Chapter 6 178 modelled in a subsequent, multivariable GBTM. We included a priori the pre-morbid risk factors age (years) at infection, sex, and BMI category as well as timing of infection (first COVID-19 wave vs. subsequent waves) as risk factors, from which the odds ratios (OR) and their 95% confidence intervals (CI) (comparing the odds of group membership between levels of the risk factors) could be estimated. Severity of initial COVID-19 disease was not included in this model as it acts lies on the pathway between pre-morbid risk factors and long COVID. To assess the association between symptom trajectory group and illness perception, we calculated the median (IQR) B-IPQ scores for each sub-domain across trajectory groups at each time-point and compared using the Kruskal-Wallis test. In a post-hoc analysis, the mean total B-IPQ scores at M1, M6, and M12 were then modelled using a linear mixedeffects model, stratified by trajectory group. Time since illness onset was included as both a fixed and random effect whilst allocated trajectory group and its interaction with time, sex and age (years) were added to the model as fixed effects. BMI category, initial COVID-19 severity and timing of SARS-CoV-2 infection were included in the model if the likelihood ratio test (LRT) indicated a significantly improved fit. In an additional post-hoc analysis, we assessed the potential of using total B-IPQ scores at month 1 as an early screening tool for later more severe long COVID. We used an ordered logit model to regress group-based symptom trajectory group on total M1 B-IPQ score (per 10-point increase), with covariates selected as in the linear mixed-effects model. Analyses were performed using Stata (v15.1, StataCorp LLC, College Station, TX, USA). GBTM were estimated using the “traj” plug-in in Stata. A two-sided p-value <0.05 was considered statistically significant. Sensitivity analyses To understand the effect of our definition of long COVID symptoms on the identified trajectories, a sensitivity analysis was performed in which all symptoms (i.e., not only those arising <1 month of overall illness onset) were included in the outcome of the GBTM. RESULTS Description of the study population Of 349 enrolled study participants by June 2021, 292 (83.7%; 86/292 [29.5%] mild, 127/292 [43.5%] moderate and 79/292 [27.1%] severe/critical COVID-19) completed
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