Tens of millions of people across the world are thought to suffer from long Covid, but four years after the pandemic was declared this elusive condition still cannot be tested for — let alone treated. However research could be finally starting to find early clues on the trail of long Covid, raising hopes of future breakthroughs that may also illuminate other stubbornly ambiguous chronic syndromes. Long Covid is 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. The most common are fatigue, shortness of breath, muscle pain and brain fog. One notable study released last month showed there were significant differences in the proteins of the blood of more than 110 long Covid patients. Onur Boyman, a Swiss researcher and senior author of the Science study, told AFP he believes this is a “central puzzle piece” in what keeps Covid raging for so long in the bodies of some people.