BACKGROUND: Risk factors for severe respiratory syncytial virus (RSV) illness include early infancy, premature birth, and underlying medical conditions. However, the clinical significance of respiratory viral co-detection is unclear. We compared the clinical outcomes of young children with RSV-only detection and those with RSV viral co-detection. METHODS: We conducted active, population-based surveillance of children with medically attended fever or respiratory symptoms at 7 US medical centers (1 December 2016-31 March 2020). Demographic and clinical data were collected through parental interviews and chart abstractions. Nasal swabs, with or without throat swabs, were systematically tested for RSV and 6 other common respiratory virus groups. We compared clinical outcomes, including hospitalization, and among those hospitalized, length of stay, intensive care unit admission, supplemental oxygen use, and intubation, between children aged <2 years with RSV-only detection and those with RSV co-detection. RESULTS: We enrolled 18 008 children aged <2 years. Of 17 841 (99.1%) tested for RSV, 5099 (28.6%) were positive. RSV was singly detected in 3927 children (77.0%) and co-detected in 1172 (23.0%). RSV co-detection with parainfluenza virus or adenovirus was associated with significantly lower odds of hospitalization (adjusted odds ratio, 0.56; 95% confidence interval [CI]: .33-.95; P = .031) and supplemental oxygen use (adjusted odds ratio, 0.66; 95% CI: .46-.95; P = .026), respectively, than RSV-only detection. For all other comparisons, we did not identify a significant association between RSV co-detection and worse clinical outcomes. CONCLUSIONS: Co-detection of RSV with another respiratory virus was not significantly associated with worse clinical outcomes compared with RSV-only detection.

IMPORTANCE: Enterovirus D68 (EV-D68) typically causes mild to severe acute respiratory illness (ARI). Testing and surveillance for EV-D68 in the US are limited, and important epidemiologic gaps remain. OBJECTIVE: To characterize the epidemiology and clinical severity of EV-D68 among US children seeking care for ARI from 2017 to 2022, using a multisite, active, systematic surveillance network. DESIGN, SETTING, AND PARTICIPANTS: This cross-sectional study collected data from the New Vaccine Surveillance Network, an active, prospective, population-based surveillance system of emergency departments (EDs) and hospitals at 7 US academic medical centers. Children with ARI and EV-D68-positive results were enrolled during platform-wide EV-D68 testing periods (July to October 2017, July to November 2018, July to November 2020, and July 2021 to December 2022). Included children were aged younger than 18 years, reported 1 or more qualifying ARI symptoms, with a symptom duration less than 14 days at enrollment. Data were analyzed from in October 2024. EXPOSURES: Laboratory-confirmed EV-D68 infection, including overall infections or those without viral codetection. MAIN OUTCOMES AND MEASURES: Trends and characteristics of EV-D68, including demographics, underlying conditions, and clinical severity by health care setting, were explored. Among hospitalized children with EV-D68-positive results without viral codetection, multivariable logistic regression was used to examine factors associated with receipt of (1) supplemental oxygen or (2) intensive care. RESULTS: From 2017 to 2022, 976 children with EV-D68-positive results were identified (median [IQR] age, 47 [18-63] months; 391 [40.1%] female); most were enrolled in 2018 (382 children) and 2022 (533 children). Among these, 856 had no viral codetection, of which 320 were discharged home from the ED (median [IQR] age, 33 [16-59] months; 180 male [56.3%]; 237 [74.1%] with no reported underlying conditions) and 536 were hospitalized (median [IQR] age, 40 [19-69] months; 330 male [61.6%]; 268 [50.0%] with no reported underlying conditions). Among those hospitalized, 199 (37.1%) reported a history of asthma or reactive airway disease (RAD) and 77 (14.4%) reported a condition other than asthma or RAD. Having an underlying condition other than asthma or RAD was associated with increased odds of receiving supplemental oxygen (adjusted odds ratio, 2.72; 95% CI, 1.43-5.18) or intensive care admission (adjusted odds ratio, 3.09; 95% CI, 1.72-5.56); neither age group nor history of asthma or RAD were associated with oxygen receipt or intensive care admission. CONCLUSIONS AND RELEVANCE: In this cross-sectional study of children with medically attended EV-D68 infections, EV-D68 was associated with severe disease in otherwise healthy children of all ages, and children with nonasthma or RAD comorbidities were at higher risk for severe outcomes when hospitalized.

