Maaike Swets

31 Surveillance of Severe Acute Respiratory Infections 2 Correlation The correlation between the different surveillance indicators can be seen in Table 1. The total number of RT-PCR results was used for the PCR surveillance indicator (as opposed to the number of positive tests). Overall, the correlation between the different surveillance indicators was highest in the third time period, especially between contact and droplet precautions and ICD-10 registration. Surveillance indicators Week 1/2017– week 8/2020 Week 9/2020– week 53/2020 Week 1/2021– week 18/2023 RT-PCR and contact and droplet precautions 0.82 0.38 0.73 Contact and droplet precautions and ICD-10 0.52 0.66 0.84 RT-PCR and ICD-10 0.57 0.56 0.64 Table 1. Pearson correlation coefficient for the correlation between combinations of surveillance indicators in the three different time periods, Leiden University Medical Center, Leiden, the Netherlands, 1 January 2017–30 April 2023 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 Virus Influenza A Influenza B Other RSV SARS−CoV−2 RT−PCR tests and positive RT−PCR results Figure 3. RT-PCR test results for respiratory viruses over time, Leiden University Medical Center, Leiden, the Netherlands, 1 January 2017–30 April 2023 (n = 117,404 hospital admissions). RSV: respiratory syncytial virus; SARS-CoV-2: severe acute respiratory syndrome coronavirus 2. Positive RT-PCR test results over time (coloured bars) and total number of RT-PCR tests done (yellow line) are shown. The total number of positive tests represented are as follows: influenza A: 274; influenza B: 114; other: 1,009; RSV: 317; SARS-CoV-2: 1,034. The ‘other’ group includes the following viruses: human adenovirus, bocavirus, human coronaviruses (229E, HKU1, NL63 and OC43), human metapneumovirus, parainfluenza virus 1–4 and human rhinovirus. The number of positive tests can be higher than the total number of tests when a patient tests positive for multiple viruses.

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