98 Chapter 4 Inclusion Influenza Patients aged ≥18 years with influenza, upper RTIs, or flu-like symptoms were identified in the ELAN registry by searching the dossiers for the International Classification of Primary Care first edition (ICPC-1) codes (Table 1). Patients were included if they had any of these codes registered during influenza seasons 2017, 2018, 2019, or 2020 (Box 1) (9,10). Box 1. Definition and dates influenza season (9,10) An influenza season is defined as more than 51 patients per 100,000 inhabitants with influenza like illness or symptoms visiting their GP. For season 2019-2020, the threshold was 58 patients per 100,000 inhabitants per week. 2017: November 28, 2016 up to including March 6, 2017. 2018: December 11, 2017 up to including April 9, 2018. 2019: December 10, 2018 up to including March 11, 2019. 2020: January 27, 2020 up to including, March 15, 2020 SARS-CoV-2 The following two definitions for diagnosis of a COVID-19 infection were accepted: (1) COVID-19 confirmed with a positive polymerase chain reaction (PCR) test and an appropriate ICPC code in the EMR (Table 1); and (2) COVID-19 highly suspected, based on symptoms (Box 2) and an appropriate ICPC code in the EMR (Table 1). The second definition was used owing to a lack of test capacity in The Netherlands from the start of the SARS-CoV-2 pandemic (February 2020) until 1 June 2020. Patients were included in the study if their PCR test or symptoms (Box 2) matched the definition of COVID-19, categorised as confirmed or suspected COVID-19, and divided in two groups according to their date of diagnosis (11). The first wave lasted from 15 February 2020–1 August 2020. The second wave lasted from 1 August 2020–1 January 2021. The SARS-CoV-2 index lineage was dominant in The Netherlands during both waves (12).
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