Maternal respiratory syncytial virus (RSV) vaccine and nirsevimab, a long-acting monoclonal antibody for infants aged 0-7 months and children aged 8-19 months who are at increased risk for severe RSV disease, became widely available for prevention of severe RSV disease among infants and young children during the 2024-25 RSV season. To evaluate the association between availability of these products and infant and child RSV-associated hospitalization rates, the rates among children aged <5 years were compared for the 2024-25 and 2018-20 RSV seasons using data from the RSV-Associated Hospitalization Surveillance Network (RSV-NET) and New Vaccine Surveillance Network (NVSN). Among infants aged 0-7 months (eligible for protection with maternal vaccination or nirsevimab), 2024-25 RSV-associated hospitalization rates were lower compared with 2018-20 pooled rates (estimated relative rate reductions of 43% [RSV-NET: 95% CI = 40%-46%] and 28% [NVSN: 95% CI = 18%-36%]). The largest estimated rate reduction was observed among infants aged 0-2 months (RSV-NET: 52%, 95% CI = 49%-56%; NVSN: 45%, 95% CI = 32%-57%) and during peak hospitalization periods (December-February). These findings support Advisory Committee on Immunization Practices' recommendations for maternal vaccination or nirsevimab to protect against severe RSV disease in infants and highlight the importance of implementing the recommendations to protect infants as early in the RSV season as possible, before peak transmission, and for infants born during the RSV season, within the first week of life, ideally during the birth hospitalization.

BACKGROUND: Respiratory syncytial virus (RSV) is a leading cause of lower respiratory tract infection in children less than 5 years of age worldwide. In the United States, RSV commonly causes hospitalization in young children and is the leading cause of hospitalizations in infants. As new RSV immunizations become available, burden estimates are critical to guide the implementation of recommendations and quantify impact. METHODS: We estimated RSV-associated hospitalization rates at a large US pediatric medical center during an 8-year period using five approaches, namely, estimation directly from active and passive surveillance systems, both a crude and stratified capture-recapture analysis of data from both systems, and estimation based on discharge diagnosis codes. The stratified analysis was performed to ensure adherence with the capture-recapture methodology assumption that samples are independent and participants have an equal probability of being observed within each system. RESULTS: Overall, estimated RSV-associated hospitalization rates per 1000 children were 4.0 (2.5, 6.1) based on adjusted estimates from active surveillance, 1.7 (2.1, 4.4) from passive surveillance, 7.9 (5.7, 13.0) from crude capture-recapture analysis, 5.0 (3.8, 7.2) from the stratified capture-recapture, and 4.4 (4.0, 4.9) from discharge diagnosis codes. CONCLUSIONS: Each method has limitations and inherent biases that may impact the estimation of the burden of RSV. Capture-recapture analysis may be a useful tool to estimate the burden of RSV, but needs to be adjusted to account for possible violation of the assumptions of independence and equal probability of capture to ensure accurate approximation of disease burden and avoid over estimation.

BACKGROUND: Human parainfluenza viruses (PIV) are a major cause of acute respiratory infection (ARI) leading to hospitalization in young children. In order to quantify the burden of PIV hospitalizations and to evaluate the characteristics of children hospitalized with PIV by virus type, we used data from the New Vaccine Surveillance Network (NVSN), a multicenter, active, prospective population-based surveillance network, enrolling children hospitalized for ARI (defined as fever and/or respiratory symptoms) at 7 U.S. children's hospitals. METHODS: The study period included December 1, 2016 through March 31, 2020. Data captured included demographic characteristics, clinical presentation, underlying medical conditions, discharge diagnoses, and virus detection by RT-PCR. Linear and logistic regression were used to compare descriptive and clinical characteristics among children. Population-based PIV-associated hospitalization rates were calculated by age group and PIV-type. RESULTS: Of the 16,791 enrolled children with PIV virologic testing, 10,488 had only one respiratory virus detected, among whom 702 (7%) had positive testing for PIV without a co-detected virus (mean age [SD], 2.2 [3.2] years). Of these 702 children, 340 (48%) had underlying comorbidities, 139 (20%) had a history of prematurity, 121 (17%) were admitted to the ICU, and 23 (3%) required intubation. Overall, PIV hospitalization rates were highest in children aged 0-5 months (1.91 hospitalizations per 1,000 children per year [95% CI, 1.61-2.23], with PIV-3 contributing to the highest rates in that age group, followed by PIV-1 and PIV-4: 1.08 [0.84-1.21], 0.42 [0.28-0.58] and 0.25 [0.15-0.37] per 1,000 children per year, respectively. Seasonal distribution of PIV-associated hospitalizations varied by type. CONCLUSIONS: PIV infection was associated with a substantial number of ARI hospitalizations in children aged 0-5 months. Results suggest that future PIV prevention strategies in the US that focus on younger children and protection against PIV-3, PIV-1, and PIV-4 might have the greatest impact on reducing PIV hospitalization burden.

