A recent study published in the Centers for Disease Control and Prevention (CDC)’s Morbidity and Mortality Weekly Report described the incidence and severity of influenza in 2022-23 among adolescents and children in the United States (US).
Study: High Influenza Incidence and Disease Severity Among Children and Adolescents Aged <18 Years ― United States, 2022–23 Season. Image Credit: CDC
Influenza activity in the 2022-23 season began in October in the US, earlier than in previous seasons, reaching levels recorded before the coronavirus disease 2019 (COVID-19) pandemic. Moreover, the season recorded high hospitalization rates in pediatric populations in the Southeast and a limited decrease in oseltamivir availability, the antiviral influenza drug.
Besides, influenza virus co-circulated with respiratory syncytial virus (RSV) and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The Centers for Disease Control and Prevention (CDC) annually assesses seasonal severity and estimates the rates and numbers of medical visits, hospitalizations, and deaths due to influenza in the US.
Seasonal severity is classified using three indicators – the percentage of outpatient visits for influenza-like illness (ILI) and influenza-related hospitalization rates and the percentage of deaths. Severity is low if ≥ 2 indicators peak below the fiftieth percentile intensity threshold (IT50), determined using data from previous seasons. It is rated as very high, high, or moderate if ≥ 2 indicators peak above IT98, IT90, or IT50, respectively.
About the study
In the present study, researchers described the incidence and severity of the 2022-23 influenza season among US adolescents and children. The incidence of influenza-associated outpatient visits, hospitalizations, and deaths was estimated. The hospitalization rates were calculated by applying hospitalization rates from the Influenza Hospitalization Surveillance Network (FluSurv-NET) to the US population, adjusting for possible under-detection.
The researchers estimated influenza-related outpatient visits and deaths. The rates were calculated for children under five and those aged 5-17. Influenza vaccination status and clinical characteristics were obtained. Data were collected for hospitalized adolescents and children from 2016-17 to 2021-22 and for an age-stratified sample in 2022-23. The present study reported findings from the 2022-23 season relative to data from 2016-17 to 2021-22 seasons.
Findings
The 2022-23 influenza season for adolescents and children was highly severe. The three severity indicators peaked between IT90 and IT98. The hospitalization rate and percentage of outpatient ILI visits peaked in November 2022, three weeks before influenza-associated deaths peaked, which was high for four weeks in December 2022.
Influenza-related medical visits, hospitalizations, and deaths during 2022-23 were higher in children under five than those aged 5-17. Further, medical visit and hospitalization rates among adolescents and children aged 5-17 were higher in 2022-23 than in previous seasons. Children under five had the second-highest rates of medical visits and hospitalization in 2022-23. However, the rates of influenza-related deaths were low, consistent with prior seasons.
FluSurv-NET reported 2,762 influenza-related hospitalizations in individuals under 18 between October 2022 and April 2023. Nearly half had underlying conditions like obesity, asthma, and neurologic disorders. More than 95% of infections were due to influenza A virus. Most hospitalizations occurred in October–November 2022, higher than in previous seasons.
About 18.3% of hospitalized subjects were vaccinated in 2022-23, compared to 35.8% to 41.8% in previous seasons. The proportion of hospitalized subjects receiving antiviral therapy was similar in 2021-22 and 2022-23 but lower during pre-COVID-19 seasons. Further, the proportions of patients requiring intensive care and mechanical ventilation and in-hospital deaths in 2022-23 were similar to previous seasons.
Conclusions
The influenza season in 2022-23 was rated as highly severe among adolescents and children, the fourth such instance since the influenza A pandemic of 2009. The severity indicators surpassed the high severity thresholds, with peaks occurring early in the season. The rates of outpatient visits and hospitalizations due to influenza were higher than during prior seasons, which strained healthcare systems.
Moreover, fewer hospitalized adolescents and children in 2022-23 were vaccinated than in previous seasons. Similarly, the proportion of pediatric hospitalized patients receiving antiviral medication was low. Overall, the findings highlight the importance of seasonal influenza vaccination for adolescents and children, emphasizing the need to increase coverage of antiviral treatment for hospitalized patients.
New insights into how IFITM3 protein protects against severe flu