As the world faces the possibility of another seasonal spike in cases of the coronavirus, medical experts are still pondering the cause of long COVID.
The mysterious ailment causes a range of symptoms long after the COVID-19 infection has cleared, such as fatigue, loss of sexual desire, loss of smell and taste, chest pain and chronic cough.
Scientists don’t yet know for certain what causes long COVID, the catchall term for about 200 widely varying symptoms.
But researchers at the University of Pennsylvania have suggested that reduced levels of serotonin — a neurotransmitter that affects mood, memory, sleep, digestion, blood clotting and sexual desire — might explain the lingering symptoms.
Between 10% and 30% of people are estimated to have experienced some form of long COVID after recovering from a coronavirus infection, a risk that has dropped somewhat since early in the pandemic.
To investigate the role that serotonin might play, researchers analyzed the blood of 58 patients who had been experiencing long COVID for up to 22 months since their infection.
Those results were compared to those of 30 people with no post-COVID symptoms and of 60 patients who were in the early stage of coronavirus infection.
Their analysis revealed that levels of serotonin were altered right after a coronavirus infection, something that also happens after other viral infections. But in people with long COVID, serotonin was the only molecule that did not recover to pre-infection levels.
Lowered levels of serotonin disrupt the vagus nerve system, which transmits signals between the body and the brain.
Serotonin plays a role in short-term memory, and the researchers proposed that depleted serotonin could lead to memory problems and other cognitive issues that plague people with long COVID.
“Even if not everybody experiences difficulties in the serotonin pathway, at least a subset might respond to therapies that activate this pathway,” Dr. Christoph Thaiss, a lead author of the study and assistant professor at the Perelman School of Medicine at the University of Pennsylvania, told the New York Times.
If low serotonin levels are to blame for long COVID, some researchers hope that increasing those levels may help.
“If we supplement serotonin or prevent the degradation of serotonin, maybe we can restore some of the vagal signals and improve memory and cognition,” said Dr. Maayan Levy, a lead author and assistant professor of microbiology at the Perelman School of Medicine.
To this end, Levy and Thaiss are planning a clinical trial to test fluoxetine, a selective serotonin reuptake inhibitor (SSRI) branded as Prozac. The trial would join other tests currently underway to solve the mystery of long COVID.