Annual influenza vaccination is recommended for all persons aged ≥6 months in the United States. Interim influenza vaccine effectiveness (VE) was calculated among patients with acute respiratory illness-associated outpatient visits and hospitalizations from four VE networks during the 2024-25 influenza season (October 2024-February 2025). Among children and adolescents aged <18 years, VE against any influenza was 32%, 59%, and 60% in the outpatient setting in three networks, and against influenza-associated hospitalization was 63% and 78% in two networks. Among adults aged ≥18 years, VE in the outpatient setting was 36% and 54% in two networks and was 41% and 55% against hospitalization in two networks. Preliminary estimates indicate that receipt of the 2024-2025 influenza vaccine reduced the likelihood of medically attended influenza and influenza-associated hospitalization. CDC recommends annual receipt of an age-appropriate influenza vaccine by all eligible persons aged ≥6 months as long as influenza viruses continue to circulate locally.

IMPORTANCE: Increasing the understanding of vaccine effectiveness (VE) against levels of severe influenza in children could help increase uptake of influenza vaccination and strengthen vaccine policies globally. OBJECTIVE: To investigate VE in children by severity of influenza illness. DESIGN, SETTING, AND PARTICIPANTS: This case-control study with a test-negative design used data from 8 participating medical centers located in geographically different US states in the New Vaccine Surveillance Network from November 6, 2015, through April 8, 2020. Participants included children 6 months through 17 years of age who were hospitalized or presented to an emergency department (ED) with acute respiratory illness. EXPOSURES: Receipt of at least 1 dose of the current season's influenza vaccine. MAIN OUTCOMES AND MEASURES: Demographic and clinical characteristics of patients presenting to the hospital or ED with or without influenza were recorded and grouped by influenza vaccination status. Estimated VE against severe influenza illness was calculated using multiple measures to capture illness severity. Data were analyzed between June 1, 2022, and September 30, 2023. RESULTS: Among 15 728 children presenting for care with acute respiratory illness (8708 [55.4%] male; 13 450 [85.5%] 6 months to 8 years of age and 2278 [14.5%] 9-17 years of age), 2710 (17.2%) had positive influenza tests and 13 018 (82.8%) had negative influenza tests (controls). Of the influenza test-positive cases, 1676 children (61.8%) had an ED visit, 896 children (33.1%) required hospitalization for noncritical influenza, and 138 children (5.1%) required hospitalization for critical influenza. About half (7779 [49.5%]) of the children (both influenza test positive and test negative) were vaccinated. Receiving at least 1 influenza vaccine dose was estimated to have a VE of 55.7% (95% CI, 51.6%-59.6%) for preventing influenza-associated ED visits or hospitalizations among children of all ages. The estimated VE was similar across severity levels: 52.8% (95% CI, 46.6%-58.3%) for ED visits, 52.3% (95% CI, 44.8%-58.8%) for noncritical hospitalization, and 50.4% (95% CI, 29.7%-65.3%) for critical hospitalization. CONCLUSIONS AND RELEVANCE: Findings from this case-control study with a test-negative design involving children with a spectrum of influenza severity suggest that influenza vaccination protects children against all levels of severe influenza illness.

BACKGROUND: Guidelines state that all hospitalized children with suspected or confirmed influenza receive prompt treatment with influenza-specific antivirals. We sought to determine the frequency of, and factors associated with, antiviral receipt among hospitalized children. METHODS: We conducted active surveillance of children presenting with fever or respiratory symptoms from 1 December 2016 to 31 March 2020 at 7 pediatric medical centers in the New Vaccine Surveillance Network. The cohort consisted of children hospitalized with influenza A or B confirmed by clinical or research testing. The primary outcome was frequency of antiviral receipt during hospitalization. We used logistic regression to obtain adjusted odds ratios (aORs) and 95% confidence intervals (CIs) for factors associated with antiviral receipt. RESULTS: A total of 1213 children with laboratory-confirmed influenza were included. Overall, 652 children (53.8%) received an antiviral. Roughly 63.0% of children received clinical influenza testing. Among those with clinical testing, 67.4% received an antiviral. Factors associated with higher odds of antiviral receipt included hematologic (aOR = 1.76; 95% CI = 1.03-3.02) or oncologic/immunocompromising (aOR = 2.41; 95% CI = 1.13-5.11) disorders, prehospitalization antiviral receipt (aOR = 2.34; 95% CI = 1.49-3.67), clinical influenza testing (aOR = 3.07; 95% CI = 2.28-4.14), and intensive care unit admission (aOR = 1.53; 95% CI = 1.02-2.29). Symptom duration >2 days was associated with lower odds of antiviral treatment (aOR = 0.40; 95% CI = .30-.52). Antiviral receipt varied by site with a 5-fold difference across sites. CONCLUSIONS: Almost half of children hospitalized with influenza did not receive antivirals. Additional efforts to understand barriers to guideline adherence are crucial for optimizing care in children hospitalized with influenza.

