Doctors have discovered an “antibody signature” that can help identify patients most at risk of developing long Covid, a condition where debilitating symptoms of the disease can persist for many months. Researchers at University hospital Zurich analysed blood from Covid patients and found that low levels of certain antibodies were more common in those who developed long Covid than in patients who swiftly recovered. When combined with the patient’s age, details of their Covid symptoms and whether or not they had asthma, the antibody signature allowed doctors to predict whether people had a moderate, high or very high risk of developing long-term illness. “Overall, we think that our findings and identification of an immunoglobulin signature will help early identification of patients that are at increased risk of developing long Covid, which in turn will facilitate research, understanding and ultimately targeted treatments for long Covid,” said Onur Boyman, a professor of immunology who led the research. The team studied 175 people who tested positive for Covid and 40 healthy volunteers who acted as a control group. To see how their symptoms changed over time, doctors followed 134 of the Covid patients for up to a year after their initial infection. Blood tests on the participants showed that those who developed long Covid – also known as post-acute Covid-19 syndrome (Pacs) – tended to have low levels of the antibodies IgM and IgG3. When Covid strikes, IgM ramps up rapidly, while IgG antibodies rise later and provide longer-term protection. The scientists combined the antibody signature with the patient’s age, whether they had asthma and details of their symptoms to produce a long Covid risk score. To confirm that the score was useful, they ran the test on a separate group of 395 Covid patients that had been followed up for six months. The test cannot predict a person’s risk of long Covid before they are infected because details of their symptoms are needed, but Dr Carlo Cervia, the first author on the study, said people with asthma and low IgM and IgG3 levels beforehand could assume they were at an increased risk. “This is expected to improve care for long Covid patients as well as motivate high-risk groups, such as asthmatic patients, to get vaccinated and thus prevent long Covid,” Cervia said. The research is published in Nature Communications. Although there is no effective cure for long Covid, being able to work out who is most at risk could help doctors direct patients to clinical trials for long Covid therapies and arrange early rehabilitation. Better control of the infection through antibody treatments, antivirals and anti-inflammatory drugs, may all help to reduce the risk and vaccines can sometimes alleviate long Covid symptoms, but more studies are needed. Another hope is that early identification of long Covid patients will help doctors work out what causes the condition in particular people. Researchers have proposed several possible drivers, from long-term damage wrought by the virus to a misfiring immune system and pockets of virus hiding out in the body. Dr Claire Steves, a clinical senior lecturer at King’s College London, welcomed the work but said it was important to replicate the findings in a larger number of patients. With cases still high, she added, many more people were at risk of developing long-term symptoms. “We urgently need to scale up research on how to prevent this happening,” she said. Dr David Strain, a clinical senior lecturer at the University of Exeter medical school and the British Medical Association’s lead on long Covid, said the study was a step towards better understanding long Covid. The antibody signature identified by the Swiss researchers was similar to that seen in myalgic encephalomyelitis, or ME, he said, a condition that affects a quarter of a million people across the UK. “Further comparisons between these diseases may allow mutual benefit and cross-pollination of ideas as the learnings from each benefit the other,” he said.