Sentinel severe acute respiratory infection (SARI) surveillance was recommended by WHO following the 2009 H1N1 influenza virus pandemic.It uses a case definition based on clinical criteria, making it relevant to monitor the severity of other respiratory viruses such as SARS-CoV-2.The Belgian sentinel SARI network is composed of six representative hospitals. The original case definition was based on the 2014 WHO SARI case definition. Following the emergence of SARS-CoV-2, a more exhaustive list of symptoms was reported by the hospitals, in addition to basic demographic data, information on known risk factors/comorbidities, influenza and SARS-CoV-2 vaccination status, administration of antiviral or antibiotics, complications during hospitalization, and outcome. Respiratory samples were sent to the National influenza center for RT-qPCR testing for influenza virus, SARS-CoV-2 and 16 other respiratory viruses.During the first phase of the pandemic (Feb–March 2020), the SARI network operated normally (covering the end of the influenza epidemic) and was able to capture the local spread of SARS-CoV-2 in Belgium at the same time as the national emergency monitoring. In phase 2 (May 2020–January 2021), coinciding with the first waves of the pandemic when the strongest Non-Pharmaceutical Interventions were in place, the SARI surveillance was disrupted due to a high workload in the hospitals dealing with the surge of hospitalized COVID-19 cases. In phase 3 (January–December 2021), the SARI surveillance resumed in a lighter mode (using comprehensive sampling on specific days to provide more flexibility to the hospitals). The adapted method enabled us to monitor the COVID-19 pandemic and the resurgence of other respiratory viruses such as RSV and parainfluenza viruses, and to contribute to European COVID-19 vaccine effectiveness studies. Phase 4 started in January 2022, with the re-emergence of seasonal influenza viruses and their co-circulation with SARS-CoV-2.
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