IMPORTANCE: During the 2023-2024 respiratory syncytial virus (RSV) season in the United States, 2 new RSV prevention products were recommended to protect infants in their first RSV season: nirsevimab and Pfizer's maternal RSV vaccine. Postlicensure studies are needed to assess prevention product impact and effectiveness. OBJECTIVE: To compare the epidemiology and disease burden of medically attended RSV-associated acute respiratory illness (ARI) among children younger than 5 years during the 2023-2024 RSV season with 3 prepandemic RSV seasons (2017-2020), estimate nirsevimab effectiveness against medically attended RSV-associated ARI, and compare nirsevimab binding site mutations among circulating RSV in infants with and without nirsevimab receipt. DESIGN, SETTING, AND PARTICIPANTS: This study included a prospective population-based surveillance for medically attended ARI with systematic molecular testing for RSV and whole-genome sequencing of RSV positive samples, as well as a test-negative case-control design to estimate nirsevimab effectiveness. The study was conducted in 7 academic pediatric medical centers in the United States with data from RSV seasons (September 1 through April 30) in 2017 through 2024. Participants were children younger than 5 years with medically attended ARI. EXPOSURE: For the nirsevimab effectiveness analyses, nirsevimab receipt among infants younger than 8 months as of or born after October 1, 2023. MAIN OUTCOME AND MEASURE: Medically attended RSV-associated ARI. RESULTS: Overall, 28 689 children younger than 5 years with medically attended ARI were enrolled, including 9536 during September 1, 2023, through April 30, 2024, and 19 153 during the same calendar period of 2017-2020. Of these children, 16 196 (57%) were male, and 12 444 (43.4) were female; the median (IQR) age was 15 (6-29) months. During 2023-2024, the proportion of children with RSV was 23% (2199/9490) among all medically attended episodes, similar to 2017-2020. RSV-associated hospitalization rates in 2023-2024 were similar to average 2017-2020 seasonal rates with 5.0 (95% CI, 4.6-5.3) per 1000 among children younger than 5 years; the highest rates were among children aged 0 to 2 months (26.6; 95% CI, 23.0-30.2). Low maternal RSV vaccine uptake precluded assessment of effectiveness. Overall, 10 of 765 case patients (1%) who were RSV positive and 126 of 851 control patients (15%) who were RSV negative received nirsevimab. Nirsevimab effectiveness was 89% (95% CI, 79%-94%) against medically attended RSV-associated ARI and 93% (95% CI, 82%-97%) against RSV-associated hospitalization. Among 229 sequenced specimens, there were no differences in nirsevimab binding site mutations by infant nirsevimab receipt status. CONCLUSIONS AND RELEVANCE: This analysis documented the continued high burden of medically attended RSV-associated ARI among young children in the US. There is a potential for substantial public health impact with increased and equitable prevention product coverage in future seasons.

Annually, tens of thousands of U.S. children and adolescents are hospitalized with seasonal influenza virus infection. Both influenza vaccination and early initiation of antiviral treatment can reduce complications of influenza. Using data from two U.S. influenza surveillance networks for children and adolescents aged <18 years with medically attended, laboratory-confirmed influenza for whom antiviral treatment is recommended, the percentage who received treatment was calculated. Trends in antiviral treatment of children and adolescents hospitalized with influenza from the 2017-18 to the 2023-2024 influenza seasons were also examined. Since 2017-18, when 70%-86% of hospitalized children and adolescents with influenza received antiviral treatment, the proportion receiving treatment notably declined. Among children and adolescents with influenza during the 2023-24 season, 52%-59% of those hospitalized received antiviral treatment. During the 2023-24 season, 31% of those at higher risk for influenza complications seen in the outpatient setting in one network were prescribed antiviral treatment. These findings demonstrate that influenza antiviral treatment is underutilized among children and adolescents who could benefit from treatment. All hospitalized children and adolescents, and those at higher risk for influenza complications in the outpatient setting, should receive antiviral treatment as soon as possible for suspected or confirmed influenza.

BACKGROUND: Previous investigations into clinical signs and symptoms associated with influenza types and subtypes have not definitively established differences in the clinical presentation or severity of influenza disease. METHODS: The study population included children 0 through 17 years old enrolled at 8 New Vaccine Surveillance Network sites between 2015 and 2020 who tested positive for influenza virus by molecular testing. Demographic and clinical data were collected for study participants via parent/guardian interview and medical chart review. Descriptive statistics were used to summarize demographic and clinical characteristics by influenza subtype. Multivariable logistic regression and Cox proportional hazard models were used to assess effects of age, sex, influenza subtype, and history of asthma on severity, including hospital admission, need for supplemental oxygen, and length of stay. RESULTS: Retractions, cyanosis, and need for supplemental oxygen were more frequently observed among patients with influenza A(H1N1)pdm09. Headaches and sore throat were more commonly reported among patients with influenza B. Children with influenza A(H1N1)pdm09 and children with asthma had significantly increased odds of hospital admission (adjusted odds ratio (AOR): 1.39, 95% CI: 1.14-1.69 and AOR: 2.14, 95% CI: 1.72-2.67, respectively). During admission, children with influenza A(H1N1)pdm09 had significantly increased use of supplemental oxygen compared to children with A(H3N2) (AOR: 0.60, 95% CI: 0.44-0.82) or B (AOR: 0.56, 95% CI: 0.41-0.76). CONCLUSIONS: Among children presenting to the emergency department and admitted to the hospital, influenza A(H1N1)pdm09 caused more severe disease compared to influenza A(H3N2) and influenza B. Asthma also contributed to severe influenza disease regardless of subtype.

