25 Surveillance of Severe Acute Respiratory Infection 2 Introduction Severe acute respiratory infection (SARI) surveillance is essential for disease control and prevention, enabling assessment of the effectiveness of community-based preventive measures, detection of unusual events, identification of risk factors and evaluating pandemic preparedness and capacity management1,2. Ideally, a SARI surveillance system should be (near) real-time, combining syndromic surveillance with pathogen testing and be automated where possible to decrease the administrative burden. European-level SARI surveillance is available, with weekly reports being published online by the European Centre for Disease Prevention and Control (ECDC) 3. However, the number of contributing countries is small, and there is often a delay in reporting because of the intensive nature of data collection. At present, there is no robust sentinel or universal SARI surveillance system in the Netherlands. The rapid developments in the field of data science and the increase in easily accessible healthcare data bring new opportunities for infectious disease surveillance 4. While manual reporting of cases was once the sole method for infectious disease surveillance, a variety of data sources is used at present4. Selected International Classification of Disease (ICD)-10 codes are used in multiple European countries for SARI surveillance, with or without virological test results5-8. Although ICD-10 codes are standardised, delays in reporting and the mix of codes being used may over- or underestimate the true number of SARI cases1. A narrow selection of codes could underestimate the true number of SARI cases, while a broad selection of codes could overestimate the true number of SARI cases. Early in the COVID-19 pandemic in 2020, the World Health Organization (WHO) issued guidelines for the protection of healthcare workers, e.g. in hospitals9. These guidelines recommended contact and droplet precautions when caring for suspected COVID-19 patients. These guidelines have been implemented rapidly and, in most hospitals in the Netherlands, these patients have a contact and droplet precaution label in their electronic medical record (EMR). During the pandemic, information about the numbers in contact and droplet isolation have been used to determine the COVID-19 impact on hospital capacity10. Both before and following the pandemic, contact and droplet isolation precautions have been used for patients with a suspected viral respiratory infection. The number of these patients is likely to reflect the number of patients who are hospitalised with a respiratory tract infection and could serve as a proxy in SARI surveillance. In addition, patients who are hospitalised with a suspected viral respiratory infection are typically tested using a reverse transcription (RT) PCR test. The number of RT-PCR tests for viral respiratory pathogens, irrespective of the test result, could also reflect the number of hospitalised patients with a respiratory tract infection, and be suitable for SARI surveillance. In this proof-of concept study, we hypothesise that both the number of RT-PCR tests and contact and droplet precaution labels are indicative of SARI and could be pragmatic indicators for monitoring of trends and capacity management in
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