Severe Influenza Strikes US Children and Adolescents in 2022-2023 Season

Based on previous seasons’ data, 2022-2023 was considered a high-severity influenza season for children and adolescents under the age of 18, according to the CDC’s Morbidity and Mortality Weekly Report.

Researchers explained that the CDC annually assesses the severity of each influenza season based on previous seasons, estimating the numbers and rates of influenza-associated medical visits, hospitalizations, and deaths in the United States. Consequently, the researchers’ report compared the impact of the 2022-2023 influenza season on children and adolescents to that of the 2016-2017 season through the 2021-2022 season; they excluded data from the 2020-2021 season as it was during the peak of the COVID-19 pandemic with minimal influenza activity.

To examine these areas, the CDC utilized 3 indicators:

• The percentage of all outpatient visits for influenza-like illness (ILI) obtained from the US Outpatient Influenza-like Illness Surveillance Network
• The rates of laboratory-confirmed influenza hospitalization, which were estimated through the Influenza Hospitalization Surveillance Network (FluSurv-NET)
• Death registry data from the National Vital Statistics System were used to calculate the percentage of all deaths due to influenza

The researchers explained that, for each severity indicator, they calculated the 50th, 90th, and 98th intensity thresholds (ITs) based on the mean of peak weekly values in previous seasons. They added that seasonal severity is classified as low if at least 2 of 3 indicators peak below IT₅₀. On the other hand, it is classified as moderate, high, or very high if at least 2 of 3 indicators peak above IT₅₀, IT₉₀, or IT₉₈, respectively.

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From this study, the researchers classified the 2022-2203 influenza season for children and adolescents as high severity because the weekly percentage of outpatient visits for ILI, the influenza-associated hospitalization rate, and the percentage of deaths due to influenza all peaked between IT₉₀ and IT₉₈. Overall, rates of influenza-associated medical visits and hospitalizations among children and adolescents aged 5 to 17 years were higher during 2022-2023 than any season since 2016-2017.

Between October 1, 2022, and April 30, 2023, FluSurv-NET identified 2762 influenza-associated hospitalizations of patients younger than 18 years, 2108 of which they sampled. Similar to recent seasons, the researchers found the median (IQR) age of patients hospitalized to be 5 (2-9) years, 57.4% were male, and 50.5% had an underlying condition, the most common being asthma, obesity, and neurologic disorders. Notably, more than half of the 2022-2023 season’s total pediatric hospitalizations occurred between October and November 2022.

The researchers hypothesized that the high hospitalization rates during the 2022-2023 season occurred due to early influenza circulation before many patients had been vaccinated. Of those hospitalized that had data within FluSurv-NET, 18.3% received an influenza vaccine; this was much lower than other seasons where 35.8% to 41.8% of hospitalized patients were vaccinated. Additionally, of the symptomatic hospitalized patients, 64.9% received influenza antiviral treatment, which was lower than in prepandemic influenza seasons (80.8%-87.1%).

They also acknowledged their study’s limitations, one being that they based the severity assessments and incidence estimation adjustments for the frequency of influenza testing and other ratios on previous seasons’ data. Because of this, the data may not reflect current testing practices or care-seeking behaviors. Another limitation mentioned was that FluSurv-NET only covers approximately 9.0% of the US population, meaning that the characteristics of hospitalized children and adolescents may not be generalizable to all US pediatric hospitalizations.

Despite these limitations, the researchers noted that the study still demonstrated the high severity of the 2022-2023 influenza season among children and adolescents, as influenza-associated medical visits and hospitalizations met or exceeded those of previous seasons.

“Taken together, these findings underscore the importance of children and adolescents receiving a seasonal influenza vaccination, ideally by the end of October, and prompt influenza antiviral treatment for those who are hospitalized,” the authors wrote.

Reference

White EB, O’Halloran A, Sundaresan D, et al. High influenza incidence and disease severity among children and adolescents aged <18 years ― United States, 2022–23 season. MMWR Morb Mortal Wkly Rep. 2023;72:1108–1114. doi:10.15585/mmwr.mm7241a2.