | Strong H1N1 predominance in elderly |
| Total Respiratory Specimens | Part of 3,978,954 tested | Part of 3,978,954 tested | 489,579 positive (12.3%) |
| Public Health Lab Positives | 44,733 identified | 39,527 identified | 94,078 influenza A total |
| Genetic Clade 5a.2a | 32.3% (646 viruses) | Not applicable | H1N1-specific classification |
| Genetic Clade 5a.2a.1 | 67.7% (1,353 viruses) | Not applicable | Matches 2024-25 vaccine strain |
| Dominant Subclade D.3.1 | 56.4% (1,128 viruses) | Not applicable | Predominant since February 2025 |
| Vaccine Match (H1N1) | 99.3% well-recognized | 54.7% well-recognized (H3N2) | Better match for H1N1 than H3N2 |
| Antiviral Resistance (H1N1) | 8 viruses with NA-H275Y | 2 viruses with mutations | Very low resistance overall |
| Regional Peak Timing | Regions 4,6,7,9: Late January | Regions 1,2,3,5,8,10: Early Feb | Varied by HHS region |
| Season Classification | Contributed to high severity | Co-contributed to severity | First high severity since 2017-18 |
Data source: CDC Influenza Division, FluView Interactive, Virus Surveillance Data 2024-2025, Genetic and Antigenic Characterization Reports
The 2024-2025 influenza season witnessed nearly equal co-circulation of H1N1pdm09 and H3N2 viruses, though H1N1 ultimately claimed the majority position with 53.1% of all subtyped influenza A specimens. This represents 44,733 confirmed H1N1pdm09 viruses out of 84,260 total subtyped seasonal influenza A viruses processed by public health laboratories across the United States. Clinical laboratories tested more than 3.9 million respiratory specimens for influenza viruses using diagnostic tests, with 489,579 testing positive for influenza overall, yielding a 12.3% positivity rate. The national weekly percentage of respiratory specimens testing positive for influenza ranged from a low of 0.4% to a remarkable peak of 31.6% during the week ending February 1, 2025, marking the highest peak percentage reported in the past nine influenza seasons since the 2015-2016 period.
Genetic characterization of circulating H1N1pdm09 viruses revealed that samples belonged to two primary clades: 5a.2a accounting for 32.3% and 5a.2a.1 representing 67.7% of characterized specimens. The A(H1N1)pdm09 component for the 2024-2025 Northern Hemisphere influenza vaccines contained hemagglutinin genes from clade 5a.2a.1, providing an excellent match with most circulating viruses. Within the dominant clade, subclade D.3.1 emerged as the predominant lineage, representing 56.4% of all characterized H1N1pdm09 viruses and becoming the dominant subclade since February 2025. Antigenic characterization demonstrated exceptional vaccine alignment, with 99.3% of H1N1pdm09 viruses tested being well-recognized by ferret antisera to cell-grown A/Wisconsin/67/2022-like reference viruses representing the vaccine component. This strong antigenic match contrasts sharply with H3N2 viruses, where only 54.7% showed good recognition by vaccine antisera, suggesting that the H1N1 vaccine component performed better than its H3N2 counterpart during this particular season. Age distribution patterns showed that while H1N1pdm09 and H3N2 circulated at nearly equal proportions among children aged 0-4 years, H1N1 was slightly more frequently reported among individuals aged 25-64 years at 55.5% and among those 65 years and older at 59.9%, indicating this strain had particular impact on adult and elderly populations.
Hospitalization Burden Due to H1N1 in the US 2025
| Hospitalization Metric | 2024-25 Season Data | Age Group Most Affected | Comparison to Previous Seasons |
|---|---|---|---|
| Total Flu Hospitalizations | 545,026 admissions nationwide | All ages affected | Highest since NHSN reporting began |
| Cumulative Hospitalization Rate | 161.5 per 100,000 population | National average | Substantially higher than 2021-24 |
| FluSurv-NET Hospitalizations | 39,319 laboratory-confirmed cases | Population-based surveillance | Covered ~9% of US population |
| FluSurv-NET Cumulative Rate | 128.3 per 100,000 population | Overall population | Highest rate since 2010-11 |
| Peak Weekly Rate (NHSN) | 16.1 per 100,000 (Week 6) | Peaked February 8, 2025 | Latest peak in four seasons |
| Peak Weekly Rate (FluSurv) | 13.7 per 100,000 (Week 6) | Tied for highest since 2010-11 | February 8, 2025 peak |
| H1N1 Among Hospitalizations | 58.9% of subtyped flu A cases | Among FluSurv-NET patients | Majority of hospitalized flu A cases |
| Age 65+ Hospitalization Rate | 407.6 per 100,000 population | Elderly most severely affected | Nearly 3x higher than next group |
| Age 50-64 Rate | 150.1 per 100,000 population | Second highest rate | Above national average |
| Age 0-4 Rate | 104.7 per 100,000 population | Young children third highest | Significant pediatric burden |
| Age 18-49 Rate | 50.7 per 100,000 population | Working-age adults | Below national average |
| Age 5-17 Rate | 40.4 per 100,000 population | School-age children lowest | Lowest cumulative rate |
| Underlying Conditions (Adults) | 94.1% had ≥1 condition | Among 8,876 hospitalized adults | Hypertension, cardiovascular most common |
| Underlying Conditions (Children) | 53.5% had ≥1 condition | Among 3,109 hospitalized children | Asthma, neurologic disease most common |
| Pregnant Women Hospitalized | 22.6% of women aged 15-49 | Among 4,135 women of childbearing age | Pregnancy as significant risk factor |
Data source: CDC FluSurv-NET, National Healthcare Safety Network (NHSN) Hospital Respiratory Data Module, 2024-2025 Influenza Hospitalization Surveillance
The 2024-2025 influenza season produced the most severe hospitalization burden documented in recent years, with H1N1pdm09 playing a dominant role in driving admissions across all age groups. The National Healthcare Safety Network reported 545,026 influenza-associated hospitalizations nationwide between September 29, 2024, and August 30, 2025, resulting in a cumulative hospitalization rate of 161.5 per 100,000 population. Hospitalizations began increasing in mid-November, experienced a slight decline around the winter holidays, and peaked during the week ending February 8, 2025, with 54,272 hospitalizations recorded in a single week, translating to 16.1 admissions per 100,000 population. This peak occurred substantially later and at much higher levels compared to the previous three seasons, marking an unprecedented burden on the American healthcare system.
