INTRODUCTION:Respiratory syncytial virus (RSV) is the leading cause of hospitalization among US infants. Characterizing service utilization during infant RSV hospitalizations may provide important information for prioritizing resources and interventions. OBJECTIVE:The objective of this study was to describe the procedures and services received by infants hospitalized during their first RSV episode in their first RSV season, in addition to what proportion of infants died during this hospitalization. METHODS:In this retrospective observational study, we analyzed three different administrative claims datasets to examine healthcare service utilization during RSV hospitalizations among infants. The study population included infants born between July 2016 and February 2020 who experienced an RSV episode during their first RSV season and had an associated inpatient hospitalization. We stratified infants into three comorbidity groups: healthy term, palivizumab-eligible, and other comorbidities. Outcomes included extracorporeal membrane oxygenation, supplemental oxygen use (in-hospital and post-discharge), mechanical ventilation (invasive and non-invasive), chest imaging, infant mortality, length of inpatient stay, intensive care unit (ICU) admission, and number of days in the ICU. RESULTS:Chest imaging was the most frequently administered procedure during RSV-associated hospitalizations, with approximately 34-38% of infants receiving it. Around one-quarter of infants were admitted to the ICU during their first RSV hospitalization. Median lengths of stay in the hospital were 3-4 days, extending to 4-6 days in the presence of ICU admission. Palivizumab-eligible infants had higher utilization of healthcare services and spent more time in the hospital or ICU compared to healthy infants or those with other comorbidities. CONCLUSIONS:This study provides insights into the utilization of healthcare services during RSV hospitalizations among infants. Understanding service utilization patterns can aid in improved management and resource allocation for infants in the United States, ultimately contributing to better outcomes and reduced healthcare costs overall. However, likely under-ascertainment of ventilation and oxygen-related services in insurance claims remains an impediment to studying these outcomes.

BACKGROUND:Respiratory syncytial virus (RSV) is the leading cause of infant hospitalization in the United States. Understanding healthcare utilization associated with medically attended (MA) RSV lower respiratory tract infection (LRTI) might inform research priorities aimed at reducing RSV-associated pediatric morbidity. We described healthcare utilization during acute MA RSV LRTI episodes within a geographically diverse cohort of infants in the United States. METHODS:We created retrospective cohorts of infants born in the United States from July 1, 2016 through February 29, 2020 in each of three de-identified insurance claims datasets: Merative MarketScan Commercial Claims and Encounters, Multi-State MarketScan Medicaid, and Optum's de-identified Clinformatics ® Data Mart. We identified infants' first MA RSV LRTI diagnosis during their first RSV season and followed them for 7 subsequent days to record outpatient, emergency department, and inpatient hospital utilization. We calculated the number of outpatient visits, emergency department visits, and inpatient hospital stays occurring during this acute episode and estimated the proportion of episodes involving ≥ 2 visits to a given healthcare setting. RESULTS:In the CCAE database, we identified 25,409 acute MA RSV LRTI episodes under the specific RSV definition and 69,068 under the sensitive definition. In the MDCD database, these totals were 67,357 and 170,744, while in the CDM database, they were 12,402 and 31,363, respectively. Across data sources, 34%-69% of infants' first acute MA RSV LRTI episodes involve 2 or more visits to a healthcare setting within 7 days. The percentage of episodes involving at least 2 visits ranged from 34-62% among healthy term infants, 38-65% for Palivizumab-eligible infants, and 38-69% for infants with other comorbidities. CONCLUSIONS:Within a week of their first MA RSV LRTI diagnosis, infants frequently experience at least 2 visits to one or more healthcare settings, regardless of their comorbidity profile. The percentage of MA RSV LRTI episodes involving at least 2 visits to a healthcare setting may vary by insurance claims database, even between commercial payers.

