As many as one-third of people who are infected with COVID-19 may suffer long-term symptoms from the virus, such as a cough, fatigue, or ongoing headache in what is known as long COVID. But until now, it has been unknown who exactly is at risk for being a long-haul COVID patient.

A new study published in the journal Nature Communications looked at the potential risk of a person developing long COVID based on a simple blood test.

The test found that people with long COVID have lower levels of certain antibodies in their blood right after they are infected with the virus.

The blood test findings can be confirmed through larger studies and could be a good indicator for medical professionals to determine who may develop long COVID after a virus infection.

Dr. Onur Boyman, a researcher in the department of immunology at University Hospital Zurich and an author of the study, told NBC News, “We want to be able to recognize and identify, as early as possible, who is at risk of developing long COVID.”

Those who suffer from long COVID have symptoms that can persist for at least a month or longer, according to the Centers for Disease Control and Prevention.

Not much is known about the reason why long-haul COVID sufferers have symptoms longer than most, even without underlying medical conditions, the agency said.

With the help of a blood test, scientists would have more power to predict these symptoms, which Charles Downs, a researcher of long COVID and an associate professor in the School of Nursing and Health Studies at the University of Miami, said was “very promising.”

He told NBC News, “There is no single test, no imaging study, that can be used to give a diagnosis” of long COVID, adding, but “this helps move us in that direction.”

For the study, researchers tested more than 500 COVID-19 patients for a year – some who went on to have long COVID. In the patients with long COVID, Boyman’s team saw a decrease in the immunoglobulins IgM and IgG3 – the antibodies that fight infection in the immune system.

Boyman told NBC News that when combined with factors, such as middle age and a history of asthma, they were 75% effective in predicting long COVID.

“These individuals might have a disadvantage from the start,” he said, “and then due to their asthmatic background, they might also react slightly differently to viruses, which then leads to a misguided immune response.”

Patients were studied from April 2020 to August 2021, before the Omicron variant spread. The study also didn’t take into account vaccination status as many of the patients were unvaccinated at the time the research took place.

A lab technician sorts blood samples inside a lab for a Covid-19 vaccine study at the Research Centers of America (RCA) in Hollywood, Florida
A lab technician sorts blood samples inside a lab for a Covid-19 vaccine study at the Research Centers of America (RCA) in Hollywood, Florida AFP / CHANDAN KHANNA