Today in Morbidity and Mortality Weekly Report, a study finds that the 2022-2023 influenza season in the United States was severe, especially for children and adolescents.
As they do each flu season, researchers from the Centers for Disease Control and Prevention (CDC) analyzed preliminary data on outpatient visits for influenza-like illness (ILI) from the US Outpatient Influenza-like Illness Surveillance Network, flu hospitalizationsfrom the Influenza Hospitalization Surveillance Network, and flu deaths from the National Vital Statistics System death registry.
Data were collected for all hospitalized children and adolescents from the 2016–2017 through 2021–2022 flu seasons and for an age-stratified sample (younger and older than 5 years) in the 2022-2023 season through September 21.
"During the 2022–23 influenza season, early increases in influenza activity, co-circulation of influenza with other respiratory viruses, and high influenza-associated hospitalization rates, particularly among children and adolescents, were observed," the study authors wrote.
The 2022-2023 flu season was classified as having high severity, with the weekly proportion of outpatient visits for ILI, flu hospitalizations, and flu deaths all peaking between the 90th and 98th percentile. Both the percentage of ILI outpatient visits and the rate of flu hospitalizations peaked in late November 2022, 3 weeks before the peak in the proportion of flu deaths. Deaths remained elevated throughout December.
During the 2022–23 influenza season, early increases in influenza activity, co-circulation of influenza with other respiratory viruses, and high influenza-associated hospitalization rates, particularly among children and adolescents, were observed.
Point estimates of flu-related medical visits, hospitalizations, and deaths were higher among children younger than 5 than among older children. Yet rates of ILI visits and hospitalizations among children aged 5 to 17 were higher in 2022-2023 than they were in 2016-2017.
During the 2022-2023 season, children younger than 5 years had the second-highest rates of flu-related medical visits (11,443 per 100,000) and hospitalizations (119 per 100,000) since 2016-2017. Flu death rates were low in 2022-2023, consistent with previous seasons, at 1.2 per 100,000 children younger than 5 years and 0.5 per 100,000 older children.
From October through April 2023, there were 2,762 flu-related hospitalizations among all children, 2,108 of which were sampled and had clinical data. The median age was 5 years, 57.4% were boys, and 50.5% had underlying conditions, consistent with recent seasons. The most common chronic conditions were asthma, neurologic disorders, and obesity.
Nearly all (95.4%) of infections were influenza A, 80.2% of the subtyped cases were A(H3N2), and 19.6% were A(H1N1)pdm09. A total of 57.1% of pediatric hospitalizations occurred in October and November 2022, a figure higher than the percentages during October and November in the 2016-2017 through 2021-2022 seasons (1.6% to 6.8%).
Of all hospitalized children, 18.3% were vaccinated against the flu, compared with 35.8% to 41.8% in recent seasons. A total of 64.9% of children with respiratory symptoms were given flu antiviral drugs in the hospital in 2022-2023, similar to the 61.5% in 2021-2022 but lower than the 80.8% to 87.1% in prepandemic seasons.
The proportions of children who were admitted to an intensive care unit (18.4%), required invasive mechanical ventilation (4.7%), or died in the hospital (0.4%) were comparable to those during recent flu seasons.
Season saw substantially lower flu vaccination rate
"The 2022–23 influenza season was classified as high severity among children and adolescents, the fourth season with that classification since the 2009 influenza A(H1N1) pandemic," the researchers wrote, adding that the high incidence strained healthcare systems, especially those also treating large numbers of COVID-19 and respiratory syncytial virus (RSV).
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 noted that a substantially lower percentage of hospitalized pediatric patients were vaccinated against flu in 2022-2023 than in previous seasons, which they said could be the result of overall low vaccine uptake in the population, high vaccine efficacy, or both.
"The National Immunization Survey estimates that when pediatric influenza-associated hospitalization rates peaked during the week ending November 26, 2022, only 41.9% of children and adolescents aged 6 months–17 years nationwide had received their annual influenza vaccination (compared with 55.1% by the end of the season)," they wrote.
By the end of November 2022, flu vaccine coverage was comparable to that in 2021-2022 (45.0%) but lower than in 2019 (51.9%) and 2020 (49.7%). Preliminary data, the researchers said, estimate that the 2022-2023 flu vaccine was 68% effective against pediatric hospitalization.
"The combination of low influenza vaccine coverage early in the season and unusually early influenza activity (57.1% of the season’s pediatric hospitalizations occurred by the end of November) likely contributed to the high observed rate of influenza-associated hospitalization, despite the moderately strong protection from the 2022–23 influenza vaccine," they wrote.
A lower percentage of hospitalized patients in 2022-2023 were given antiviral drugs than those in prepandemic flu 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.