BACKGROUND: The coronavirus disease 2019 pandemic disrupted respiratory syncytial virus (RSV) seasonality resulting in early, atypical RSV seasons in 2021 and 2022, with an intense 2022 peak overwhelming many pediatric healthcare facilities. METHODS: We conducted prospective surveillance for acute respiratory illness during 2016-2022 at 7 pediatric hospitals. We interviewed parents, reviewed medical records, and tested respiratory specimens for RSV and other respiratory viruses. We estimated annual RSV-associated hospitalization rates in children aged <5 years and compared hospitalization rates and characteristics of RSV-positive hospitalized children over 4 prepandemic seasons (2016-2020) to those hospitalized in 2021 or 2022. RESULTS: There was no difference in median age or age distribution between prepandemic and 2021 seasons. Median age of children hospitalized with RSV was higher in 2022 (9.6 months vs 6.0 months, P < .001). RSV-associated hospitalization rates were higher in 2021 and 2022 than the prepandemic average across age groups. Comparing 2021 to 2022, RSV-associated hospitalization rates were similar among children <2 years of age; however, children aged 24 to 59 months had significantly higher rates of RSV-associated hospitalization in 2022 (rate ratio 1.68 [95% confidence interval 1.37-2.00]). More RSV-positive hospitalized children received supplemental oxygen and there were more respiratory virus codetections in 2022 than in prepandemic seasons (P < .001 and P = .003, respectively), but there was no difference in the proportion hypoxemic, mechanically ventilated, or admitted to intensive care. CONCLUSIONS: The atypical 2021 and 2022 RSV seasons resulted in higher hospitalization rates with similar disease severity to prepandemic seasons.

IMPORTANCE: Studies of influenza in children commonly rely on coded diagnoses, yet the ability of International Classification of Diseases, Ninth Revision codes to identify influenza in the emergency department (ED) and hospital is highly variable. The accuracy of newer International Statistical Classification of Diseases and Related Health Problems, Tenth Revision (ICD-10) codes to identify influenza in children is unknown. OBJECTIVE: To determine the accuracy of ICD-10 influenza discharge diagnosis codes in the pediatric ED and inpatient settings. DESIGN, SETTING, AND PARTICIPANTS: Children younger than 18 years presenting to the ED or inpatient settings with fever and/or respiratory symptoms at 7 US pediatric medical centers affiliated with the Centers for Disease Control and Prevention-sponsored New Vaccine Surveillance Network from December 1, 2016, to March 31, 2020, were included in this cohort study. Nasal and/or throat swabs were collected for research molecular testing for influenza, regardless of clinical testing. Data, including ICD-10 discharge diagnoses and clinical testing for influenza, were obtained through medical record review. Data analysis was performed in August 2023. MAIN OUTCOMES AND MEASURES: The accuracy of ICD-10-coded discharge diagnoses was characterized using molecular clinical or research laboratory test results as reference. Measures included sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV). Estimates were stratified by setting (ED vs inpatient) and age (0-1, 2-4, and 5-17 years). RESULTS: A total of 16 867 children in the ED (median [IQR] age, 2.0 [0.0-4.0] years; 9304 boys [55.2%]) and 17 060 inpatients (median [IQR] age, 1.0 [0.0-4.0] years; 9798 boys [57.4%]) were included. In the ED, ICD-10 influenza diagnoses were highly specific (98.0%; 95% CI, 97.8%-98.3%), with high PPV (88.6%; 95% CI, 88.0%-89.2%) and high NPV (85.9%; 95% CI, 85.3%-86.6%), but sensitivity was lower (48.6%; 95% CI, 47.6%-49.5%). Among inpatients, specificity was 98.2% (95% CI, 98.0%-98.5%), PPV was 82.8% (95% CI, 82.1%-83.5%), sensitivity was 70.7% (95% CI, 69.8%-71.5%), and NPV was 96.5% (95% CI, 96.2%-96.9%). Accuracy of ICD-10 diagnoses varied by patient age, influenza season definition, time between disease onset and testing, and clinical setting. CONCLUSIONS AND RELEVANCE: In this large cohort study, influenza ICD-10 discharge diagnoses were highly specific but moderately sensitive in identifying laboratory-confirmed influenza; the accuracy of influenza diagnoses varied by clinical and epidemiological factors. In the ED and inpatient settings, an ICD-10 diagnosis likely represents a true-positive influenza case.

