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. 2025 Jan;97(1):e70167.
doi: 10.1002/jmv.70167.

Systematic Molecular Influenza A/B Screening Upon Hospital Admission in Belgium, January-April 2022: Positivity Ratios and Viral Loads According to Symptomatology, Age, and Vaccination Status

Evelyne Huyghe  1   2 , Steven Abrams  3   4 , Emmanuel André  5   6 , Kurt Anseeuw  7 , Eva Bernaert  2 , Peggy Bruynseels  1   2 , Lize Cuypers  5   6 , Pieter De Schouwer  1 , Petra Hilkens  8 , Els Keyaerts  5   6 , Lies Laenen  5   6 , Justine Maes  8 , Koen Magerman  8   9 , Otto Van de Gaer  5   6 , Ann Verdonck  5   6 , Walter Verstrepen  1 , Sien Ombelet  5   6 , Reinout Naesens  1   2
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Systematic Molecular Influenza A/B Screening Upon Hospital Admission in Belgium, January-April 2022: Positivity Ratios and Viral Loads According to Symptomatology, Age, and Vaccination Status

Evelyne Huyghe et al. J Med Virol. 2025 Jan.

Abstract

Three hospitals implemented molecular point-of-care tests (POCTs) to screen patients for SARS-CoV-2 infection upon admission during the 2021/2022 influenza season, which in Belgium lasted from January to April 2022. The samples were simultaneously tested for influenza A/B. Influenza positivity at admission was examined in relation to patient characteristics and symptomatology. Influenza POCTs were performed on all patients requiring urgent hospitalization, regardless of the admission reason. A total of 9327 patients were included in the study, of which 411 (4.4%) tested positive for influenza A/B. Asymptomatic infection and mild illness accounted for respectively 11.2% (95% CI: 8.5%-14.6%), and 43.3% (95% CI: 38.6%-48.1%) of the cases. A total of 66% (95% CI: 60%-72%) of all patients in these symptom categories (asymptomatic and mild illness) showed a high viral load (cycle threshold [Ct] < 24). Only in 30 (7.3%, 95% CI: 5.2%-10.2%) of all cases and in two (4.4%, 95% CI: 1.2%-14.5%) of the asymptomatic cases, the symptomatology worsened during hospital stay. Coinfections with both influenza and SARS-CoV-2 occurred in 35 patients (8.5% of all influenza positive patients). There was no difference in symptomatology between patients with co-infections and those with an influenza mono-infection. Patients could not be reliably categorized into carriers with low versus high viral loads based on symptomatology, age, and vaccination status. More than half of the influenza-positive individuals were either asymptomatic or had mild symptoms upon admission, while often carrying high viral loads. Our results show that without screening of patients at hospital admission, a considerable number of patients with a high viral load may be incorrectly classified as being not infectious.

Keywords: LIAT; POCT; epidemiology; influenza.

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