BACKGROUND:Respiratory syncytial virus (RSV) is a substantial cause of infant morbidity and mortality due to seasonal peaks of bronchiolitis across the United States. Clinical and viral surveillance plays a pivotal role in helping hospital systems prepare for expected surges in RSV bronchiolitis. Existing surveillance efforts have shown a geographic pattern of RSV positivity across the United States, with cases typically starting in the southeast and spreading north and west. Public health measures implemented due to the COVID-19 pandemic disrupted viral transmission across the nation and altered the expected seasonality of RSV. The impact of these changes on the geographic progression of infant RSV bronchiolitis across the United States has not been described. METHODS:Here, we used clinical and viral surveillance data from four health care systems located in different regions of the United States to describe the geographic progression of infant RSV bronchiolitis across the country from 2015 to 2023. RESULTS:Prior to widespread circulation of SARS-CoV-2, infant RSV bronchiolitis followed an established geographic pattern associated with seasonal epidemics originating in Florida and spreading north (North Carolina and New York) and later westward (Nevada). Although public health and social measures implemented during the COVID-19 pandemic disrupted the seasonality of RSV disease, infant RSV bronchiolitis epidemics progressed across the nation in a pattern identical to the prepandemic era. CONCLUSIONS:Our findings highlight the importance of ongoing clinical and viral surveillance to optimally track the onset of RSV epidemics and allow health care systems to prepare for expected RSV bronchiolitis surges.

BackgroundBronchiolitis due to respiratory syncytial virus (RSV) is the leading cause of hospitalization among American infants. The overall burden of RSV among infants has been historically under-estimated due to variable testing practices, particularly in the outpatient setting. Universal masking and social distancing implemented during the coronavirus disease 2019 (COVID-19) pandemic altered RSV seasonality, however potential consequences on RSV testing practices across different healthcare settings and sociodemographic groups have not been described. Variable testing practices could also affect accurate assessment of the effects of two recently approved RSV preventative agents targeting infants.MethodsUtilizing real-time clinical and viral surveillance, we examined RSV testing practices among infants with bronchiolitis within four United States healthcare systems across different healthcare settings and sociodemographic groups pre- and post-COVID-19.ResultsRSV testing among infants with bronchiolitis increased since 2015 within each healthcare system across all healthcare settings and sociodemographic groups, with a more dramatic increase since the COVID-19 pandemic. Outpatient testing remained disproportionately low compared to hospital-based testing, although there were no major differences in testing frequency among sociodemographic groups in either setting.ConclusionsAlthough RSV testing increased among infants with bronchiolitis, relatively low outpatient testing rates remain a key barrier to accurate RSV surveillance.

BACKGROUND:Respiratory syncytial virus (RSV) is a major cause of morbidity and mortality among US infants. A child's calendar birth month determines their age at first exposure(s) to RSV. We estimated birth month-specific risk of medically attended (MA) RSV lower respiratory tract infection (LRTI) among infants during their first RSV season and first year of life (FYOL). METHODS:We analyzed infants born in the USA between July 2016 and February 2020 using three insurance claims databases (two commercial, one Medicaid). We classified infants' first MA RSV LRTI episode by the highest level of care incurred (outpatient, emergency department, or inpatient), employing specific and sensitive diagnostic coding algorithms to define index RSV diagnoses. In our main analysis, we focused on infants' first RSV season. In our secondary analysis, we compared the risk of MA RSV LRTI during infants' first RSV season to that of their FYOL. RESULTS:Infants born from May through September generally had the highest risk of first-season MA RSV LRTI-approximately 6-10% under the specific RSV index diagnosis definition and 16-26% under the sensitive. Infants born between October and December had the highest risk of RSV-related hospitalization during their first season. The proportion of MA RSV LRTI events classified as inpatient ranged from 9% to 54% (specific) and 5% to 33% (sensitive) across birth month and comorbidity group. Through the FYOL, the overall risk of MA RSV LRTI is comparable across birth months within each claims database (6-11% under the specific definition, 17-30% under the sensitive), with additional cases progressing to care at outpatient or ED settings. CONCLUSIONS:Our data support recent national recommendations for the use of nirsevimab in the USA. For infants born at the tail end of an RSV season who do not receive nirsevimab, a dose administered prior to the onset of their second RSV season could reduce the incidence of outpatient- and ED-related events.

