A recent study published within the Centers for Disease Control and Prevention (CDC)’s Morbidity and Mortality Weekly Report described the incidence and severity of influenza in 2022-23 amongst adolescents and youngsters in the US (US).

Study: High Influenza Incidence and Disease Severity Amongst Children and Adolescents Aged <18 Years ― United States, 2022–23 Season. Image Credit: CDC

Influenza activity within the 2022-23 season began in October within the US, sooner than in previous seasons, reaching levels recorded before the coronavirus disease 2019 (COVID-19) pandemic. Furthermore, the season recorded high hospitalization rates in pediatric populations within 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 attributable to influenza within the US.

Seasonal severity is classed using three indicators ­– the proportion of outpatient visits for influenza-like illness (ILI) and influenza-related hospitalization rates and the proportion of deaths. Severity is low if ≥ 2 indicators peak below the fiftieth percentile intensity threshold (IT₅₀), determined using data from previous seasons. It’s rated as very high, high, or moderate if ≥ 2 indicators peak above IT₉₈, IT₉₀, or IT₅₀, respectively.

In regards to the study

In the current study, researchers described the incidence and severity of the 2022-23 influenza season amongst US adolescents and youngsters. 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 youngsters under five and people aged 5-17. Influenza vaccination status and clinical characteristics were obtained. Data were collected for hospitalized adolescents and youngsters from 2016-17 to 2021-22 and for an age-stratified sample in 2022-23. The current 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 youngsters 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 4 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 amongst adolescents and youngsters 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. Nonetheless, 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. Greater than 95% of infections were attributable 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, in comparison with 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 just like previous seasons.

Conclusions

The influenza season in 2022-23 was rated as highly severe amongst adolescents and youngsters, the fourth such instance for the reason that influenza A pandemic of 2009. The severity indicators surpassed the high severity thresholds, with peaks occurring early within the season. The rates of outpatient visits and hospitalizations attributable to influenza were higher than during prior seasons, which strained healthcare systems.

Furthermore, fewer hospitalized adolescents and youngsters 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 youngsters, emphasizing the necessity to increase coverage of antiviral treatment for hospitalized patients.