OBJECTIVE: To assess medical costs of hospitalizations and emergency department (ED) care associated with respiratory syncytial virus (RSV) disease in children enrolled in the New Vaccine Surveillance Network. STUDY DESIGN: We used accounting and prospective surveillance data from six pediatric health systems to assess direct medical costs from laboratory-confirmed RSV-associated hospitalizations (n=2,007) and ED visits (n=1,267) from 2016 through 2019 among children aged <5 years. We grouped costs into categories relevant to clinical care and administrative billing practices. We examined RSV-associated medical costs by care setting using descriptive and bivariate analyses. We assessed associations between known RSV risk factors and hospitalization costs and length of stay (LOS) using chi-square tests of association. RESULTS: The median cost was $7,100 (IQR: $4,006-$13,355) per hospitalized child and $503 (IQR: $387-$930) per ED visit. Eighty percent (n=2,628) of our final sample were children aged <2 years. Fewer weeks' gestational age (GA) was associated with higher median costs in hospitalized children [p<0.001, ≥37 weeks' GA: $6,840 ($3,905-$12,450); 29-36 weeks' GA: $7,721 ($4,362-$15,274); <29 w weeks' GA: $9,131 ($4,518-$19,924)]. Full-term infants accounted for 70% of the total expenditures in our sample. Almost three quarters of the healthcare dollars spent originated in children under 12 months of age; the primary age group targeted by recommended RSV prophylactics. CONCLUSIONS: Reducing the cost burden for RSV-associated medical care in young children will require prevention of RSV in all young children, not just high-risk infants. Newly available maternal vaccine and immunoprophylaxis products could substantially reduce RSV-associated medical costs.

Respiratory syncytial virus (RSV) is the leading cause of hospitalization among infants in the United States. In August 2023, CDC's Advisory Committee on Immunization Practices recommended nirsevimab, a long-acting monoclonal antibody, for infants aged <8 months to protect against RSV-associated lower respiratory tract infection during their first RSV season and for children aged 8-19 months at increased risk for severe RSV disease. In phase 3 clinical trials, nirsevimab efficacy against RSV-associated lower respiratory tract infection with hospitalization was 81% (95% CI = 62%-90%) through 150 days after receipt; post-introduction effectiveness has not been assessed in the United States. In this analysis, the New Vaccine Surveillance Network evaluated nirsevimab effectiveness against RSV-associated hospitalization among infants in their first RSV season during October 1, 2023-February 29, 2024. Among 699 infants hospitalized with acute respiratory illness, 59 (8%) received nirsevimab ≥7 days before symptom onset. Nirsevimab effectiveness was 90% (95% CI = 75%-96%) against RSV-associated hospitalization with a median time from receipt to symptom onset of 45 days (IQR = 19-76 days). The number of infants who received nirsevimab was too low to stratify by duration from receipt; however, nirsevimab effectiveness is expected to decrease with increasing time after receipt because of antibody decay. Although nirsevimab uptake and the interval from receipt of nirsevimab were limited in this analysis, this early estimate supports the current nirsevimab recommendation for the prevention of severe RSV disease in infants. Infants should be protected by maternal RSV vaccination or infant receipt of nirsevimab.

In the United States, annual influenza vaccination is recommended for all persons aged ≥6 months. Using data from four vaccine effectiveness (VE) networks during the 2023-24 influenza season, interim influenza VE was estimated among patients aged ≥6 months with acute respiratory illness-associated medical encounters using a test-negative case-control study design. Among children and adolescents aged 6 months-17 years, VE against influenza-associated outpatient visits ranged from 59% to 67% and against influenza-associated hospitalization ranged from 52% to 61%. Among adults aged ≥18 years, VE against influenza-associated outpatient visits ranged from 33% to 49% and against hospitalization from 41% to 44%. VE against influenza A ranged from 46% to 59% for children and adolescents and from 27% to 46% for adults across settings. VE against influenza B ranged from 64% to 89% for pediatric patients in outpatient settings and from 60% to 78% for all adults across settings. These findings demonstrate that the 2023-24 seasonal influenza vaccine is effective at reducing the risk for medically attended influenza virus infection. CDC recommends that all persons aged ≥6 months who have not yet been vaccinated this season get vaccinated while influenza circulates locally.

