Chapter 6 190 Strengths of our study include its long prospective follow-up from illness onset onwards, representation of individuals with mild to critical COVID-19, and detailed symptom data. Nonetheless, our study also has several limitations. Without a SARS-CoV-2-negative or pre-pandemic historical control group, it is difficult to determine which symptoms are truly attributable to long COVID and which symptoms are due to a background prevalence of pre-existing complaints. In an attempt to avoid counting non-specific, transient symptoms as long COVID, we deemed symptoms arising more than 1 month after acute illness onset unrelated to long COVID. In doing so, we may have ignored some long COVID symptoms with a later date of onset. However, a sensitivity analysis removing this assumption demonstrated no difference in trajectory shapes, only in total number of symptoms. In addition, without validated thresholds to indicate the significance of B-IPQ values, we cannot examine clinically relevant outcomes. Furthermore, we did not include vaccination or reinfection as time-varying covariates in our GBTM. Reinfection may have altered the course of symptoms in a small proportion of individuals[23], although the impact of vaccination on long COVID symptoms appears to be negligible[24]. In conclusion, we identified distinct two-year trajectories of long COVID symptoms, which were strongly congruent with illness perception over time. Additionally, we observed groups of individuals with diverse patterns of specific symptom progression, suggesting a need for individualised clinical management. Female sex and overweight or obesity are risk factors of having persistently high number of symptoms and more negative B-IPQ scores. Both researchers and clinicians should recognise that long COVID symptoms may fluctuate over time, particularly when making estimating the occurrence of the condition.
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