Elke Wynberg

Evolution of COVID-19 symptoms during the first 12 months after illness onset 4 101 Definitions COVID-19 illness onset was defined as the first day of symptoms; for asymptomatic patients, date of SARS-CoV-2 diagnosis was used. Complete recovery was defined as resolution of all COVID-19 symptoms. As per National Institute for Health and Care Excellence (NICE) guidelines[15], the acute phase of disease was defined as the first four weeks after illness onset, and post-COVID syndrome as symptoms persisting at least 12 weeks after illness onset. Clinical severity groups were defined based on WHO COVID-19 disease severity criteria[16] using physical measurements from D0 and D7 study visits. Mild disease was defined as having a RR<20/min and SpO2>94% on room air at both D0 and D7 study visits; moderate disease as having a RR20-30/min and SpO2 90-94% on room air (or receiving oxygen therapy, if no off-oxygen measurement available) at either visit; severe disease as having a RR>30/min and SpO2<90% on room air (or receiving oxygen therapy) at either visit; critical disease as requiring ICU admission as a result of COVID-19 at any point. Symptom severity was measured on a four-point scale, with the exception of dyspnoea, measured using the six-point modified Medical Research Council (mMRC) breathlessness scale[17]. Comorbidities at illness onset were those listed by the WHO as associated with severe COVID-19[18]: cardiovascular disease (CVD), diabetes mellitus (DM), chronic lung disease (CLD), liver disease, chronic kidney disease, immunodeficiency, cancer, cerebrovascular disease, dementia or psychiatric illness. Obesity was excluded from the comorbidity variable because body mass index (BMI) was defined separately, categorised in kg/m2 as: <25, underweight or normal weight; 25-30, overweight; >30, obese. Ethnicity was based on the country of birth of the study participant and their parents[19]. Loss to follow-up (LTFU) was defined as active withdrawal from the study or two consecutive no-show appointments despite three attempts to establish contact. Date of LTFU was defined as the date of last contact with the participant. Statistical analyses Socio-demographic and clinical characteristics of participants were compared between clinical severity groups. The incidence proportions for 18 different symptoms (based on the WHO/ISARIC Case Report Forms [CRF][14]) at one, four and twelve weeks after illness onset were calculated as the number of participants reporting each symptom since illness onset over the total

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