32 Chapter 2 The Pearson correlation coefficient reflects the correlation between two measures, with a value of −1 or 1 reflecting perfect positive or negative linear correlation, and a value of 0 indicating that there is no relation between the two measures. Intensive care unit population A total of 4,847 ICU (including the neonatal ICU, NICU) admissions, with or without one of the three surveillance indicators, were registered in our study period. A sensitivity analysis including only ICU admitted patients was performed. In contrast to the total hospital population, the three different surveillance indicators followed a relatively similar trajectory over time in both the absolute number (provided in Supplementary Figure 5) and the surveillance indicators as a proportion of the number of ICU admissions (provided in Supplementary Figure 6). Result by age group The three surveillance indicators show a relatively similar pattern when splitting the results by age group (provided in Supplementary Figures 7 (RT-PCR-tests), 8 (contact and droplet precautions) and 9 (ICD-10 diagnostic codes)). An increase in the number of RSV infections was reported in the summer of 202114, which can be seen in the 0–4-year age group for all surveillance indicators, but most clearly with contact and droplet precautions (provided in Supplementary Figure 8) and ICD-10 registration (provided in Supplementary Figure 9). Using the positive RT-PCR test results provided in Supplementary Figure 10, the increase in RSV infections in young children can also be seen. Discussion We compared three different surveillance indicators for their potential use in SARI surveillance. Generally, the three surveillance indicators followed a similar pattern between 2017 and 2023, with differences in the absolute values. There are two important exceptions to this, both in late 2020. The first was a divergence of the absolute count of contact and droplet precaution labels from the other two indicators, explained by a change in registration policy. This registration policy change involved an expansion of the number of healthcare workers who could register contact and droplet precautions. Prior to this change, only staff in the hospital hygiene and infection prevention department were able to register these labels. As registration of contact and droplet precaution labels is now primarily done by nurses, the registration is more complete as it is no longer limited to working hours, i.e. 9am–5pm Monday to Friday. The second exception is that during the second COVID-19 wave in the Netherlands (first surge started week 41/202015), there was an increase in ICD-10 registrations and positive RT-PCR tests, but not in the number of RT-PCR tests. The lack of increase in the number of RT-PCR tests might be explained by a relatively large number of patients who tested at home or were transferred to our hospital,
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