Long Covid remains a mystery, though theories are emerging

Between 10 to 20 percent of people who contract coronavirus are estimated to have long Covid symptoms.

Millions of people around the world are believed to suffer from long Covid yet little remains known about the condition -- though research has recently proposed several theories for its cause.

Between 10 to 20 percent of people who contract coronavirus are estimated to have long Covid symptoms -- most commonly fatigue, breathlessness and a lack of mental clarity dubbed brain fog -- months after recovering from the disease.

The US-based Institute for Health Metrics and Evaluation (IHME) estimates that nearly 145 million people worldwide had at least one of those symptoms in 2020 and 2021.

In Europe alone, 17 million people had a long Covid symptom at least three months after infection during that time, according to IHME modelling for the World Health Organization (WHO) published earlier this month.

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These millions "cannot continue to suffer in silence", WHO Europe director Hans Kluge said, calling for the world to act quickly to learn more about the condition.

Researchers have been racing to catch up but the vast array -- and inconsistency -- of symptoms has complicated matters.