Summaries in English and Dutch 10 301 and the United States. These health inequalities are likely to be due to a complex interplay of biological, social, cultural and psychological factors – many of which existed prior to COVID-19 – which must be understood and addressed urgently, especially before facing a next pandemic. Chapter 4 highlights the evolution of COVID-19 symptoms observed in the RECoVERED cohort, a prospective cohort study of adults with mild (ambulatory) to critical (intensive care unit [ICU] admission) laboratory-confirmed SARS-CoV-2 infection. In survival analyses, approximately one-third of participants with initially mild COVID-19, two-thirds of participants with moderate COVID-19 and four-fifths of participants with severe or critical COVID-19 reported one or more ongoing symptoms 12 weeks after illness onset (i.e., ‘long COVID’). One year after illness onset, almost half (40%) of participants continued to report at least one persistent symptom. Participants with female sex and obesity experienced a slower recovery from COVID-19 symptoms. These data provided initial insight into the occurrence and determinants of persistent symptoms following SARS-CoV-2 infection. Chapter 5 explores the prevalence and determinants of the most commonly reported persistent symptom: fatigue. To overcome reporting bias in symptom questionnaires, a validated questionnaire (the Short Fatigue Questionnaire [SFQ]) was used to determine the prevalence of severe fatigue at months 1, 3, 6, 9 and 12 after illness onset. Among our study participants, 17.4%, 21.6% and 44.8% with initially mild, moderate or severe/critical COVID-19, respectively, were severely fatigued 12 months after COVID-19 onset. Determinants of severe fatigue 6 months after illness onset included initially severe/critical COVID-19 and reporting low mood during the first 2 weeks of illness. Little recovery from fatigue beyond 6 months after illness onset was observed, suggesting poor prognosis among participants with ongoing fatigue half a year after infection. In the absence of a SARS-CoV-2-negative control group or pre-COVID-19 data on fatigue, however, the extent to which severe fatigue can be explained by SARSCoV-2 infection remains uncertain. Part 2 of this thesis features an in-depth exploration of long COVID, aiming to provide the reader with a thorough overview of what we currently understand about the condition. Chapter 6 identifies groups of participants with similar patterns of COVID-19 symptoms over time. Four trajectories of the total number of symptoms were identified, with 8.9% of participants consistently reporting 6 or more symptoms whilst a quarter (24.5%) reported no persistent symptoms over a 24-month period after illness onset. Interestingly, the total number of reported symptoms in each trajectory did not appear to fluctuate substantially over time. However, when exploring trajectories of individual
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