Influenza virus is a leading cause of human morbidity (‘how unhealthy we are’) and mortality (‘death rate’) worldwide. On average, influenza viruses infect 5 to 15% of the global population, resulting in approximately 500,000 deaths annually. Each year, an influenza epidemic occurs and about three or four times a century, a new influenza virus emerges and causes a pandemic. The severity of the epidemic or pandemic varies, depending on the type and subtype of circulating influenza virus and the degree of vulnerability of the population to the virus. Influenza surveillance is therefore a process that must be repeated every year. Since the A(H1N1) pandemic in 2009, special attention is being paid to a closer monitoring of the severity of influenza cases. The Belgian National Influenza Centre (NIC) has extended, since 2010, its surveillance to SARI (Severe Acute Respiratory Infection) cases.
In Belgium, the influenza surveillance is performed by the National Influenza Centre, in collaboration with the Unit of Epidemiology of Infectious Diseases of Sciensano. A network of sentinel general practices (SGPs) has been involved since 2007 in the clinical and virological influenza surveillance. The main purposes of the surveillance are the early detection of an influenza epidemic, the study of the intensity and duration of the epidemic, the identification and characterisation of circulating viruses and the participation to the selection of next-season influenza vaccine strains.
The development of capability to detect new emerging viruses, the estimation of vaccine effectiveness and the monitoring of the antiviral susceptibility are also important tasks.
Influenza surveillance is based primarily on physician data that collect weekly the number of influenza-like illnesses (ILI) and acute respiratory infections seen in consultation and also collect nasopharyngeal samples from a part of their patients.
On the basis of this information, the epidemic curve is established and the intensity of the circulation of the virus in the population is measured. These data can provide a very good indication of influenza in the general population.
However, following the A(H1N1)2009 pandemic, it was clear that the ILI surveillance was not sufficient to give adequate information on the severity of influenza disease and the virulence of circulating viruses. Therefore, WHO and ECDC recommended to also install hospital-based surveillance of severe acute respiratory infections (SARI) as a tool to monitor severe diseases caused by influenza. This should complement surveillance of outpatient monitoring of influenza-like illness by the network of SGPs to cover the full spectrum of influenza-related diseases.
As a result, in Belgium, a SARI surveillance network has been implemented since 2010 and includes 6 hospitals. About 1000 to 2000 samples of SARI cases per year are sent for analysis to the NRC Influenza at Sciensano. This surveillance is based on the recording of clinical data by the hospitals, detection of influenza viruses by real-time PCR at the NRC and the comparison of different indicators of severity, such as treatment in the intensive care unit (ICU), acute respiratory distress syndrome (ARDS), extracorporeal membrane oxygenation (ECMO), death, etc.
The main goal of this surveillance is to contribute to the early detection of clinical and/or virological signals of severity during the influenza epidemic or pandemic periods, to describe the genotypic and phenotypic characteristics of influenza viruses associated with severe forms of infection and to analyse the presence and implication of other respiratory viruses.