Introduction: New extended half-life antibodies for the single-dose prevention of medically attended (MA) respiratory syncytial virus (RSV) lower respiratory tract infection (LRTI) have been developed for administration to all infants before or during their first RSV season. For infants born during the season, administration as soon as feasible after birth would provide optimal protection and minimize access disparities. The objective of this study was to assess the time from birth hospitalization discharge to the first outpatient visit (FOV) among US infants in order to determine optimal site of administration for the extended half-life antibody. Material and methods: This retrospective, observational, time-to-event analysis uses the Merative (TM) MarketScan (R) Commercial and Multi-State Medicaid Databases. Time to FOV is reported separately for the COVID-19 and recent pre-COVID-19 eras and for commercially insured and Medicaid infants. Results: Overall, 73.8 % of Medicaid infants had an FOV within 5 days as compared to 84.7 % of commercially insured infants. Estimates were higher during the COVID-19 era. Urban commercially insured infants had much higher FOV completion than their counterparts. Among Medicaid infants, urban Black and rural White infants were least likely to complete their FOV within 5 days of birth hospitalization discharge. Discussion and conclusion: FOV within 5 days after birth hospitalization discharge for Medicaid infants is substantially lower than that of commercially insured infants. Approximately 1 in 4 Medicaid infants and 1 in 8 infants with commercial insurance did not have an outpatient visit within 5 days of birth hospitalization discharge. For US infants born during the RSV season, administration of extended half-life RSV antibodies in the newborn nursery prior to discharge would ensure optimal uptake and minimize access disparities.

AbstractBackground and AimsPolicymakers need data about the burden of respiratory syncytial virus (RSV) lower respiratory tract infections (LRTI) among infants. This study estimates quality of life (QoL) for otherwise healthy term US infants with RSV‐LRTI and their caregivers, previously limited to premature and hospitalized infants, and corrects for selective testing.MethodsThe study enrolled infants <1 year with a clinically diagnosed LRTI encounter between January and May 2021. Using an established 0–100 scale, the 36 infants’ and caregivers’ QoL at enrollment and quality‐adjusted life year losses per 1000 LRTI episodes (quality‐adjusted life years [QALYs]/1000) were validated and analyzed. Regression analyses examined predictors of RSV‐testing and RSV‐positivity, creating modeled positives.ResultsMean QoL at enrollment in outpatient (n = 11) LRTI‐tested infants (66.4) was lower than that in not‐tested LRTI infants (79.6,p = 0.096). For outpatient LRTI infants (n = 23), median QALYs/1000 losses were 9.8 and 0.25 for their caregivers. RSV‐positive outpatient LRTI infants (n = 6) had significantly milder QALYs/1000 losses (7.0) than other LRTI‐tested infants (n = 5)(21.8,p = 0.030). Visits earlier in the year were more likely to be RSV‐positive than later visits (p = 0.023). Modeled RSV‐positivity (51.9%) was lower than the observed rate (55.0%). Infants’ and caregivers’ QALYs/1000 loss were positively correlated (rho = 0.34,p = 0.046), indicating that infants perceived as sicker imposed greater burdens on caregivers.ConclusionsThe overall median QALYs/1000 losses for LRTI (9.0) and RSV‐LRTI (5.6) in US infants are substantial, with additional losses for their caregivers (0.25 and 0.20, respectively). These losses extend equally to outpatient episodes. This study is the first reporting QALY losses for infants with LRTI born at term or presenting in nonhospitalized settings, and their caregivers.

Supplementary Figure S5. BET bromodomain inhibitors and proteasome inhibitors exert synergistic anticancer effects against TERT-rearranged neuroblastoma cells.

Supplementary Figure S2. BRD4 is required for TERT expression and cell proliferation in TERT-rearranged neuroblastoma cells.

BACKGROUND:Respiratory syncytial virus (RSV) bronchiolitis is the leading cause of hospitalizations among infants in the United States. Unpredictability in RSV seasonality has occurred following the onset of the coronavirus disease 2019 (COVID-19) pandemic. Local surveillance networks can enhance the ability to appropriately time prophylaxis when exposure risk is highest. METHODS:A retrospective, cohort study was conducted to describe the epidemiologic patterns of RSV disease among outpatient, emergency department and inpatient encounters in children <5 years in Central New York before and after the onset of the COVID-19 pandemic. Local data were collected from October 2015 to January 2023 and compared to state-level data. Linear regression models were used to identify clinical and sociodemographic differences before and after the pandemic. RESULTS:Local variation in RSV seasonality was noted prior to the COVID-19 pandemic, however highly atypical circulation patterns appeared in the post-COVID-19 era. Since March 2020, patterns for local and state-defined RSV seasons have remained atypical (local season onset in 2021: week 27 and 2022: week 27; state season onset in 2021: week 31 and 2022: week 38). After adjusting for increases in testing, RSV bronchiolitis cases were not significantly different during pre- and post-pandemic eras. In comparison to the 2021 bronchiolitis season, the 2022 season had a higher proportion of RSV cases despite decreased testing. CONCLUSIONS:Temporal patterns for RSV have shifted during the COVID-19 pandemic. Local surveillance networks may be advantageous in trending community-level RSV activity to optimize prophylaxis administration. Changes in RSV testing patterns occurred throughout the study period and should be accounted for when describing infant and childhood RSV disease.

