The transfer of the Influenza virus testing to partner laboratories was promoted to increase the diagnostic capacity required to meet the increased demands. This allowed the NIC to focus its activity again on the population based surveillance, the antigenic and genetic characterisation of the strains and the anti-viral resistance monitoring.
Although the decentralisation of the activities to the participating laboratories improves the analysis turnaround time and thus the patient care, it reduces the completeness of the cohort database. The missing information for gender, residence and date of birth can be explained by the priorities defined by the individual laboratories. Therefore the decentralisation reduced the capacity and the power of the epidemiological surveillance and this information can only be provided by community-based surveillance programmes such as the SGP network [3
Patients included in this study were not equally distributed throughout the different regions in Belgium. Since one Walloon laboratory did not report the residence of its patients, the representation of the patients group from the Walloon Region is slightly underestimated. However, the heterogeneous distribution should still be taken into consideration in the generalisation of the results to the diagnosed patients in Belgium.
The distribution curve of the number of infected cases as a function of time had the same shape as the one observed by other implemented surveillance systems such as the surveillance SGP network [3
] and the paediatric surveillance [5
]. The peak of the number of positive cases was observed in week 43, after which, in week 44, the decline of the number of cases was initiated.
There is some evidence that the start of school holidays reduces the influenza transmission and that the return to school slightly accelerates the epidemic. As the decline in prevalence occurred before the autumn holidays (week 45) we could not confirm this hypothesis.
The similarity in the age distribution of patients at the different laboratories demonstrates the age-independent inclusion criteria for these laboratories (Figure ).
The vulnerability for Influenza A(H1N1)2009 of the younger age groups as shown in this report, was also observed by the national surveillance SGP network [3
] and is consistent with other investigations [6
]. The very low presence of people over 65 years of age is consistent with other investigations (Health Protection Agency, Centers for Disease Control and Prevention). Data suggest that the elderly may to some extent be protected from infection [8
As in many other countries, the Influenza A(H1N1)2009 virus was the predominant circulating influenza virus [6
]. However 8 Influenza B strains were detected by 2 out of 7 laboratories performing this typing assay. Although these 2 laboratories reported the majority of samples (53.7%), this prevalence of subtype B could still be an underestimation. This is of notice as no Influenza B strain was detected from the samples provided by the SGP network [4
]. Whether this difference was due to the selection of patient samples or to the sensitivity or specificity of the assays should still be investigated.
These data are an additional tool for the virological surveillance of Influenza A(H1N1)2009 during the epidemic in Belgium. While the epidemiological surveillance by the SGP network selects patients with influenza-like illness (ILI) attending the general practitioner's, this surveillance study is focussed on hospitalised patients and patients at risk of severe diseases. Therefore, the obtained laboratory results provide information on the identification of the infection waves during an epidemic and can be implemented as a confirmation tool of the population-based surveillance.
To avoid inappropriate assignment of morbidity to the Influenza A(H1N1)2009 virus, some efforts to detect other respiratory viral agents are warranted. Therefore, it would be of particular interest if this virological surveillance could be enlarged with the monitoring of co-circulating viral agents associated with ILI during the Influenza season. Similarly the link between the cases and the clinical data is lacking in this study and should be considered in a future study.
Quality assessments have proven their usefulness to demonstrate similar performances be-tween the different laboratories [12
]. Considering the heterogeneous PCR procedures used by the different laboratories, the organisation of such a quality assessment will be of upmost importance to compare the relatedness of the performances between the laboratories. Meanwhile such a quality assessment was organised after the epidemic period by the MolecularDiagnostics.be workgroup for their members including most of the participating laboratories from our study. Results are available for these members through their website [14
This study demonstrated the fluent decentralisation of the confirmatory analysis from the NIC towards the participating laboratories. The network and websites were successfully organised and implemented allowing the collection of virological data. This network allowed the reduction of the workload at the NIC as well and made the diagnosis more widely available. Despite the differences in testing algorithm and inclusion criteria, similar positivity rates as well as age distributions of the confirmed cases were notified between the laboratories.