29 Surveillance of Severe Acute Respiratory Infections 2 Absolute counts On average, the absolute count in the pre-COVID-19 years (2017–19) was lower than the absolute count during and at the end of the pandemic (see figure 1 and Supplementary Table 1, which provides the mean count per week for the three surveillance indicators). Prior to the COVID-19 pandemic, all three surveillance indicators had relatively similar absolute counts, with the number of RT-PCR tests being slightly higher than the other two surveillance indicators, especially during the traditional influenza-like illness (ILI) season. In 2018, for example, there were on average 4.5 contact and droplet precaution labels, 10.2 ICD-10 codes and 15.1 RT PCR tests each week, see Supplementary Table 1. From spring 2020 onwards, the number of RT-PCR tests was consistently higher than the number of the other two surveillance indicators e.g. in 2021, there were 34.5 contact and droplet precaution labels, 18.2 ICD-10 codes and 69.6 RT-PCR tests each week (see Supplementary Table 1). The number of contact and droplet precaution labels remained low throughout most of 2020, but increased steeply at the end of 2020 and remained higher than ICD-10 registrations in 2021, 2022 and 2023 (up to and including week 18). During this study period, 11,959 PCR tests were done, and 4,683 contact and droplet isolation labels and 3,908 ICD-10 codes were registered. The total number of registrations for the three surveillance indicators combined is 20,550 for 117,404 hospital admissions. 2023 2021 2022 2019 2020 2017 2018 0 5 101520253035404550 0 5 101520253035404550 0 25 50 75 100 0 25 50 75 100 0 25 50 75 100 0 25 50 75 100 Weeknumber Absolute count Contact and droplet precautions ICD−10 RT−PCR Figure 1. Absolute count per week for the three surveillance indicators over time, Leiden University Medical Center, Leiden, the Netherlands, 1 January 2017–30 April 2023 (n = 117,404 hospital admissions).
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