Re - Revised Supplementary Materials & Methods

Supplementary Figure S6. OTX015 and carfilzomib exert synergistic anticancer effects partly by inducing oxidative stress and endoplasmic reticulum stress.

Supplementary Table S1. Primary screening of the Food and Drug Administration-Approved Oncology Drugs (AODs) Set IV from the US National Cancer Institute for BET bromodomain inhibitor enhancers.

Abstract Background Respiratory syncytial virus (RSV) is the leading cause of US infant hospitalization [1]. The objective of this work is to describe the rate of US infant RSV hospitalization (RSVH) and related charges by race/ethnicity and type of insurance payer. Methods Using the US national Healthcare Cost and Utilization Project (HCUP) National Inpatient Sample (NIS) for 2018-2020, we identified US infant RSVH and charges by RSV diagnosis in any position, race and Hispanic origin of the mother (race/ethnicity), and type of insurance payer. Results RSVH Rates and Burden: Rates of US infant RSVH are highest among Native American infants, are similar for White, Black, and Hispanic infants, and are lower among Other/Unknown and Asian/Pacific Islander infants. The trend of RSVH rates being higher among infants covered by Medicaid is consistent across each race/ethnicity group (Figure 1). This results in a greater proportion of the US infant RSVH burden among Black, Hispanic, Asian/Pacific Islander, Native American, and Other/Unknown infants being among infants covered by Medicaid (Figure 2). Overall, 60% of US infant RSVH is among those covered by Medicaid. RSVH Charges: For White, Black, Native American and Other/Unknown race/ethnicity infants, mean RSVH total charges are higher for Medicaid insurance (Figure 3), despite Medicaid reimbursing at lower levels. For Hispanic and Asian/Pacific Islander race/ethnicity infants the opposite is true, charges are higher for infants with other/unknown insurance (Figure 3). This results in a greater proportion of the US infant RSVH total charges among all infants except White infants being among infants that pay with Medicaid (Figure 4). Overall, 63% of RSVH total charges is among infants that pay with Medicaid (Figure 4). Conclusion The prevention of RSVH in US infants covered by Medicaid would have a disproportionate benefit for Black, Hispanic, Asian/Pacific Islander, Native American and Other/Unknown race/ethnicity infants and contribute towards economic and race/ethnicity equity. Because 60% of US infant RSVHs are covered by Medicaid accounting for 63% of RSVH total charges, the prevention of RSVH among infants who paid through Medicaid would also have a substantial impact on overall RSVH burden and related costs. Disclosures Christopher B. Nelson, PhD MPH, Sanofi: employee|Sanofi: employee|Sanofi: Stocks/Bonds|Sanofi: Stocks/Bonds Xiaohui Jiang, MS, Sanofi: Grant/Research Support Mina Suh, MPH, International Health, AstraZeneca: Grant/Research Support|Sanofi: Grant/Research Support|Sobi: Grant/Research Support Jon Fryzek, PhD, MPH, Sanofi: Grant/Research Support