Population-based surveillance through the Influenza Hospitalization Surveillance Network revealed that from October 1, 2024, to April 30, 2025, 39,319 laboratory-confirmed influenza-related hospitalizations were reported across participating sites covering approximately 9% of the US population. The overall cumulative hospitalization rate reached 128.3 per 100,000 population, representing the highest cumulative rate for all seasons since 2010-2011. Among these hospitalizations, the vast majority at 95.9% were associated with influenza A virus, with only 3.8% linked to influenza B. Critically, among influenza A cases with subtype information available, 58.9% were identified as A(H1N1)pdm09 while 41.1% were A(H3N2), demonstrating that H1N1 accounted for the majority of hospitalized influenza A patients. Age-stratified data revealed striking disparities, with adults aged 65 years and older experiencing a cumulative hospitalization rate of 407.6 per 100,000 population, nearly three times higher than any other age group. Adults aged 50-64 years followed with 150.1 per 100,000, while children aged 0-4 years recorded 104.7 per 100,000. Among hospitalized adults with documented underlying medical conditions, an overwhelming 94.1% had at least one pre-existing condition, with hypertension, cardiovascular disease, metabolic disorders, and obesity being the most commonly reported. For hospitalized children and adolescents, 53.5% had at least one underlying condition, with asthma, neurologic disease, and obesity predominating. Notably, among 4,135 hospitalized women of childbearing age, 22.6% were pregnant, highlighting pregnancy as a significant risk factor for severe influenza requiring hospitalization.
Mortality Statistics Related to H1N1 in the US 2025
| Mortality Indicator | 2024-25 Season Data | Age Group Impact | Historical Context |
|---|---|---|---|
| Total Deaths (All Causes) | 2,784,349 deaths reported nationally | All ages | September 2024 – August 2025 |
| Influenza-Associated Deaths | 18,399 (0.7% of all deaths) | All ages | Underlying or contributing cause |
| Peak Weekly Death Percentage | 2.8% (Week 7) | February 15, 2025 | Highest peak since 2015-16 |
| Pediatric Influenza Deaths | 279 laboratory-confirmed deaths | Children <18 years | Highest seasonal epidemic total ever |
| Pediatric Deaths Since 2004 | Previous range: 37-207 deaths/season | Excluding 2020-21 (low circulation) | 2024-25 exceeded all prior seasons |
| H1N1 Pediatric Deaths | 95 (56.2%) of subtyped deaths | Among 169 subtyped influenza A | H1N1 majority among children |
| H3N2 Pediatric Deaths | 73 (43.2%) of subtyped deaths | Among 169 subtyped influenza A | H3N2 contributed substantially |
| Mean Age at Death (Children) | 7 years (range: 2 weeks-17 years) | Across all pediatric deaths | Wide age range affected |
| Deaths After Hospital Admission | 51.6% (143 children) | Among 277 with known location | Majority occurred in hospital |
| Emergency Room Deaths | 26.4% (73 children) | Among 277 with known location | Significant ED mortality |
| Out-of-Hospital Deaths | 22.0% (61 children) | Among 277 with known location | Deaths before hospitalization |
| Underlying Conditions Present | 56.5% (147 children) | Among 260 with known history | Majority had risk factors |
| Unvaccinated Pediatric Deaths | 89.4% (185 children) | Among 207 vaccine-eligible with status | Overwhelming majority unvaccinated |
| Cumulative Death Percentage | 0.7% of all deaths flu-related | Entire 2024-25 season | Highest cumulative % since 2015-16 |
| Deaths Prevented by Vaccination | 12,000-39,000 deaths averted | All age groups | Estimated vaccine impact |
| Overall Disease Burden Deaths | 38,000-99,000 influenza deaths | Preliminary estimates | Modeled total mortality burden |
Data source: CDC National Center for Health Statistics (NCHS) Mortality Surveillance System, Influenza-Associated Pediatric Mortality Surveillance System, Burden Estimates 2024-2025
Mortality data from the 2024-2025 influenza season painted a sobering picture of the disease’s impact across all age groups, with H1N1pdm09 contributing significantly to death tolls. According to the National Center for Health Statistics Mortality Surveillance System, between September 29, 2024, and August 30, 2025, 18,399 deaths had influenza listed as either an underlying or contributing cause, representing 0.7% of all deaths reported nationally during this period. The weekly percentage of deaths attributed to influenza ranged from 0.02% to 2.8%, peaking during the week ending February 15, 2025, at 2.8%, which represents the highest peak recorded since the 2015-2016 season when mortality surveillance data were first incorporated into influenza monitoring systems. This peak occurred substantially later than in recent post-COVID seasons and at much higher levels than the 2023-2024 season’s 1.4% peak or the 2022-2023 season’s 1.6% peak.
