Evolution of COVID-19 symptoms during the first 12 months after illness onset 4 111 be recognised. Questionnaires in languages other than English and Dutch were not offered, therefore individuals with a migration background, who have been disproportionally affected by COVID-19, also in Amsterdam[29, 30], were underrepresented in this cohort. Furthermore, as the majority of our study participants were enrolled when wild-type SARS-CoV-2 was the dominant variant in the Netherlands, the progression of disease reported in our cohort may not be representative for patients infected with other SARSCoV-2 variants[31]. In addition, certain symptoms that are frequently linked to post-COVID syndrome (e.g. ‘brain fog’, sleep disturbance) were not recorded. A further limitation was the effect of survival bias among retrospectively-enrolled participants (although sensitivity analysis of prospectively-enrolled participants rendered comparable results). In addition, those who were in a life-threatening situation when admitted to hospital were less likely to be enrolled in the study (as demonstrated by only 2 COVID-19 deaths in our cohort and significantly older median age of non-enrolled hospitalised patients [Supplementary Figure S1]). Our study population might not be generalizable to those with extremely severe disease; however, these individuals have a high risk of death and symptom recovery is not applicable to deceased individuals. Finally, pre-COVID symptomatology was not recorded, making it difficult to accurately estimate the proportion of persistent symptoms directly attributable to post-COVID syndrome. We demonstrated that post-COVID syndrome is common, even after mild disease. Symptoms persisted for twelve months after illness onset in one-sixth of participants with mild disease and in approximately half of participants with moderate and severe/critical disease. Female sex and obesity were the most important predictors of slow recovery, showing that creating an environment which facilitates healthy living behaviours is of utmost importance, even during a pandemic. Next steps in post-COVID syndrome research must include assessing the public health and socioeconomic impact, identifying further predictive and prognostic characteristics, and exploring the underlying biological mechanisms of disease in order to develop effective interventions.
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