Millions of people around the world are thought to be suffering from long-term Covid, but little is still known about the disease – although recent research has suggested several theories as to its cause.

It is estimated that between 10 and 20 per cent of people who contract coronavirus months after recovering from the disease have long-lasting Covid symptoms – most commonly fatigue, shortness of breath and a lack of mental clarity known as brain fog.

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

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

Those millions “cannot continue to suffer in silence,” said WHO Europe Director Hans Kluge, urging the world to act quickly to learn more about the disease.

Researchers have been trying to catch up, but the wide range — and inconsistency — of symptoms have complicated things.

More than 200 different symptoms have been attributed to the long Covid so far, according to a study by University College London.

“There are no symptoms that are really specific to Long Covid, but it does have certain characteristics that fluctuate,” said Olivier Robineau, Long Covid coordinator at France’s Emerging Infectious Diseases Research Agency.

“Fatigue stays in the background,” he told AFP, while symptoms “seem to get worse after mental or physical exertion — and become less frequent over time.”

One thing we do know is that people with more severe initial cases, including those requiring hospitalization, are more likely to get long Covid, according to the IHME.

Researchers have followed several clues as to what exactly might be behind the condition.

A study published in September in the journal Clinical Infectious Diseases found that Covid’s infamous spike protein – the key that lets the virus into the body’s cells – was still present in patients a full year after infection.

This suggests virus reservoirs can persist in some people and potentially cause inflammation that could lead to long-lasting Covid-like symptoms, the researchers said.

If they’re right, a test could be developed to identify the spike, potentially leading to one of the grand and elusive goals of long Covid research – a clear path to diagnosing the condition.

However, their findings have not been corroborated by other research, and several other causes have been suggested.

A leading theory is that tissue damage from severe Covid cases triggers permanent immune system dysfunction.

Another suggests the initial infection causes tiny blood clots that could be linked to long-lasting Covid symptoms.

However, “the data for each of these hypotheses is not yet very robust,” said Robineau.

It is very likely that “we will not find a single cause to explain Covid for a long time,” he added.

“The causes may not be exclusive. They could be related in the same person or even follow each other or be different in different people.”

A way to treat the condition also remains elusive.

For a year now, the Hotel-Dieu hospital in Paris has been offering a half-day treatment course to long Covid patients.

“You meet an infectious disease specialist, a psychiatrist, and then a doctor who specializes in sports rehabilitation,” said Brigitte Ranque, who leads the CASPER protocol.

“In the team’s experience, a majority of the symptoms can be traced back to functional somatic syndromes,” she said. This is a group of chronic conditions like chronic fatigue and fibromyalgia that have no known cause.

Cognitive-behavioral therapy, a psychological approach commonly used for these syndromes, is used to treat long-term Covid alongside monitored physical activity, Ranque said.

“Patients are brought back three months later. The majority of them are better off. More than half say they are cured,” she told AFP.

“But about 15 percent haven’t improved at all.”