The pediatric mortality burden proved particularly devastating, with 279 laboratory-confirmed influenza-associated deaths reported among children younger than 18 years of age, marking the highest number ever reported during a seasonal influenza epidemic since pediatric influenza deaths became a nationally notifiable condition in 2004. Prior to this season, the total number of pediatric influenza deaths during typical seasonal epidemics ranged from a low of 37 during the 2011-2012 season to 207 during the 2023-2024 season, excluding the 2020-2021 season which saw unusually low influenza circulation. Among the 279 pediatric deaths, 239 were associated with influenza A viruses, 38 with influenza B, and 1 with co-infection. Of the 169 influenza A virus infections with reported subtype information, 95 deaths (56.2%) were attributed to influenza A(H1N1), while 73 deaths (43.2%) involved A(H3N2), and 1 death (0.6%) represented co-infection with both subtypes. The mean age at death among these children was 7 years, with cases ranging from infants as young as 2 weeks old to adolescents 17 years of age. Location of death data revealed that approximately half (51.6% or 143 children) died after hospital admission, 26.4% (73 children) died in emergency rooms, and 22.0% (61 children) died outside of hospital settings, often before medical care could be accessed. Among the 260 children and adolescents with known medical history, 147 children (56.5%) had at least one underlying medical condition associated with higher risk for developing serious influenza-related complications, meaning that 43.5% of pediatric deaths occurred in previously healthy children. Most alarmingly, among the 207 children who were eligible for influenza vaccination and had known vaccination status, 185 children (89.4%) were not fully vaccinated against influenza, demonstrating a critical missed opportunity for prevention.
Vaccine Effectiveness Against H1N1 in the US 2025
| Effectiveness Measure | Vaccine Effectiveness (VE) | Study Population | Setting Type |
|---|---|---|---|
| Children Outpatient VE – Network 1 | 32% (95% CI) | Ages 6 months-17 years | US Flu VE Network |
| Children Outpatient VE – Network 2 | 59% (95% CI) | Ages 6 months-17 years | NVSN Network |
| Children Outpatient VE – Network 3 | 60% (95% CI) | Ages 6 months-17 years | VISION Network |
| Children Hospitalization VE – Network 1 | 63% (95% CI) | Ages 6 months-17 years | IVY Network |
| Children Hospitalization VE – Network 2 | 78% (95% CI) | Ages 6 months-17 years | NVSN Network |
| Adults Outpatient VE – Network 1 | 36% (95% CI) | Ages ≥18 years | US Flu VE Network |
| Adults Outpatient VE – Network 2 | 54% (95% CI) | Ages ≥18 years | VISION Network |
| Adults Hospitalization VE – Network 1 | 41% (95% CI) | Ages ≥18 years | IVY Network |
| Adults Hospitalization VE – Network 2 | 55% (95% CI) | Ages ≥18 years | VISION Network |
| H1N1-Specific VE (Overall) | 37-56% range | All age groups | Combined outpatient settings |
| H1N1-Specific VE (Hospitalization) | 39-62% range | All age groups | Combined inpatient settings |
| Antigenic Match (H1N1) | 99.3% well-recognized | Virus characterization | Laboratory ferret antisera testing |
| Genetic Clade Match | 67.7% viruses matched vaccine | Clade 5a.2a.1 viruses | Vaccine contained 5a.2a.1 strain |
| Study Period | October 2024 – February 2025 | Interim estimates | Four VE networks combined |
| Overall Influenza VE (Any Type) | 32-60% (children outpatient) | Children and adolescents | Three surveillance networks |
Data source: CDC MMWR February 2025, Interim Estimates of 2024-2025 Seasonal Influenza Vaccine Effectiveness, Four Vaccine Effectiveness Networks
Vaccine effectiveness monitoring during the 2024-2025 influenza season provided comprehensive data on how well the H1N1 vaccine component protected Americans against medically attended influenza and severe outcomes requiring hospitalization. The Centers for Disease Control and Prevention derived interim effectiveness estimates from four major US vaccine effectiveness networks spanning the period from October 2024 through February 2025, encompassing the peak months of influenza activity. Among children and adolescents aged less than 18 years, vaccine effectiveness against any influenza ranged from 32% in one network to 59-60% in two other networks for preventing outpatient visits related to acute respiratory illness. When examining protection against the most severe outcomes, vaccine effectiveness against influenza-associated hospitalization proved substantially higher at 63% in one network and 78% in another network, demonstrating that vaccination provided strong protection against life-threatening complications even when effectiveness against milder illness appeared moderate.
