General discussion 9 285 decision-making during a crisis often relies on imperfect and incomplete data. Evaluating how surveillance data were collected, interpreted and utilised in the Netherlands during the first months of the crisis may help guide decision-making when next confronted with a pathogen with pandemic potential. At the beginning of the COVID-19 pandemic, public health recommendations were overly reliant on notification of laboratory-confirmed cases and thus subject to detection bias. Although the European Centre for Disease Prevention and Control (ECDC) recognised in February 2020 that substantial uncertainty existed around case data, the risk associated with SARS-CoV-2 infection to the wider European population was still deemed to be low[96]. In the absence of sufficient laboratory capacity to test widely, certain existing tools could have been utilised to help overcome blind spots in our laboratory surveillance. For instance, early integration of existing syndromic surveillance systems in monitoring the outbreak may have helped provide initial warning of widespread disease transmission, overcoming initial barriers to testing in the context of limited PCR testing capacity. For instance, Nivel sentinel stations in the Netherlands offer a representative sample of primary care practices that, since 1970[97], have reported on the number and basic characteristics of respiratory syndromes seen in the clinic in order to monitor the incidence of influenza-like illness. Timely evaluation of these data may have allowed for early detection of an increase in respiratory illnesses in the wider community at a time when testing was restricted to those with severe disease, allowing for earlier initiation of NPIs. In addition, InfectieRadar (a voluntary symptom survey, part of the InfluenzaNet project[98]), may have helped assess outbreak dynamics of non-hospitalised infections during the first few months of the pandemic[99] when test capacity was limited to those with severe disease. Indeed, syndromic surveillance has been previously recognised as an early warning tool in the context of disease outbreaks[100]. The Netherlands could therefore utilise existing networks more extensively in the future to, when appropriate, initiate more aggressive early containment strategies. During the pandemic, other surveillance tools were also scaled up to help monitor SARS-CoV-2 dynamics in the Netherlands, especially of non-hospitalised infections in the community. Sewage surveillance, for instance, had previously used to monitor polioviruses[101] and proved to be an invaluable way to overcome biases in testing access and uptake during the COVID-19 pandemic (although still prone to sampling bias). Sewage surveillance has been recognised as a vital component of population-level data on infectious disease spread and its value in monitoring a wide range of different pathogens is being explored[102]. Another form of surveillance that proved to be crucial
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