What exactly is Long-COVID?

What is Long COVID and how can it be diagnosed?

What exactly is Long COVID?

If symptoms persist for more than twelve weeks after a coronavirus infection, the condition is referred to as Post-COVID or Long COVID. The term, however, covers a wide range of different health issues.

Some individuals experience lasting organ damage — for example, in the lungs, heart, or other organs — which can often be detected through imaging procedures (such as CT scans).

Others suffer from severe fatigue, concentration problems, sleep disturbances, or pain, even though medical tests show no clear physical cause. This form closely resembles an illness known for many years: ME/CFS (Myalgic Encephalomyelitis / Chronic Fatigue Syndrome).

The challenge of diagnosis

Both Long COVID and ME/CFS are serious conditions, but so far there is no single medical test that can definitively identify them.

There is no specific blood marker or biomarker that proves Long COVID, and imaging studies (such as MRI or CT) usually show no characteristic changes.

Diagnosis is therefore based on symptoms and their duration. Physicians use various symptom checklists or criteria that were not originally developed for Long COVID. As a result, diagnoses can vary depending on the system used. This inconsistency poses a major challenge for research, since studies often focus on very different patient groups.

Possible causes and mechanisms

Why some people remain ill for weeks or months after a COVID infection while others fully recover is still not fully understood. Several theories attempt to explain the persistence of symptoms:

• Viral persistence: Fragments of the virus may remain in body tissues and repeatedly trigger inflammation.

• Reactivation of other viruses: The coronavirus could reactivate dormant viruses such as the Epstein–Barr virus, which can also cause symptoms.

• Autoimmune reactions: The immune system may remain overactive after infection and mistakenly attack the body’s own structures, leading to chronic inflammation.

• Vascular damage and microclots: There is evidence of tiny blood clots (microclots) that block small vessels and impair circulation. Reduced blood flow has been observed in the brains of some Long COVID patients.

• Mitochondrial dysfunction: The mitochondria — the “powerhouses” of the cells — may not function properly, leading to an energy deficit that could explain the profound fatigue many people experience.

All these mechanisms are currently scientific hypotheses — none has yet been definitively proven. It is likely that several factors interact and that the underlying causes vary from person to person.

For more information on the latest scientific findings about Long COVID diagnostics, see the

Science Cops video (German).

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