Avian Flu Diary: High Influenza Incidence & Disease Severity Among Children and Adolescents Aged < 18 Years ― U.S., 2022

#17,722

Last year, between likely post-pandemic`vaccine fatigue' and the unusually early arrival of flu season, fewer kids (aged 6 mos - 17 years) got a flu shot than in recent years (see Flu Vaccination Coverage, United States, 2022–23 Influenza Season), which likely contributed to seeing a higher severity influenza season among children and adolescents.

Every flu season is different, of course, but the CDC would like to avoid a repeat and is once again urging that parents talk to their doctors about the flu vaccine for their kids.

This week the CDC's MMWR breaks down the numbers from last year's flu season, and its impact on children. Due to its length, I've only posted some excerpts. I'll have a postscript after the break.

High Influenza Incidence and Disease Severity Among Children and Adolescents Aged <18 Years ― United States, 2022–23 Season

Weekly / October 13, 2023 / 72(41);1108–1114
Print

Elizabeth B. White, PhD1,2; Alissa O’Halloran, MSPH1; Devi Sundaresan, MPH1; Matthew Gilmer, MS1,3; Ryan Threlkel, MPH1; Arielle Colón, MPH1; Katie Tastad, PhD1; Shua J. Chai, MD4,5; Nisha B. Alden, MPH6; Kimberly Yousey-Hindes, MPH7; Kyle P. Openo, DrPH8,9,10; Patricia A. Ryan, MS11; Sue Kim, MPH12; Ruth Lynfield, MD13; Nancy Spina, MPH14; Brenda L. Tesini, MD15; Marc Martinez, BA16; Zachary Schmidt, MPH17; Melissa Sutton, MD18; H. Keipp Talbot, MD19; Mary Hill, MPH20; Matthew Biggerstaff, ScD1; Alicia Budd, MPH1; Shikha Garg, MD1; Carrie Reed, DSc1; A. Danielle Iuliano, PhD1; Catherine H. Bozio, PhD1 (VIEW AUTHOR AFFILIATIONS)View suggested citation

Summary

What is already known about this topic?

The 2022–23 influenza season began early, coinciding with circulation of other respiratory viruses. High hospitalization rates among children and adolescents were observed.

What is added by this report?

Among children and adolescents aged <18 years, 2022–23 was a high severity influenza season compared with thresholds based on previous seasons’ data; influenza-associated medical visits and hospitalizations met or exceeded incidence in previous seasons.

What are the implications for public health practice?

CDC recommends that all persons aged ≥6 months without contraindications should receive the annual seasonal influenza vaccine, ideally by the end of October.

Abstract
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. This report describes the 2022–23 influenza season among children and adolescents aged <18 years, including the seasonal severity assessment; estimates of U.S. influenza-associated medical visits, hospitalizations, and deaths; and characteristics of influenza-associated hospitalizations.

The 2022–23 influenza season had high severity among children and adolescents compared with thresholds based on previous seasons’ influenza-associated outpatient visits, hospitalization rates, and deaths. Nationally, the incidences of influenza-associated outpatient visits and hospitalization for the 2022–23 season were similar for children aged <5 years and higher for children and adolescents aged 5–17 years compared with previous seasons. Peak influenza-associated outpatient and hospitalization activity occurred in late November and early December.

Among children and adolescents hospitalized with influenza during the 2022–23 season in hospitals participating in the Influenza Hospitalization Surveillance Network, a lower proportion were vaccinated (18.3%) compared with previous seasons (35.8%–41.8%). Early influenza circulation, before many children and adolescents had been vaccinated, might have contributed to the high hospitalization rates during the 2022–23 season. Among symptomatic hospitalized patients, receipt of influenza antiviral treatment (64.9%) was lower than during pre–COVID-19 pandemic seasons (80.8%–87.1%). CDC recommends that all persons aged ≥6 months without contraindications should receive the annual influenza vaccine, ideally by the end of October.

(SNIP)

Discussion

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. Further, all three severity indicators not only surpassed intensity levels for high severity, but the peaks also occurred early in the season (late November and early December) (1). National estimates of the rates of influenza-associated medical visits and hospitalizations were higher than those during most previous seasons for children aged <5 years and children and adolescents aged 5–17 years. This high incidence strained health care systems, particularly with the co-circulation of SARS-CoV-2 and RSV.†††

Among children and adolescents hospitalized with influenza during 2022–23, a substantially lower proportion were vaccinated compared with previous seasons, which could be related to low vaccination coverage in the population, high vaccine effectiveness, 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). Influenza vaccination coverage by the end of November was similar in 2022 and in 2021 (45.0%), but lower than in 2019 (51.9%) and 2020 (49.7%).

Preliminary assessments have shown that the 2022–23 influenza vaccine provided moderately strong (68%) protection 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.

In addition, a lower proportion of symptomatic hospitalized patients in 2022–23 received influenza antiviral medication compared with that during pre–COVID-19 pandemic seasons. Taken together, these findings underscore the importance of children and adolescents receiving a seasonal influenza vaccination, ideally by the end of October (9), and prompt influenza antiviral treatment for those who are hospitalized.****

(Continue . . . )

While what happens in the Southern Hemisphere's flu season doesn't necessarily carry over into the Northern Hemisphere, this year Australia reported an H1N1 dominant flu season (see chart below).

It is quite early in the season, and things could certainly change, but the CDC's FluView is reporting that among influenza A viruses, H1N1 holds a 10-to-1 lead over H3N2 in week 40.

If this trend holds, we could be looking at the first H1N1 dominant season since the truncated (by the emergence of COVID) 2019-2020 flu season. While generally milder in adults than H3N2, H1N1 can sometimes impact children and adolescents unusually hard.