Adult populations aged 18 years and older experienced similar effectiveness patterns across surveillance networks. In outpatient settings, vaccine effectiveness against medically attended influenza was 36% in one network and 54% in another, while protection against influenza-associated hospitalization reached 41% and 55% in two separate networks. The relatively consistent effectiveness ranges across multiple independent surveillance systems strengthened confidence in these estimates and provided robust evidence that the 2024-2025 influenza vaccine delivered meaningful protection despite not reaching the ideal targets health officials always strive toward. Laboratory characterization data supported these real-world effectiveness findings, with 99.3% of H1N1pdm09 viruses tested showing excellent recognition by ferret antisera raised against cell-grown A/Wisconsin/67/2022-like reference viruses, which represented the A(H1N1)pdm09 component of both cell-based and recombinant-based influenza vaccines. This exceptional antigenic match between vaccine strain and circulating viruses contrasted sharply with A(H3N2) performance, where only 54.7% of tested viruses showed good recognition, suggesting that the slightly lower overall vaccine effectiveness figures resulted primarily from H3N2 mismatch rather than poor H1N1 vaccine performance. Genetic analysis confirmed that 67.7% of characterized H1N1pdm09 viruses belonged to clade 5a.2a.1, precisely matching the genetic clade of the vaccine strain, while the remaining 32.3% from clade 5a.2a still demonstrated good cross-reactivity with vaccine-induced antibodies.
Vaccine Coverage and Distribution in the US 2025
| Coverage Metric | 2024-25 Season Data | 2023-24 Comparison | Change from Prior Year |
|---|---|---|---|
| Total Vaccine Doses Distributed | 147.6 million doses | Data through March 8, 2025 | Final season distribution |
| Projected Supply Availability | 148 million doses planned | Manufacturer projections | Sufficient supply achieved |
| Children Vaccination Coverage | 49.2% (95% CI: 48.2-50.1) | 53.4% at same timepoint | -4.2 percentage point decline |
| Adult Vaccination Coverage | 46.7% (95% CI: 46.0-47.5) | 47.4% at same timepoint | Similar to previous season |
| Adults 65+ Coverage | 71% overall | Higher than younger adults | Best coverage among age groups |
| Adults 18-49 Coverage | 35% approximate | Lowest adult age group | Substantial gap in young adults |
| Physician Office Doses | 21.5 million doses administered | Through April 12, 2025 | ~4 million fewer than 2023-24 |
| Coverage by Poverty Status | Varies significantly | Data shows disparities | Lower coverage in low-income |
| Coverage by Race/Ethnicity | Varies by demographic | Disparities documented | Gaps persist across groups |
| Rural vs Urban Coverage | ~40% rural vs 48% urban | Adults in different settings | 8 percentage point gap |
| Vaccine Composition | Trivalent only (3 components) | First all-trivalent season | B/Yamagata lineage removed |
| Thimerosal-Free Proportion | Majority of supply | Single-dose formulations | Multi-dose vials phased out |
| Self-Administration Approval | FluMist approved Sept 2024 | Ages 2-49 years | Available 2025-26 season |
| VFC Program Coverage | No-cost vaccines available | Eligible children through age 18 | Continued program support |
| Insurance Coverage Status | Most plans cover at no cost | Preventive care inclusion | ACA mandate coverage |
Data source: CDC Weekly Flu Vaccination Dashboard, FluVaxView, National Immunization Survey-Flu (NIS-Flu), Immunization Information Systems
Vaccine distribution and coverage during the 2024-2025 influenza season fell short of public health goals despite adequate supply availability, with particularly concerning declines observed among pediatric populations. By March 8, 2025, manufacturers had distributed 147.6 million doses of influenza vaccine throughout the United States, approaching the projected supply of 148 million doses that manufacturers had committed to producing for the season. This distribution represented one of the largest vaccine deployment efforts in recent years and ensured that supply shortages did not serve as barriers to vaccination. However, actual vaccination uptake told a different story, with coverage rates stagnating or declining across most demographic groups.
Among children aged 6 months through 17 years, only 49.2% had received a flu vaccination by April 26, 2025, according to data from the National Immunization Survey-Flu, representing a 4.2 percentage point decline compared to the 53.4% coverage achieved at the same timepoint during the 2023-2024 season. This drop in pediatric vaccination occurred during what would become the deadliest flu season on record for children, with 279 pediatric deaths reported and 89.4% of eligible deceased children not having been fully vaccinated. Adult vaccination coverage remained similarly suboptimal at 46.7% as of late April 2025, essentially unchanged from the 47.4% observed during the previous season. Coverage varied substantially by age group, with adults aged 65 years and older achieving the highest rates at approximately 71%, while young adults aged 18-49 years recorded the lowest coverage at around 35%. Physicians’ offices administered approximately 21.5 million doses through mid-April 2025, representing roughly 4 million fewer doses than had been administered through the same timepoint during the 2023-2024 season, suggesting reduced healthcare system engagement with flu vaccination efforts.
