New studies have provided novel insights into long COVID, which continues to affect millions of people worldwide. The exact rates of the disease are hard to determine because of variations in clinical criteria, and other differences such as differing standards for who is included or excluded from a long COVID diagnosis or study. According to a July 2024 study in The Lancet, the rates of long COVID following an acute infection range from 50 to as much as 85 percent of people who were hospitalized for COVID-19 and unvaccinated; 10 to 35 percent of people who were not hospitalized and not vaccinated, and about ten percent of vaccinated people.
Scientists are still working to determine the causes of long COVID, which may vary or include several factors depending on who is affected and what their health history might be. Some research has found evidence of active infection in people with long COVID. But in that study, half of the long COVID patients did not test negative for viral proteins, suggesting that there are multiple causes of the illness.
Symptoms of long COVID include but are not limited to: fatigue, fever, malaise, coughing, chest pain, headaches, sleep problrms, anxiety, diarrhea or constipation, joint pain, and rash. Long term complications may include increased risk of diabetes, blood clots, and heart problems.
A 2022 study reported in Nature Medicine determined that heart dysfunction could be one reason for the symptoms of long COVID, even after a mild infection.
A recent report published in Nature Microbiology has shown that there were abnormally high levels of inflammatory signaling molecules called cytokines in patients with long COVID.
The scientists found that these cytokines could damage heart calls called cardiomyocytes. Since these cells are responsible for the pumping action of the heart, problems with those cells may help explain long COVID symptoms like chest pain and heart palpitations. A heart condition known as POTS (postural orthostatic tachycardia syndrome) has also been associated with long COVID.
Unrelated, recent work reported in the Journal of Autoimmunity and Frontiers in Immunology, has discovered two proteins that could serve as biomarkers for long COVID, and may help explain the mechanisms underlying some cases of the disease. This work showed that people with long COVID carry abnormally high levels of proteins called galectin-9 and artemin in their blood.
These patients also had unusually high levels of certain immune cells known as neutrophils and monocytes, which may cause inflammation. There were also abnormally high levels of killer T cells that were exhausted. Long COVID patients were also deficient in immune cells that can fight infection, called lymphocytes. The researchers noted that neutrophils that are under stress often release galactin-9, which can boost inflammation.
Another recent study from this group, reported in The Lancet Microbe found no evidence of persistent infection in long COVID patients, challenging studies that have found the opposite and highlighting the complexities and sometimes confounding nature of this highly variable disorder.
Sources: Nature Microbiology, Journal of Autoimmunity, Frontiers in Immunology, The Lancet Microbe