Viral aetiology of influenza-like illness in Belgium during the influenza A(H1N1)2009 pandemic

The purpose of this investigation was to determine the proportion of influenza-like illness (ILI) attributable to specific viruses during the influenza A(H1N1)2009 pandemic and to describe the demographic and clinical characteristics of ILI due to respiratory viruses in Belgium. Nasopharyngeal swabs were collected from ILI patients by general practitioners (GPs) and paediatricians (PediSurv) and analysed for viruses. Of 139 samples collected from children <5 years of age by PediSurv, 86 were positive, including 28 influenza (20%), 27 respiratory syncytial virus (RSV) (19%), 21 rhinovirus (17%), 12 human metapneumovirus (hMPV) (9%) and ten parainfluenza virus (PIV) (7%). Of 810 samples received from GPs, 426 were influenza (53%). Of 312 influenza-negative samples, 41 were rhinovirus (13%), 13 RSV (4%), 11 PIV (4%) and three hMPV (1%). Influenza mostly affected the 6–15 years old age group. Other respiratory viruses were commonly detected in the youngest patients. Similar clinical symptoms were associated with different respiratory viruses. Influenza A(H1N1)2009 was the most detected virus in ILI patients during the 2009–2010 winter, suggesting a good correlation between ILI case definition and influenza diagnosis. However, in children under 5 years of age, other respiratory viruses such as RSV were frequently diagnosed. Furthermore, our findings do not suggest that the early occurrence of the influenza A(H1N1)2009 epidemic impacted the RSV epidemic in Belgium.