Significant disparities persisted across multiple demographic dimensions throughout the season. Coverage varied by children’s age, mother’s education level, poverty status, race and ethnicity, and urbanicity, with children from disadvantaged backgrounds consistently showing lower vaccination rates. Among adults, similar patterns emerged, with vaccination coverage varying by age, disability status, health insurance status, poverty level, race and ethnicity, sex, and geographic location. Geographic disparities proved particularly striking, with approximately 40% of rural adults receiving vaccination compared to 48% of urban residents, representing an 8 percentage point coverage gap that likely contributed to differential disease burden across communities. The 2024-2025 season marked the first year that all influenza vaccines manufactured for the United States were trivalent formulations containing only three virus components rather than the quadrivalent vaccines used in prior seasons, following the global disappearance of the influenza B/Yamagata lineage after March 2020. This simplification of vaccine composition did not appear to negatively impact supply or access, though it coincided with the unfortunate decline in overall uptake rates.
Disease Burden and Vaccine-Prevented Outcomes in the US 2025
| Burden Estimate | Without Vaccination | With Vaccination | Prevention Impact |
|---|---|---|---|
| Symptomatic Illnesses | 52.4-89 million estimated | 43-73 million occurred | 9.4-16 million prevented |
| Medical Visits | 23.4-39 million estimated | 19-32 million occurred | 4.4-7.1 million prevented |
| Hospitalizations | 730,000-1.46 million estimated | 560,000-1.1 million occurred | 170,000-360,000 prevented |
| Deaths | 50,000-138,000 estimated | 38,000-99,000 occurred | 12,000-39,000 prevented |
| Older Adult Hospitalizations | 57% of total admissions | Ages ≥65 years | Disproportionate elderly burden |
| Vaccine Coverage Range | 35% (adults 18-49) to 71% (65+) | Varied by age group | Coverage influenced prevention |
| Outpatient VE Range | 37-56% effectiveness | Against H1N1 specifically | Moderate protection achieved |
| Inpatient VE Range | 39-62% effectiveness | Against H1N1 hospitalization | Higher protection vs severe disease |
| Economic Impact | Billions in healthcare costs | Medical and indirect costs | Substantial savings from prevention |
| Burden Estimate Period | Sept 29, 2024 – April 26, 2025 | Preliminary estimates | Updated later in 2025 |
| Severity Classification | High severity season | All age groups affected | First since 2017-18 season |
| H1N1 Contribution | 53.1% of flu A burden | Among subtyped viruses | Dominant strain impact |
| Peak Burden Period | Early February 2025 | Week 6 of 2025 | Later than recent seasons |
| Season Duration | 17 weeks above baseline | ILI surveillance metric | Similar to pre-pandemic seasons |
| Multiple Indicator Impact | High across all metrics | Hospitalizations, deaths, outpatient | Comprehensive severity assessment |
Data source: CDC Preliminary Burden Estimates 2024-2025, Disease Burden and Prevention Models, FluView Surveillance Summary
The Centers for Disease Control and Prevention’s mathematical modeling revealed the enormous public health impact of the 2024-2025 influenza season and the substantial benefit provided by vaccination despite suboptimal coverage rates. Preliminary end-of-season estimates for the period from September 29, 2024, through April 26, 2025, indicate that influenza virus infections likely resulted in between 43 million and 73 million symptomatic illnesses, 19 million to 32 million medical visits, 560,000 to 1.1 million hospitalizations, and 38,000 to 99,000 deaths. These staggering figures positioned the 2024-2025 season among the most severe on record, surpassing all recent influenza seasons except for death counts during the high severity 2017-2018 season. Older adults aged 65 years and older accounted for 57% of all influenza hospitalizations, underscoring the disproportionate burden carried by elderly populations despite their higher vaccination rates of approximately 71%.
The vaccine-prevented burden calculations demonstrated that even with moderate effectiveness and incomplete coverage, influenza vaccination saved thousands of lives and prevented hundreds of thousands of severe outcomes. CDC’s compartmental modeling approach estimated that without any vaccination, the 2024-2025 season would have produced between 52.4 million and 89 million symptomatic illnesses, 23.4 million to 39 million medical visits, 730,000 to 1.46 million hospitalizations, and 50,000 to 138,000 deaths. By comparing these estimates with what actually occurred, researchers determined that influenza vaccination prevented between 9.4 million and 16 million symptomatic illnesses, 4.4 million to 7.1 million medical visits, 170,000 to 360,000 hospitalizations, and 12,000 to 39,000 deaths during the season. These prevention numbers translate to enormous healthcare system savings, reduced suffering, and preserved lives despite vaccine effectiveness figures that ranged from 32% to 60% in outpatient settings and 39% to 78% against hospitalization depending on age group and surveillance network. The economic implications extend beyond direct medical costs to include reduced work absenteeism, decreased caregiver burden, and avoided productivity losses across the American economy. With H1N1pdm09 accounting for 53.1% of subtyped influenza A viruses and contributing substantially to overall disease burden, the vaccine’s good antigenic match against this strain likely drove a significant proportion of the prevented outcomes, particularly among adult and elderly populations where H1N1 predominated at rates of 55.5% and 59.9% respectively.
