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New studies confirm that fatigue remains the most common feature of so-called long COVID-19, and in a substantial fraction it is the most stubborn.

In fact, researchers are finding that a diagnosis of chronic fatigue -- with all that implies -- appears inevitable for many.

Among 239 individuals belonging to online support groups for "long COVID," 178 (85%) said they had severe fatigue when surveyed 11 weeks after symptom onset, according to Maarten Van Herck, a PhD student at the University of Hasselt in the Netherlands, who spoke at the European Respiratory Society's virtual annual meeting.

At week 24, the fraction indicating they suffered from severe fatigue stood at 79% -- with 15 of the 35 who said they had mild or no fatigue at week 11 now saying it was severe, even as 31 of the 178 with initially severe fatigue reported it had become milder or disappeared.

Responses to survey items on specific fatigue symptoms showed an even distribution between mental and physical features.

Perhaps most alarming was that, by week 24, most of the cohort had consulted at least one medical professional as well as one or more allied specialists such as physiotherapists, psychologists, or dietitians -- yet most of those with severe fatigue apparently had no benefit.

Only a few -- 4% at week 11 and 13% at week 24 -- had received formal rehabilitation therapy.

"A large share is progressing toward chronic fatigue," said Van Herck, defined as severe fatigue lasting more than 6 months. And, he added, "it remains unclear whether and to what extent fatigue will resolve spontaneously" as time goes on.

The same group reported additional data from the cohort in a poster presentation titled "Recovery from COVID-19: a sprint or a marathon?"

As might be expected from a group experiencing severe fatigue, the mean percentage of work time missed because of poor health was 52% at week 24, and mean impairment while at work was 60%.

There was one encouraging finding in the study: about 11% of the cohort at week 24 indicated they were in overall poor health, down from 26% at the 11-week survey.

On the other hand, responses to standard survey questions on quality of life indicated that more than one-quarter of the group at week 24 were at less than the fifth percentile compared with the general population. And their self-rated health status was only fair on average, scoring 56 on a 100-point visual analog scale. (Nearly 90% retroactively reported good health prior to COVID-19 infection, Van Herck said.)

The study had substantial limitations, of course. Because it was drawn from online support groups, the cohort was not necessarily representative of all "long COVID" patients, and the survey response rate was not reported. Additionally, patients rated their own fatigue, health status, and other data, which thus were subject to various potential biases.