Abstract Background Respiratory syncytial virus (RSV) bronchiolitis (Br) is the leading cause of US infant hospitalizations and lower respiratory tract infections [1]. The historical seasonality of RSV in the US across all ages using passive laboratory surveillance has been described by the CDC (2014-17) [2] [3] and more recently by Staadegaard et al [4]. The objective of this analysis is to describe local variation in infant RSV-Br seasonality since the onset of COVID-19 in March 2020, across all facilities and all settings in four health systems across the US, and compare to trends from the recent pre-COVID-19 era. Methods From October 2015-April 2023 we conducted real-time surveillance across all facilities and all settings in the State University of New York (SUNY) Upstate Medical University Health System in Syracuse, NY; the Duke University Health System (DUHS) in Durham, NC; the University of South Florida Health and Tampa General Hospital (TGH) in Tampa, FL; and the Renown Regional Medical Center Health System (Renown) in Reno, NV. We assessed electronic health records (EHR) of infants (0-11 months) using ICD-10 codes for Br and RSV-Br and laboratory testing data to describe infant RSV-Br encounters. For additional methods see Movva N et al [5]. Results During the recent pre-COVID-19 era, there was regional variation in seasonal onset and peak disease burden of infant RSV-Br (Figure 1). Onset and peak disease was earliest at TGH in the southeast and latest at Renown in the west. While occurring outside the historical seasonality timeframe, interseasonal 2021 and early seasonal 2022 infant RSV-Br was similar in duration and followed the same geographic pattern of onset to historical disease. Conclusion After the US onset of COVID-19 in March 2020, the geographic pattern of RSV-Br onset and peaks was preserved, despite the shift to interseasonal and early seasonal disease in 2021 and 2022, respectively. RSV surveillance is useful to determine whether RSV seasonality is returning to typical historic patterns. These results highlight the need for health system-level surveillance and can guide discussion regarding administration of candidate RSV vaccines and immunoprophylaxis and assessment of their impact. Disclosures Mina Suh, MPH, International Health, AstraZeneca: Grant/Research Support|Sanofi: Grant/Research Support|Sobi: Grant/Research Support ZACHARY Wolf, MBA, MS, AstraZeneca: Grant/Research Support|Sanofi: Grant/Research Support Emory Waddell, n/a, Sanofi: Research support Christopher B. Nelson, PhD MPH, Sanofi: employee|Sanofi: employee|Sanofi: Stocks/Bonds|Sanofi: Stocks/Bonds

Background Limited data are available on the economic costs of respiratory syncytial virus (RSV) infections among infants and young children in the United States. Methods We performed a systematic literature review of 10 key databases to identify studies published between 1 January 2014 and 2 August 2021 that reported RSV-related costs in US children aged 0-59 months. Costs were extracted and a systematic analysis was performed. Results Seventeen studies were included. Although an RSV hospitalization (RSVH) of an extremely premature infant costs 5.6 times that of a full-term infant ($10 214), full-term infants accounted for 82% of RSVHs and 70% of RSVH costs. Medicaid-insured infants were 91% more likely than commercially insured infants to be hospitalized for RSV treatment in their first year of life. Medicaid financed 61% of infant RSVHs. Paying 32% less per hospitalization than commercial insurance, Medicaid paid 51% of infant RSVH costs. Infants' RSV treatment costs $709.6 million annually, representing $187 per overall birth and $227 per publicly funded birth. Conclusions Public sources pay for more than half of infants' RSV medical costs, constituting the highest rate of RSVHs and the highest expenditure per birth. Full-term infants are the predominant source of infant RSVHs and costs.

In the United States, respiratory syncytial virus (RSV) infects two-thirds of infants by the age of 12 months [1] and has been identified as the leading cause of infant hospitalization [2]. RSV is historically seasonal, with cases accumulating rapidly over a few weeks at the start of the winter respiratory disease season [3] and with notable geographic variation in the timing of onset [4, 5]. RSV is not a reportable disease in the United States [6]. However, the US Centers for Disease Control and Prevention (CDC) National Respiratory and Enteric Virus Surveillance System monitors trends in RSV positivity using laboratory data [7, 8] and provided notification of interseasonal circulation during the summer of 2021 [9, 10] after the onset of coronavirus disease 2019 (COVID-19) in early 2020. No RSV prevention strategy is currently available for all infants, but with several candidates scheduled to complete phase III clinical development in the coming years [11] universal immunization against infant RSV lower respiratory tract infection (LRTI) may soon be feasible. RSV prevention in infants first became possible in the 1990s with the licensure and availability of RSV intravenous immunoglobulin (RespiGam; MedImmune) and then the monoclonal antibody (mAb) palivizumab (Synagis; MedImmune), both administered monthly during the RSV season [12, 13]. Although the CDC Advisory Committee on Immunization Practices has not made a recommendation for the use of palivizumab, the American Academy of Pediatrics Committee on Infectious Diseases recommends a risk-based strategy for the prevention of RSV LRTI in a targeted population of high-risk infants [14]. The limited benefit of palivizumab for the prevention of severe disease in the general population of infants has prevented expansion to the entire infant cohort [14]. However, the substantial burden of RSVdisease in infantsmakes it clear that the adoption of a broad public health aimof reducingRSVLRTI among all infants could have a significant impact on healthcare utilization. To support the upcoming policy discussion, this supplement demonstrates the burden of RSV-associated medically attended LRTIs (RSV MA-LRTIs) in all infants across all healthcare settings.

