Markers in our blood -- fingerprints of infection -- could help identify individuals who have been infected by SARS-CoV-2, the virus that causes Covid-19, several months after infection, even if the individual had only mild symptoms or showed no symptoms at all, say researchers.A team of researchers at the University of Cambridge and the Cambridge University Hospital NHS Foundation Trust has received 370,000 pounds from the National Institute for Health Research (NIHR) to develop a Covid-19 diagnostic test that would complement existing antibody tests and will objectively diagnose and monitor long Covid.Using the funding, the team developed a pilot project. They recruited 85 patients and collected blood samples from patients when they were first diagnosed and then followed up over several months. The team now hopes to expand their cohort to 500 patients.In their initial findings, the team identified a biomarker -- a biological fingerprint -- in the blood of patients who had previously had Covid-19. This biomarker is a molecule known as a cytokine produced by T cells in response to infection. As with antibodies, this biomarker persists in the blood for a long time after infection."We need a reliable and objective way of saying whether someone has had Covid-19. Antibodies are one sign we look for, but not everyone makes a very strong response and this can wane over time and become undetectable," said Dr Mark Wills, from the Department of Medicine at the University of Cambridge."We've identified a cytokine that is also produced in response to infection by T cells and is likely to be detectable for several months -- and potentially years -- following infection. We believe this will help us develop a much more reliable diagnostic for those individuals who did not get a diagnosis at the time of infection," Wills added.By following patients for up to 18 months post-infection, the team hopes to address several questions, including whether immunity wanes over time.As part of their pilot study, the team also identified a particular biomarker found in patients with long Covid. Their work suggests these patients produce a second type of cytokine, which persists in patients with long Covid compared to those that recover quickly and might be one of the drivers behind the many symptoms that patients experience. This might therefore prove to be useful for diagnosing long Covid.The team plans to publish their results shortly.