SARS-CoV-2 infection in young children is often mild or asymptomatic; however, some children are at risk for severe disease. Data describing the protective effectiveness of COVID-19 mRNA vaccines against COVID-19-associated emergency department (ED) visits and hospitalization in this population are limited. Data from the New Vaccine Surveillance Network, a prospective population-based surveillance system, were used to estimate vaccine effectiveness using a test-negative, case-control design and describe the epidemiology of SARS-CoV-2 in infants and children aged 6 months-4 years during July 1, 2022-September 30, 2023. Among 7,434 children included, 5% received a positive SARS-CoV-2 test result, and 95% received a negative test result; 86% were unvaccinated, 4% had received 1 dose of any vaccine product, and 10% had received ≥2 doses. When compared with receipt of no vaccines among children, receipt of ≥2 COVID-19 mRNA vaccine doses was 40% effective (95% CI = 8%-60%) in preventing ED visits and hospitalization. These findings support existing recommendations for COVID-19 vaccination of young children to reduce COVID-19-associated ED visits and hospitalization.

On February 1, 2022, the U.S. Virgin Islands (USVI) Department of Health (VIDOH) was notified of a confirmed case of Legionnaires disease in an adult U.S. resident (Figure). The patient, a man aged 55 years, returned to his U.S. state of residence from leisure travel in USVI on January 22 and developed a cough, shortness of breath, and fatigue on January 23. On January 29, he was hospitalized for shortness of breath and received a positive SARS-CoV-2 test result at admission. The combination of the patient’s symptoms and recent travel history prompted administration of a urinary antigen test (UAT) for Legionnaires disease specific to Legionella pneumophila serogroup 1 (Lp1); a positive result was returned on January 31. Inpatient treatment administered for COVID-19 pneumonia and Legionnaires disease included remdesivir, oral levofloxacin, oral and intravenous steroid therapy, and as-needed use of a bronchodilator inhaler and an expectorant. Remdesivir was discontinued during inpatient treatment because of elevated liver enzymes. The patient recovered and was discharged on February 2.

BACKGROUND: Descriptions of changes in invasive bacterial disease (IBD) epidemiology during the COVID-19 pandemic in the United States are limited. METHODS: We investigated changes in the incidence of IBD due to Streptococcus pneumoniae, Haemophilus influenzae, group A Streptococcus (GAS), and group B Streptococcus (GBS). We defined the COVID-19 pandemic period as March 1-December 31, 2020. We compared observed IBD incidences during the pandemic to expected incidences, consistent with January 2014-February 2020 trends. We conducted secondary analysis of a healthcare database to assess changes in testing by blood and cerebrospinal fluid (CSF) culture during the pandemic. RESULTS: Compared with expected incidences, the observed incidences of IBD due to S. pneumoniae, H. influenzae, GAS, and GBS were 58%, 60%, 28%, and 12% lower during the pandemic period of 2020, respectively. Declines from expected incidences corresponded closely with implementation of COVID-19-associated non-pharmaceutical-interventions (NPIs). Significant declines were observed across all age, race groups and surveillance sites for S pneumoniae and H influenzae. Blood and CSF culture testing rates during the pandemic were comparable to previous years. CONCLUSIONS: NPIs likely contributed to the decline in IBD incidence in the United States in 2020; observed declines were unlikely to be driven by reductions in testing.

Increases in severe respiratory illness and acute flaccid myelitis (AFM) among children and adolescents resulting from enterovirus D68 (EV-D68) infections occurred biennially in the United States during 2014, 2016, and 2018, primarily in late summer and fall. Although EV-D68 annual trends are not fully understood, EV-D68 levels were lower than expected in 2020, potentially because of implementation of COVID-19 mitigation measures (e.g., wearing face masks, enhanced hand hygiene, and physical distancing) (1). In August 2022, clinicians in several geographic areas notified CDC of an increase in hospitalizations of pediatric patients with severe respiratory illness and positive rhinovirus/enterovirus (RV/EV) test results.* Surveillance data were analyzed from multiple national data sources to characterize reported trends in acute respiratory illness (ARI), asthma/reactive airway disease (RAD) exacerbations, and the percentage of positive RV/EV and EV-D68 test results during 2022 compared with previous years. These data demonstrated an increase in emergency department (ED) visits by children and adolescents with ARI and asthma/RAD in late summer 2022. The percentage of positive RV/EV test results in national laboratory-based surveillance and the percentage of positive EV-D68 test results in pediatric sentinel surveillance also increased during this time. Previous increases in EV-D68 respiratory illness have led to substantial resource demands in some hospitals and have also coincided with increases in cases of AFM (2), a rare but serious neurologic disease affecting the spinal cord. Therefore, clinicians should consider AFM in patients with acute flaccid limb weakness, especially after respiratory illness or fever, and ensure prompt hospitalization and referral to specialty care for such cases. Clinicians should also test for poliovirus infection in patients suspected of having AFM because of the clinical similarity to acute flaccid paralysis caused by poliovirus. Ongoing surveillance for EV-D68 is critical to ensuring preparedness for possible future increases in ARI and AFM.

