Scientists claim to have found a "central puzzle piece" on the trail of long covid, raising hopes of future breakthroughs that may also illuminate other stubbornly ambiguous chronic syndromes.

Millions of people across the world are thought to be suffering from long Covid - the name given to a wide variety of symptoms still being suffered by people weeks and months after they first contracted the SARS-CoV-2 virus, or COVID-19.

The most common symptoms of long covid are fatigue, shortness of breath, muscle pain and brain fog, but other symptoms are also linked.

Several studies have been carried out since the pandemic to try to find out more about long covid and how it affects the human body.

READ MORE

Turkiye earthquake: One year on

Why a Chinese dragon isn't a dragon

'Final victory' the only solution for peace: Netanyahu

One notable study released last month showed there were significant differences in the proteins of the blood of more than 110 long Covid patients.

Swiss researcher Onur Boyman, a senior author of the study, believes this finding is an integral reason as to what keeps Covid raging for so long in the bodies of some people.

Part of the body's immune system called the complement system, which normally fights off infection by killing infected cells, remains active in people with long Covid, continuing to attack healthy targets and causing tissue damage, the researchers said.

Boyman said that when people recovered from long Covid, their complement system also improved, suggesting a strong link between the two.

"It shows that long Covid is a disease and you can actually measure it," Boyman said, adding the team hopes this could lead to a future test.

Researchers not involved in the study cautioned that this complement system "dysregulation" could not explain all the different ways that long Covid seems to attack patients. 

"It is great to see papers coming out now showing signals which might start to explain long Covid", said Claire Steves, professor of ageing and health at King's College London.

Lucia, a US-based long Covid sufferer who preferred not to give her last name, said "studies like these bring us a lot closer to understanding" the condition.

She pointed to another recent paper which found damage and fewer mitochondria in the muscles of long Covid patients, which could indicate why many patients become exhausted after even a small amount of exercise. 

Meanwhile, Ziyad Al-Aly, a clinical epidemiologist at Washington University in St Louis, said long Covid has been so elusive because it is a "multi-system disease."

"Our minds are trained to think about diseases based on organ systems" such as heart or lung disease, he said, but understanding the mechanisms behind long Covid could more broadly answer "why and how acute infections cause chronic disease."

Subscribe to Storyboard: A weekly newsletter bringing you the best of CGTN every Friday