Long COVID is defined by persistent symptoms following COVID-19 infection. Approximately 71% of individuals with long COVID experience ongoing fatigue, postexertional malaise, and cognitive impairments, which share pathological similarities with myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS). This similarity has prompted studies to explore the characteristics of long COVID to gain a better understanding of ME/CFS. To gain insights, we investigated the clinical and laboratory characteristics of individuals with fatigue-dominant long COVID.

We enrolled 100 subjects (36 males, 64 females) with long COVID who had a higher score than 60 in the modified Korean version of the Chalder Fatigue Scale (mKCFQ11) and higher than 5 in a fatigue-focused visual analogue scale. To investigate fatigue symptoms, the mKCFQ11, the Multidimensional Fatigue Inventory, a visual analogue scale for fatigue and brain fog, along with the Short-Form survey, were employed. We also measured 3 cytokines and cortisol levels for immunological and endocrinological indicators. As a cross-sectional observational study, the data were collected at a single point in time.

The mean scores on the measurements showed severe fatigue, and these scores were significantly correlated, with no differences based on sex, the post-COVID period, or age. Among the laboratory tests, plasma cortisol levels had a significant negative correlation with fatigue scores and a positive correlation with living quality. The negative correlation between cortisol levels and mKCFQ11 scores appeared to be more specific to mental fatigue than physical, which conflicted with other measurements.

Our findings provide the first insights into the characteristics of fatigue in individuals with long COVID, particularly in terms of fatigue severity and cortisol levels. These results serve as valuable reference data for clinicians dealing with fatigue symptoms in long-COVID patients and for researchers exploring postviral fatigue symptoms, including ME/CFS, in the future.