IMPORTANCE: Understanding risk factors for hospitalization in vaccinated persons and the association of COVID-19 vaccines with hospitalization rates is critical for public health efforts to control COVID-19. OBJECTIVE: To determine characteristics of COVID-19-associated hospitalizations among vaccinated persons and comparative hospitalization rates in unvaccinated and vaccinated persons. DESIGN, SETTING, AND PARTICIPANTS: From January 1, 2021, to April 30, 2022, patients 18 years or older with laboratory-confirmed SARS-CoV-2 infection were identified from more than 250 hospitals in the population-based COVID-19-Associated Hospitalization Surveillance Network. State immunization information system data were linked to cases, and the vaccination coverage data of the defined catchment population were used to compare hospitalization rates in unvaccinated and vaccinated individuals. Vaccinated and unvaccinated patient characteristics were compared in a representative sample with detailed medical record review; unweighted case counts and weighted percentages were calculated. EXPOSURES: Laboratory-confirmed COVID-19-associated hospitalization, defined as a positive SARS-CoV-2 test result within 14 days before or during hospitalization. MAIN OUTCOMES AND MEASURES: COVID-19-associated hospitalization rates among vaccinated vs unvaccinated persons and factors associated with COVID-19-associated hospitalization in vaccinated persons were assessed. RESULTS: Using representative data from 19509 hospitalizations (see Table 1 for demographic information), monthly COVID-19-associated hospitalization rates ranged from 3.5 times to 17.7 times higher in unvaccinated persons than vaccinated persons regardless of booster dose status. From January to April 2022, when the Omicron variant was predominant, hospitalization rates were 10.5 times higher in unvaccinated persons and 2.5 times higher in vaccinated persons with no booster dose, respectively, compared with those who had received a booster dose. Among sampled cases, vaccinated hospitalized patients with COVID-19 were older than those who were unvaccinated (median [IQR] age, 70 [58-80] years vs 58 [46-70] years, respectively; P<.001) and more likely to have 3 or more underlying medical conditions (1926 [77.8%] vs 4124 [51.6%], respectively; P<.001). CONCLUSIONS AND RELEVANCE: In this cross-sectional study of US adults hospitalized with COVID-19, unvaccinated adults were more likely to be hospitalized compared with vaccinated adults; hospitalization rates were lowest in those who had received a booster dose. Hospitalized vaccinated persons were older and more likely to have 3 or more underlying medical conditions and be long-term care facility residents compared with hospitalized unvaccinated persons. The study results suggest that clinicians and public health practitioners should continue to promote vaccination with all recommended doses for eligible persons.

In April 2021, we assessed mRNA vaccine effectiveness (VE) in the context of a COVID-19 outbreak in a skilled nursing facility. Among 28 cases, genomic sequencing was performed on four specimens on four different patients, and all were classified by sequence analysis as the Beta (B.1.351) variant. Adjusted VE among residents was 65% (95% Confidence Interval: 25-84%). These findings underscore the importance of vaccination for prevention of COVID-19 in skilled nursing facilities.

Clinical trials of COVID-19 vaccines currently authorized for emergency use in the United States (Pfizer-BioNTech, Moderna, and Janssen [Johnson & Johnson]) indicate that these vaccines have high efficacy against symptomatic disease, including moderate to severe illness (1-3). In addition to clinical trials, real-world assessments of COVID-19 vaccine effectiveness are critical in guiding vaccine policy and building vaccine confidence, particularly among populations at higher risk for more severe illness from COVID-19, including older adults. To determine the real-world effectiveness of the three currently authorized COVID-19 vaccines among persons aged ≥65 years during February 1-April 30, 2021, data on 7,280 patients from the COVID-19-Associated Hospitalization Surveillance Network (COVID-NET) were analyzed with vaccination coverage data from state immunization information systems (IISs) for the COVID-NET catchment area (approximately 4.8 million persons). Among adults aged 65-74 years, effectiveness of full vaccination in preventing COVID-19-associated hospitalization was 96% (95% confidence interval [CI] = 94%-98%) for Pfizer-BioNTech, 96% (95% CI = 95%-98%) for Moderna, and 84% (95% CI = 64%-93%) for Janssen vaccine products. Effectiveness of full vaccination in preventing COVID-19-associated hospitalization among adults aged ≥75 years was 91% (95% CI = 87%-94%) for Pfizer-BioNTech, 96% (95% CI = 93%-98%) for Moderna, and 85% (95% CI = 72%-92%) for Janssen vaccine products. COVID-19 vaccines currently authorized in the United States are highly effective in preventing COVID-19-associated hospitalizations in older adults. In light of real-world data demonstrating high effectiveness of COVID-19 vaccines among older adults, efforts to increase vaccination coverage in this age group are critical to reducing the risk for COVID-19-related hospitalization.