DOI: 10.1097/inf.0000000000005350 ISSN: 0891-3668

Comparison of Epidemiology, Risk Factors, Clinical Presentation and Outcomes of COVID-19 and Seasonal Influenza Among Hospitalized Swiss Children: A Nationwide Prospective Surveillance Study

Petra Zimmermann, Carlos Sanchez, Michael Buettcher, Ulrich Heininger, Julia A. Bielicki,
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Background:

Although many children infected with severe acute respiratory syndrome coronavirus type 2 (SARS-CoV-2) or seasonal influenza virus experience a mild course of disease, both infections can lead to hospitalization and, in a subset of children, severe disease requiring intensive care unit (ICU) admission for respiratory or cardiac support. To compare the epidemiology, risk factors, clinical presentation and outcomes of children hospitalized with COVID-19 or seasonal influenza, and to identify factors associated with severe disease.

Methods:

Data were collected prospectively within a nationwide surveillance system across Swiss hospitals between December 2018 and November 2023. A comparative analysis was performed among children and adolescents (<18 years) hospitalized with laboratory-confirmed SARS-CoV-2 or seasonal influenza infection. SARS-CoV-2 data were analyzed according to four time periods defined by the predominantly circulating virus variant: wild type (T1), Alpha (T2), Delta (T3) and Omicron (T4). Influenza data were analyzed across two periods: “pre-COVID” (December 2018 to April 30, 2020) and “during COVID” (May 1, 2020, to November 30, 2023).

Results:

Of 4662 included children, 3132 (67.2%) were hospitalized with SARS-CoV-2 infection and 1530 (32.8%) with influenza. Age distribution differed by virus and period ( P < 0.001): infants accounted for one-third to almost half of SARS-CoV-2 admissions (32.2%–46.7%), with adolescents also contributing notably during T1 (22.7%), whereas toddlers and school-aged children were more frequently hospitalized with influenza infection. Fever, cough and rhinitis were the most common symptoms for both viruses. Among periods with symptom data available, fever was less frequent during SARS-CoV-2 T4 (47.3%) than during earlier SARS-CoV-2 waves (62.8% in T1, 55.7% in T2 and 64.9% in T3) and was also less frequent among children with influenza during the COVID period (26.0%). Anosmia was recorded only among children with SARS-CoV-2 infection (most often during T1: 1.2%; P < 0.001). Proportions of severe symptoms were also similar, except respiratory distress being more common during T4 (12.6%) and with influenza (18.1%; P = 0.02). Acute respiratory distress syndrome was recorded only among children hospitalized with influenza and not among those hospitalized with SARS-CoV-2. ICU admissions were highest during influenza seasons (pre-COVID 10.5% and during COVID 11.8%) compared with 5.0% to 10.3% with SARS-CoV-2 ( P = 0.001). Complications were more frequent in children with seasonal influenza ( P < 0.001), most commonly lobar pneumonia, which occurred in up to 20% of children with influenza. During T3, deconditioning syndrome was more frequently reported (5.6%) than in other periods. Antibiotic use was higher in children with influenza (40.7% vs. 14.4%; P < 0.001). Factors independently associated with severe disease were neurological, cardiovascular and other underlying conditions in children with COVID-19, and neurological and respiratory conditions in children with influenza.

Conclusions:

Among hospitalized children with laboratory-confirmed infection, influenza was associated with more frequent ICU admission, complications, antibiotic use and respiratory support than SARS-CoV-2. Although underlying conditions were associated with an increased risk for severe disease, most children had no preexisting underlying conditions. These findings support consideration of broader pediatric influenza vaccination strategies beyond traditional high-risk groups and the need for ongoing surveillance to monitor evolving disease patterns and inform clinical and public health decisions.

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