Pediatric Impact of H1N1 in the US 2025
| Pediatric Metric | 2024-25 Season Data | Age Distribution | Clinical Outcomes |
|---|---|---|---|
| Pediatric Deaths Total | 279 confirmed deaths | Ages 2 weeks – 17 years | Highest seasonal epidemic ever |
| H1N1 Pediatric Deaths | 95 (56.2% of subtyped) | Among 169 subtyped flu A | Majority of subtyped deaths |
| H3N2 Pediatric Deaths | 73 (43.2% of subtyped) | Among 169 subtyped flu A | Substantial contribution |
| Mean Age at Death | 7 years | Range: 2 weeks to 17 years | Affects all pediatric ages |
| Hospital Admission Deaths | 143 (51.6%) | Among 277 with known location | Majority died in hospital |
| Emergency Room Deaths | 73 (26.4%) | Among 277 with known location | Significant ED mortality |
| Out-of-Hospital Deaths | 61 (22.0%) | Among 277 with known location | Rapid progression cases |
| Underlying Conditions Present | 147 (56.5%) | Among 260 with known history | Medical complexity present |
| Previously Healthy Children | 113 (43.5%) | Among 260 with known history | No prior risk factors |
| Unvaccinated Deaths | 185 (89.4%) | Among 207 vaccine-eligible | Overwhelmingly unvaccinated |
| Vaccination Coverage (All Children) | 49.2% coverage rate | Ages 6 months – 17 years | Below optimal levels |
| ILI Peak (Ages 0-4) | 18.1% of visits | Week ending Feb 8, 2025 | Highest pediatric rate |
| ILI Peak (Ages 5-24) | 12.6% of visits | Week ending Feb 8, 2025 | Second highest rate |
| Hospitalization Rate (Ages 0-4) | 104.7 per 100,000 | Cumulative season rate | Third highest age group |
| Hospitalization Rate (Ages 5-17) | 40.4 per 100,000 | Cumulative season rate | Lowest overall rate |
| Underlying Conditions (Hospitalized) | 53.5% had ≥1 condition | Among 3,109 hospitalized children | Asthma most common |
Data source: CDC Influenza-Associated Pediatric Mortality Surveillance System, FluSurv-NET Pediatric Data, ILINet Age-Stratified Reports
The pediatric impact of the 2024-2025 influenza season proved catastrophic, with H1N1pdm09 playing a leading role in child mortality and severe illness. The season produced 279 laboratory-confirmed influenza-associated deaths among children younger than 18 years, representing the highest number ever reported during a seasonal influenza epidemic since pediatric influenza deaths became nationally notifiable in 2004. This tragic toll exceeded the previous record of 207 pediatric deaths during the 2023-2024 season and far surpassed the lowest recorded toll of 37 deaths during the 2011-2012 season. Among the 169 pediatric influenza A deaths with available subtype information, 95 deaths (56.2%) were attributed to H1N1pdm09, while 73 deaths (43.2%) involved H3N2 viruses, confirming that H1N1 claimed the majority of identifiable influenza A pediatric fatalities.
The clinical circumstances surrounding these deaths revealed concerning patterns about disease progression and healthcare access. Among the 277 children with known location of death, approximately half at 143 children (51.6%) died after hospital admission despite receiving medical care, while 73 children (26.4%) died in emergency rooms, and 61 children (22.0%) died outside hospital settings before medical intervention could occur. The mean age at death was 7 years, but the range spanned from infants as young as 2 weeks old to adolescents 17 years of age, demonstrating that no pediatric age group remained safe from severe influenza complications. Medical history data available for 260 children showed that 147 (56.5%) had at least one underlying medical condition associated with increased risk for serious influenza complications, including asthma, neurologic disease, and obesity as the most common conditions. However, the remaining 113 children (43.5%) were previously healthy with no identified risk factors, shattering the misconception that only medically vulnerable children face life-threatening influenza outcomes. The most preventable aspect of this pediatric mortality crisis involved vaccination status: among the 207 vaccine-eligible children aged 6 months or older with known vaccination history, 185 children (89.4%) had not been fully vaccinated against influenza at the time of their illness, representing a staggering 89.4% of potentially preventable deaths. This finding occurred in the context of declining pediatric vaccination coverage, which dropped to 49.2% for the 2024-2025 season compared to 53.4% during the previous year, creating a large pool of susceptible children vulnerable to severe disease.
