New Study Identifies a Potential Biomarker for Long COVID Fatigue

Fatigue is one of the most common and debilitating symptoms reported by people with Long COVID, yet the biological mechanisms behind it have remained difficult to pinpoint. New research led by scientists at the University of Cambridge offers important insight into what may be driving this persistent exhaustion. The study suggests that a protein produced by the immune system, called interferon gamma (IFN-γ), may play a key role and could potentially serve as a biomarker for Long COVID–related fatigue.

Interferon gamma is part of the body’s natural antiviral defense system. When a virus such as SARS-CoV-2 enters the body, immune cells release signaling molecules called cytokines to help coordinate the immune response. IFN-γ is one of these molecules, and it helps activate immune cells that identify and destroy virus-infected cells. Normally, levels of IFN-γ rise during an active infection and then fall once the virus has been cleared. However, the new research found that in some people with Long COVID, production of this protein continues long after the initial infection has resolved.

The study followed a group of patients experiencing Long COVID fatigue for more than two and a half years, making it one of the longer-term investigations into the condition. Researchers collected blood samples from patients at multiple time points and analyzed the activity of immune signaling molecules. They observed that individuals who continued to experience fatigue often had persistently elevated levels of IFN-γ. In contrast, patients whose symptoms improved over time tended to show a decrease in the protein.

Because IFN-γ has been used therapeutically in other viral diseases, scientists are familiar with the symptoms that can accompany elevated levels of the protein. These can include fatigue, fever, muscle aches, headaches, and changes in mood—symptoms that closely resemble those reported by many Long COVID patients. The overlap between known effects of IFN-γ and the symptom profile of Long COVID fatigue led researchers to investigate the protein more closely as a possible biological driver of the condition.

To understand where the IFN-γ was coming from, the research team examined the immune cells responsible for producing it. Their analysis pointed to CD8+ T cells, a type of white blood cell that plays an important role in targeting virus-infected cells. The study also found that these T cells appeared to require interaction with another immune cell type, called monocytes, in order to release interferon gamma. This interaction may help explain how immune signaling could continue long after the acute infection phase has ended.

Another important observation from the study was that IFN-γ levels appeared to fall when patients began to recover. Over the course of the follow-up period, more than half of the participants experienced improvement in their symptoms, and this improvement often coincided with a drop in IFN-γ production. This pattern suggests that the protein could potentially act as a measurable signal of disease activity, helping researchers track the course of the condition.

The researchers also examined immune activity before and after COVID-19 vaccination in some participants. They observed that in patients whose symptoms improved after vaccination, IFN-γ levels also decreased. While the findings do not prove that vaccination directly resolves Long COVID symptoms, they raise the possibility that the immune response triggered by vaccination may help the body eliminate lingering viral triggers in certain cases.

The study’s findings also contribute to a growing body of research suggesting that Long COVID is unlikely to be a single uniform condition. Instead, it may consist of several subtypes with different underlying biological mechanisms. Identifying markers such as IFN-γ could help researchers classify these subtypes more clearly and eventually develop more targeted treatments.

For patients, one of the biggest challenges has been the lack of objective diagnostic markers for Long COVID. Many symptoms are subjective and overlap with other conditions, making it difficult for clinicians to confirm a diagnosis or determine why some individuals recover while others remain ill for years. A measurable biomarker like IFN-γ could potentially help provide clearer answers and guide future research into therapies.

Although more work is needed to confirm and expand on these findings, the study represents an important step toward understanding the immune processes that may contribute to long-term fatigue following viral infections. By identifying persistent immune signaling as a possible driver of symptoms, researchers are moving closer to unraveling the biological basis of Long COVID and improving care for those affected.