A new report shows the continued impact of influenza on the US healthcare systems for the past 13 flu seasons, with the impact varying from year to year but peaking in 2017-18, with more than 29,000 hospital cases.
The study, published today in Morbidity and Mortality Weekly Report, shows significant illness and hospitalization rates for influenza each season and a trend to less prescribing of antiviral drugs.
Data on laboratory-confirmed influenza-associated hospitalizations among Americans of all ages were analyzed from information gleaned from the Influenza Hospitalization Surveillance Network (FluSurv-NET) database from the 2010-11 through 2022-23 flu seasons.
FluSurv-NET has been collecting data on influenza trends since 2003, but this study is the first comprehensive summary of findings.
2017-18 had most hospital cases
FluSurv-NET included 13 to 16 participating sites each season, with prespecified geographic areas that covered 27 million to 29 million people and included an estimated 8.8% to 9.5% of the US population.
The number of laboratory-confirmed influenza-associated hospitalization cases ranged from a low of 2,415 cases during 2011-12 to a high of 29,695 during 2017-18.
Hospitalizations for flu dropped significantly during the 2020-21 season, and did not return to pre–COVID-pandemic levels until 2022-23.
During each flu season, adults 65 years and older had the highest flu-related hospitalization rate across all age-groups, followed in most seasons by children ages 0 to 4 years. Also, in all seasons, 95.4% of flu patients ages 65 and older had at least one other underlying medical condition.
"Persons with underlying medical conditions represented a large proportion of patients hospitalized with influenza," the authors wrote. The most common underlying medical conditions among adults were high blood pressure, obesity, chronic metabolic disease, chronic lung disease, and heart disease.
Among children, asthma was the most common underlying condition in those hospitalized with flu; 14.9% of case-patients ages 0 to 4 years had asthma, rising to 39.0% of patients ages 5 to 17 years.
Substantial ICU admissions
Among patients of all ages hospitalized with flu, the proportion admitted to the intensive care unit ranged from 14.1% during 2021-22 to 22.3% during 2013-14.
"The proportion of hospitalized patients who died in the hospital ranged from 2.2% during 2021–22 to 3.5% during 2013–14, although adults, particularly those aged ≥65 years, had the highest proportions of in-hospital death," the authors wrote.
Among those hospitalized, young adults aged 18 to 49 years consistently had the lowest percentage of patients receiving the current season's flu vaccine—15.4% during 2022-23 to 31.1% during 2014-15.
Drop in antiviral prescribing
Between 2010-11 and 2018-19, the rates of antiviral prescribing also increased among all ages, but decreased from 90.2% of all patients treated with antivirals in 2018-29 to 79.1% in 2022-23
Children younger than 5 years and those 5 to 17 years had the biggest drop in antiviral prescribing, from 84.4% and 82.3%, respectively, during 2018-19 to 60.9% and 59.2%, respectively, during 2022–23.
"The decline in antiviral treatment of hospitalized patients with influenza from 90% during the 2018–19 influenza season to 79% during the 2022–23 season represents a concerning trend.
"The decline in antiviral treatment of hospitalized patients with influenza from 90% during the 2018–19 influenza season to 79% during the 2022–23 season represents a concerning trend and highlights missed opportunities to prevent influenza-associated complications among those at increased risk for severe influenza complications," the authors concluded.