Antiviral Resistance Patterns for H1N1 in the US 2025
| Resistance Metric | H1N1pdm09 Data | H3N2 Comparison | Clinical Significance |
|---|---|---|---|
| Total Viruses Tested (NA) | 1,962 H1N1 specimens | 2,244 H3N2 specimens | Sequence-based analysis |
| Functional Assay Testing | 487 total viruses (9.6%) | Subset of sequenced viruses | Neuraminidase inhibition assay |
| NA-H275Y Mutations | 8 viruses (0.4% of H1N1) | Not applicable | Oseltamivir/peramivir resistance |
| NA-S247R Mutation | 1 virus (<0.1% of H1N1) | Not applicable | Multi-drug reduced inhibition |
| NA-I223V + S247N | 1 virus (<0.1% of H1N1) | Not applicable | Oseltamivir reduced inhibition |
| Oseltamivir Resistance | 8 highly reduced viruses | 2 H3N2 highly reduced | Low overall resistance |
| Peramivir Resistance | 9 highly reduced viruses | 0 H3N2 highly reduced | Primarily H1N1 issue |
| Zanamivir Resistance | 1 reduced inhibition | 1 H3N2 reduced | Very low for both subtypes |
| Baloxavir Testing (PA) | 1,813 H1N1 specimens | 2,197 H3N2 specimens | Endonuclease inhibitor testing |
| PA-I38T Mutation (H1N1) | 1 virus (<0.1%) | 1 H3N2 with PA-I38T | Baloxavir reduced susceptibility |
| Normal Susceptibility Rate | >99% of H1N1 viruses | >99% of all viruses | Antivirals remain effective |
| Resistance Surveillance Period | Sept 29, 2024 – Aug 30, 2025 | Entire 2024-25 season | Continuous monitoring |
| Clinical Impact | Minimal | Sporadic resistance only | Treatment guidelines unchanged |
| FDA-Approved Antivirals | 4 drugs recommended | Oseltamivir, peramivir, zanamivir, baloxavir | Multiple options available |
| Treatment Recommendation | Initiate promptly | High-risk patients priority | Early treatment critical |
Data source: CDC Antiviral Resistance Surveillance, Virus Characterization Reports, Neuraminidase Inhibitor Susceptibility Testing 2024-2025
Antiviral resistance monitoring throughout the 2024-2025 influenza season provided reassuring evidence that treatment options remained highly effective against circulating H1N1pdm09 viruses, with resistance detected in only a tiny fraction of tested specimens. Between September 29, 2024, and August 30, 2025, the Centers for Disease Control and Prevention characterized 5,078 viruses for susceptibility to neuraminidase inhibitors using sequence-based analysis, including 1,962 H1N1pdm09 specimens. A representative subset of 487 viruses (9.6%) underwent functional neuraminidase inhibition assay testing to phenotypically confirm resistance patterns. Among H1N1pdm09 viruses, sequence analysis identified 12 specimens with known or suspected mutations associated with reduced inhibition by neuraminidase inhibitors, representing less than 1% of all tested H1N1 samples.
The most significant resistance mutation detected involved the NA-H275Y amino acid substitution, found in 8 H1N1pdm09 viruses (0.4%). When six of these viruses underwent functional assay testing, they demonstrated highly reduced inhibition by oseltamivir and peramivir as expected, though they remained susceptible to zanamivir. One H1N1 virus carried the NA-S247R mutation, displaying reduced inhibition by oseltamivir and zanamivir plus highly reduced inhibition by peramivir, representing a more concerning multi-drug resistance pattern. Another H1N1 specimen contained a combination of NA substitutions I223V and S247N, showing reduced inhibition specifically by oseltamivir. Despite these isolated resistance cases, more than 99% of H1N1pdm09 viruses tested displayed normal susceptibility to all neuraminidase inhibitors, meaning that oseltamivir, peramivir, and zanamivir remained highly effective treatment options throughout the season. Testing for susceptibility to baloxavir, a polymerase acidic endonuclease inhibitor with a different mechanism of action, involved 4,862 viruses through sequence-based analysis and 486 viruses (10%) through phenotypic testing. Only 1 H1N1pdm09 virus contained the PA-I38T amino acid substitution associated with reduced baloxavir susceptibility, confirming that this newer antiviral agent also remained broadly effective. The remarkably low resistance rates observed throughout the 2024-2025 season meant that CDC treatment guidelines remained unchanged, with healthcare providers continuing to have four FDA-approved influenza antiviral drugs available for patient management: oseltamivir, peramivir, zanamivir, and baloxavir. The agency continued recommending prompt initiation of antiviral treatment for patients with confirmed or suspected influenza who have severe, complicated, or progressive illness, who require hospitalization, or who are at increased risk for influenza-associated complications, regardless of vaccination status or time since symptom onset.
Regional Variations of H1N1 Activity in the US 2025
| HHS Region | Peak Timing | Hospitalization Rate | ILI Peak Percentage |
|---|---|---|---|
| Region 1 (New England) | Early February 2025 | Data by region available | 22 weeks elevated activity |
| Region 2 (NY/NJ/PR) | Week 5 (Feb 1, 2025) | 218.3 per 100,000 (highest) | Same peak: weeks 4, 5, 7 |
| Region 3 (Mid-Atlantic) | Week 6 (Feb 8, 2025) | Within national range | 22 weeks elevated activity |
| Region 4 (Southeast) | Late January/Week 5 | Regional rate available | Peaked week 5 |
| Region 5 (Midwest) | Week 7 (Feb 15, 2025) | Regional rate available | Peaked week 6 |
| Region 6 (South Central) | Late January/Week 5 | Regional rate available | 10.9% ED visits (highest) |
| Region 7 (Central) | Week 7 (Feb 15, 2025) | Regional rate available | 5.5% ED visits (lowest) |
| Region 8 (Mountain) | Week 6 (Feb 8, 2025) | Regional rate available | 5.5% ED visits (lowest) |
| Region 9 (West Coast) | Late January | Regional rate available | ED peak week 52 (Dec 28) |
| Region 10 (Pacific NW) | Week 6 (Feb 8, 2025) | 117.8 per 100,000 (lowest) | 19 weeks elevated activity |
| National Peak | Week 6 (Feb 8, 2025) | 161.5 per 100,000 cumulative | 7.9% ILI peak |
| Regional Rate Range | Varied by 2-3 weeks | 117.8 to 218.3 per 100,000 | Wide geographic variation |
| Earliest Peak Region | Region 9 (West Coast) | Week 52 (December) | Earlier than national trend |
| Latest Peak Regions | Regions 5, 7 | Week 7 (mid-February) | Latest among all regions |
| H1N1 Distribution | Similar across regions | 53.1% national average | Co-circulation with H3N2 |
Data source: CDC FluView Interactive Regional Data, NHSN Hospital Respiratory Data by Region, ILINet Regional Surveillance Reports
Geographic analysis of the 2024-2025 influenza season revealed substantial regional variation in timing, intensity, and duration of H1N1 activity across the ten Health and Human Services regions. The West Coast experienced the earliest peak activity, with Region 9 reaching its maximum influenza burden during the week ending December 28, 2024, approximately five to seven weeks earlier than most other regions. In contrast, the Central and Midwest regions (Regions 5 and 7) peaked latest during the week ending February 15, 2025. The majority of regions, including Southeast, South Central, Mid-Atlantic, Mountain, and Pacific Northwest (Regions 4, 6, 3, 8, and 10), reached peak activity during the week ending February 8, 2025, aligning with the national peak.
