Statistical analyses
SPSS versions 25 and 27 were used for statistical analyses. Chi square tests were used to compare groups for categorical variables and t-tests (equal variances not assumed) for comparing means of continuous variables. Normality was tested using the Shapiro-Wilk´s test. FMD in ME/CFS patients and in healthy controls were normally distributed, and differences in mean between groups at baseline were compared by analyses of covariance (ANCOVA) in order to adjust for sex and baseline arterial diameter. For PORH, data from the ME/CFS group showed normal distribution, while data were not normally distributed in the healthy control group, and Mann-Whitney test was used to assess difference between groups. FMD and PORH by ME/CFS severity groups were tested by ANOVA and Kruskal-Wallis tests, respectively.The variables that describe patient function (SF-36-PF, FSS, self-reported function level and mean steps per 26h) were not normally distributed, therefore, Wilcoxon signed rank tests for related samples were used to compare means at baseline and 18 months. Three patients, for whom we only had baseline vascular values, were excluded from these analyses. General Linear Models (GLM) for repeated measures were used to assess differences in changes of the outcomes measures (FMD or PORH), from baseline to 18 months, between the randomization groups (rituximab and placebo). The analysis syntax for GLM repeated measures for FMD and PORH, with treatment group (rituximab or placebo) as between-subjects factor, is included as a supporting file (