A blood test may someday help determine a person's risk for long Covid, new research suggests.
The study, published Tuesday in the journal Nature Communications, found that people who go on to develop long Covid have lower levels of certain antibodies in their blood soon after they are infected with the coronavirus.
If confirmed through larger studies, the findings could help scientists develop a test to predict who may continue to suffer from symptoms weeks, months and even years following infection.
"We want to be able to recognize and identify, as early as possible, who is at risk of developing long Covid," said Dr. Onur Boyman, an author of the new study and a researcher in the department of immunology at University Hospital Zurich.
Long Covid, a poorly understood condition for which there is no standard definition, diagnosis or treatment, has vexed doctors and researchers worldwide since the pandemic began.
The precise number of long Covid patients is unclear, though it's been estimated that one-third of Covid patients overall may experience symptoms for at least a month.
Any early insights into which patients may end up becoming so-called long-haulers are welcome, outside experts said.
Charles Downs, a researcher into long Covid and an associate professor in the School of Nursing and Health Studies at the University of Miami, called the research "very promising."
"There is no single test, no imaging study, that can be used to give a diagnosis" of long Covid, he said. "This helps move us in that direction."
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Boyman's research began in early 2020, during the first wave of the pandemic. His team followed patients through the acute phase of infection, then for six months and then for a year as the phenomenon of long Covid became apparent.
Comparing more than 500 Covid patients — some of whom went on to have long Covid and others whose symptoms resolved — several key differences emerged, he said.
The most glaring was how immune systems in patients who went on to develop long Covid initially reacted to the virus.
Such patients in Boyman's study showed marked decreases in levels of two immunoglobulins, IgM and IgG3, which are antibodies that the immune system produces to fight infections. In healthy immune systems, levels of these immunoglobulins tend to rise when faced with infection.
Those antibody levels, when combined with other factors, such as middle age and a history of asthma, were 75 percent effective in being able to predict long Covid, Boyman said.
Because researchers knew which patients were suffering from long Covid, more research is needed to determine whether the criteria would be as accurate from the onset of illness.
"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."
Downs, of the University of Miami, said that in his experience, many long Covid patients tend to have either asthma or some other history of underlying allergy-related illness, such as a chronic runny nose linked to seasonal allergies.
If confirmed in larger studies, the research could be "an important step forward towards directing resources in post Covid-19 clinics to those who need them the most," Dr. Kartik Sehgal, a long Covid researcher and medical oncologist at the Dana-Farber Cancer Institute in Boston, wrote in an email.
Several caveats apply to the new research. Patients in the study were infected between April 2020 and August 2021, before the omicron variant took hold.
It is uncertain, therefore, whether the findings would apply to those who may develop long Covid following an omicron infection.
What's more, the study did not take into account the vaccination status of participants. Many of the long Covid patients became ill early in 2020, before vaccines were available.
"It would be important to look to see whether these markers are still predictive in vaccinated people as more of the world is vaccinated or has prior infection," Claire Steves, a senior clinical lecturer at Kings College London, said in a statement.
But "with cases high still, more people are at risk of developing long-term symptoms," Steves, who was not involved in the new research, said. "We urgently need to scale up research on how to prevent this happening."