Cumulative hospitalization rates demonstrated remarkable geographic disparities, ranging from a low of 117.8 per 100,000 population in Region 10 (Pacific Northwest) to a high of 218.3 per 100,000 in Region 2 (New York/New Jersey/Puerto Rico), representing nearly a two-fold difference in severe disease burden between regions. Emergency department data from the National Syndromic Surveillance System showed that Region 6 (South Central) experienced the highest peak of influenza-related ED visits at 10.9% during the week ending February 1, 2025, while Regions 7 and 8 (Central and Mountain) recorded the lowest peaks at 5.5% during the week ending February 8, 2025. The duration of elevated influenza-like illness activity also varied considerably, with Regions 1 and 3 (New England and Mid-Atlantic) experiencing the longest span at 22 consecutive weeks above baseline, while Region 8 (Mountain) had the shortest period at just 12 weeks. These regional differences likely reflected variations in population density, climate patterns, school calendars, community mitigation measures, vaccination coverage rates, and the specific timing of H1N1pdm09 versus H3N2 predominance in different geographic areas. Despite these variations, H1N1pdm09 maintained its status as the dominant influenza A subtype at approximately 53% across nearly all regions, though the relative proportions of H1N1 versus H3N2 fluctuated throughout the season and varied somewhat by location, contributing to the overall high severity classification observed nationally.
Vaccine Recommendations and Policy Updates for the US 2025-2026
| Policy Element | 2025-26 Recommendation | Change from 2024-25 | Implementation Details |
|---|---|---|---|
| Vaccine Composition | Trivalent formulation | No change | H1N1, H3N2, B/Victoria only |
| H1N1 Component (Egg-based) | A/Victoria/4897/2022-like | No change | Same strain continued |
| H1N1 Component (Cell/Recombinant) | A/Wisconsin/67/2022-like | No change | Same strain continued |
| H3N2 Component (Egg-based) | A/Croatia/10136RV/2023-like | Changed | Updated from Thailand/8/2022 |
| H3N2 Component (Cell/Recombinant) | A/District of Columbia/27/2023-like | Changed | Updated from Massachusetts/18/2022 |
| B/Victoria Component | B/Austria/1359417/2021-like | No change | Same for all vaccine types |
| Age Eligibility | ≥6 months of age | Universal recommendation | All persons without contraindications |
| Vaccination Timing | September-October optimal | Before flu activity increases | Can give throughout season |
| FluMist Self-Administration | Ages 2-49 years | New for 2025-26 | FDA approved September 2024 |
| Vaccine Types Available | Multiple formulations | Standard, high-dose, adjuvanted | Options by age and preference |
| Two-Dose Pediatric Schedule | Children 6 months-8 years | If first-time or <2 prior doses | 4 weeks apart minimum |
| Pregnant Women Priority | Recommended any trimester | Protects mother and infant | Antibody transfer documented |
For the 2025–26 U.S. influenza season, health authorities continue to recommend a trivalent vaccine formulation, covering H1N1, H3N2, and the B/Victoria lineage. The H1N1 components remain unchanged, with the same A/Victoria/4897/2022-like strain for egg-based vaccines and A/Wisconsin/67/2022-like for cell-based and recombinant vaccines. However, the H3N2 strains have been updated to improve protection, with new Croatia and District of Columbia variants replacing older lineages from Thailand and Massachusetts. The B/Victoria component remains B/Austria/1359417/2021-like. The recommendation continues to apply to all individuals aged 6 months and older, with vaccination ideally given during September–October, before peak flu activity. Multiple formulation options — including standard-dose, high-dose, and adjuvanted vaccines — ensure age-appropriate and preference-based coverage.
Several practical policy updates strengthen access to influenza protection in 2025–26. The self-administered FluMist nasal spray option becomes newly available for ages 2–49 years, expanding convenience and coverage opportunities. Young children aged 6 months to 8 years who have never been vaccinated or have received fewer than two prior doses are recommended a two-dose schedule, spaced a minimum of four weeks apart for reliable immunity. Pregnant women are prioritized at any trimester, with proven benefits in protecting both the mother and newborn. These updated strain selections, broader delivery options, and continued universal vaccination guidance aim to reduce seasonal influenza risk across the United States and better protect high-risk populations.
Disclaimer: The data research report we present here is based on information found from various sources. We are not liable for any financial loss, errors, or damages of any kind that may result from the use of the information herein. We acknowledge that though we try to report accurately, we cannot verify the absolute facts of everything that has been represented.