Abstract Background Healthcare utilization during acute medically attended (MA) RSV-associated lower respiratory tract infection (LRTI) episodes remains poorly characterized, particularly among term infants without comorbidities. Describing the care incurred during these episodes may provide important information regarding the impact of MA RSV LRTI. Objective: Estimate the occurrence and average number of outpatient, emergency department (ED), and inpatient visits during infants' first RSV season. Methods Using deidentified insurance claims data (MarketScan Commercial ®, MSC; MarketScan Medicaid ®, MSM; Optum Clinformatics ®, OC), we assembled a cohort of infants born in the United States between April 1, 2016 and February 29, 2020 and identified their first MA RSV LRTI episode during their first RSV season. We defined an RSV episode as the 7 days following an index RSV diagnosis (inclusive of the diagnosis date), allowing for two alternative definitions of the index diagnosis—a specific definition, based on ICD-10 codes explicitly indicating RSV, and a sensitive definition, including codes for unspecified bronchiolitis. We calculated the average number of outpatient, ED, and inpatient visits during this episode, stratifying estimates by gestational age and the presence/absence of comorbidities. We also calculated the proportion of episodes involving a given place of service. Results Using the specific (sensitive) definitions, infants averaged 1.11 (1.19), 0.90 (0.89), and 1.56 (1.39) outpatient visits during their first acute RSV episode, in the MSC, MSM, and OC datasets, respectively (Table). They averaged 0.35 (0.23), 0.51 (0.47), and 0.39 (0.23) visits to the ED and 0.28 (0.14), 0.25 (0.14), and 0.23 (0.11) inpatient stays. While MA RSV LRTI episodes among infants who were preterm and/or had other comorbidities (comorbidity groups B and C) were more likely to involve an ED or inpatient visit, up to 21% (10%) of episodes among otherwise healthy term infants involved an inpatient stay. Conclusion Up to 1 in 5 infants experiencing an MA RSV LRTI episode during their first RSV season visited an inpatient setting, including between 9% and 21% of otherwise healthy term infants. This study was funded by Sanofi and AstraZeneca. Disclosures Jason Gantenberg, PhD, MPH, Sanofi: Grant/Research Support Robertus van Aalst, PhD, MSc, Sanofi: Stocks/Bonds David R. Diakun, BS, Sanofi: Employed by IBM Watson Health which was contracted by Sanofi to perfom outcomes research|Sobi: Employed by IBM Watson Health which was contracted by Sobi to conduct the study Christopher B. Nelson, PhD MPH, Sanofi: employee|Sanofi: Stocks/Bonds David A. Savitz, PhD, Sanofi-Pasteur: Grant/Research Support|Sanofi-Pasteur: Honoraria Andrew R. Zullo, PharmD, PhD, Sanofi: Grant/Research Support.

Background This study describes leading causes of hospitalization, including respiratory syncytial virus (RSV), in United States infants (<1 year) from 2009 through 2019. Methods Within the National (Nationwide) Inpatient Sample (NIS) data, hospitalizations were determined by primary diagnosis using International Classification of Diseases, Ninth or Tenth Revision codes. RSV was defined as 079.6, 466.11, 480.1, B97.4, J12.1, J20.5, or J21.0. Bronchiolitis was defined as 466.19, J21.8, or J21.9. Leading causes overall and by sociodemographic variables were identified. The Kids' Inpatient Database (KID) was used for confirmatory analyses. Results Acute bronchiolitis due to RSV (code 466.11 or J21.0) was the leading primary diagnosis, accounting for 9.6% (95% confidence interval [CI], 9.4%-9.9%) and 9.3% (95% CI, 9.0%-9.6%) of total infant hospitalizations from January 2009 through September 2015 and October 2015 through December 2019, respectively; it was the leading primary diagnosis in every year accounting for >10% of total infant hospitalizations from December through March, reaching >15% in January-February. From 2009 through 2011, acute bronchiolitis due to RSV was the leading primary diagnosis in every birth month. Acute bronchiolitis due to RSV was the leading cause among all races/ethnicities, except Asian/Pacific Islanders, and all insurance payer groups. KID analyses confirmed these results. Conclusions Acute bronchiolitis due to RSV is the leading cause of US infant hospitalizations.