The novel coronavirus has infected more than 300 million people around the world since being first identified from an outbreak in the Chinese city of Wuhan in late 2019.
COVID-19 does not typically cause life-threatening disease in healthy individuals, but symptoms can sometimes persist for months, causing long-term health issues.
These long-lasting symptoms are described as long-haul COVID, post-acute COVID-19, or simply long COVID.
A new study published in the journal Nature Communications answers some important questions about this poorly-understood condition.
To conduct the study, researchers recruited more than 500 COVID patients -- for some, the symptoms resolved relatively quickly, but others went on to have long COVID.
One key difference between these two groups emerged: a group of patients that experienced long COVID showed "marked decreases" in levels of two immunoglobulins, IgM and IgG3.
The level of these antibodies -- which the immune system produces to fight infections -- combined with factors such as age and history of asthma, proved 75 percent effective in predicting long COVID.
Hint: It's Not An Overall!
Dr. Onur Boyman, one of the authors of the study and a researcher in the department of immunology at University Hospital Zurich, told NBC News that "we want to be able to recognize and identify, as early as possible, who is at risk of developing long COVID."
"These individuals might have a disadvantage from the start, and then due to their asthmatic background, they might also react slightly differently to viruses, which then leads to a misguided immune response," Boyman added.
Like all studies, this one has some limitations.
Patients who participated in the study were infected between April 2020 and August 2021, before omicron became the dominant variant.
Furthermore, the study did not take into account the vaccination status of participants, since many of them became ill before vaccines were widely available.
Claire Steves, a senior clinical lecturer at Kings College London, stressed that further research into long COVID is necessary.
"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," Steves said.
She noted that "with cases high still, more people are at risk of developing long-term symptoms."
"We urgently need to scale up research on how to prevent this happening," Steves added.
Long COVID researcher Charles Downs, meanwhile, called the results of this study "very promising."
"There is no single test, no imaging study, that can be used to give a diagnosis [of long COVID].This helps move us in that direction," she said.
