Influenza A and other respiratory viruses show distinct associations with acute respiratory presentations and temporal correlation with hospital admissions

Introduction. Influenza A is the major cause of hospitalization due to acute respiratory virus infections (ARIs) worldwide, while other respiratory viruses are thought to play lesser roles. However, viral co-infection, host demographics and environmental factors, including air quality, may influence clinical presentation and hospital admissions. This study characterizes their clinical associations during the last influenza A outbreak in Qatar before the emergence of severe acute respiratory syndrome coronavirus 2. Gap statement. The relative contributions of host demographics, viral co-infection and environmental factors to ARI hospital admissions remain incompletely understood. Aim. To assess the association of host demographics, viral co-infection and air quality on influenza A admissions for pneumonia, bronchiolitis and reactive airway exacerbations. Methodology. Between 1 November 2019 and 28 March 2020, 1,801 patients admitted with ARIs were classified using ICD-10 codes into bronchiolitis, reactive airway exacerbations and pneumonia. Respiratory samples were analysed by real-time PCR (RT-PCR) for influenza A and other common respiratory viruses, with non-influenza A grouped collectively as other respiratory viruses (ORVs). Air quality metrics at admission were ozone, sulphur dioxide, nitrogen dioxide, carbon monoxide, nitrogen oxide and fine particulate matter. Associations between influenza A, air quality, viral co-infection, age, gender and nationality were assessed. Results. Distinct waves of influenza A and respiratory syncytial virus (RSV) were observed, while human rhinovirus (HRV) transmission persisted at a high level, with remaining viruses at low levels. Influenza A was significantly associated with pneumonia, while ORVs were the main causes of bronchiolitis (RSV) and reactive airway exacerbations (HRV); viral co-infections were uncommon. Bronchiolitis predominantly affected infants <12 months, whereas reactive airway exacerbations were seen across wider age bands. No consistent independent associations between air quality metrics and ARIs were observed after multivariable adjustment during the 5-month study period. Combining all the respiratory viruses showed a strong correlation with patterns of hospital admissions ( r =0.88, 95% confidence interval: 0.72, 0.95). Conclusion. Age and viral type were the most significant determinants of admission. While the associations of influenza A, HRV and RSV with pneumonia, acute exacerbations and bronchiolitis, respectively, were confirmed, all viruses had similar associations, and the strongest correlation was observed when all viruses were treated as a single wave.