Long Covid: A Persistent Public Health Conundrum

Since the initial waves of the COVID-19 pandemic, a significant proportion of infected individuals continue to experience persistent symptoms months, or even years, after acute infection. This clinical entity, now referred to as Post-Acute Sequelae of Sars-CoV-2 infection (PASC), or long Covid, represents a major public health challenge. Prevalence estimates vary between 3% and over 30% of those infected, depending on the studies and definitions used. CDC

Understanding the Complexity of Long Covid

Although severe forms of COVID-19 have been extensively studied, particularly in hospitalized patients, the biological mechanisms underlying long Covid in individuals with mild or moderate initial infections remain poorly understood. Proposed hypotheses include viral persistence, chronic inflammation, immune dysregulation, endothelial dysfunction, microvascular disorders, and autoimmune phenomena. However, published results are often contradictory and struggle to establish a clear link between biological abnormalities and clinical symptoms. Mayo Clinic

The COPER Cohort Study: Unraveling the Mysteries

The COPER cohort, a large French study from the general population, was designed to explore the connections between persistent long Covid symptoms and blood biomarkers, considering the time elapsed since infection and the clinical heterogeneity of the condition. Results, published in July 2025 in eBiomedecine, offer new insights. The study was nested within the SAPRIS-Sero cohort, utilizing data from three large French general population cohorts (CONSTANCES, E3N-Générations and NutriNet-Santé).

Study Design and Participants

Between June and November 2022, 1,000 participants were included. After exclusions for recent infection, missing data, or lack of documented infection, the analysis focused on 801 individuals with confirmed Sars-CoV-2 infection: 490 considered cured without persistent symptoms, and 311 presenting at least one persistent symptom (the PASC group). Importantly, no participants had been hospitalized during the acute infection.

Participants underwent two home visits six months apart, including blood, saliva, and urine sample collection, along with standardized clinical questionnaires. Fourteen blood biomarkers were analyzed, covering inflammation, immune activation, endothelial activation, and tissue damage. Statistical analyses adjusted for age, sex, body mass index, and time since infection.

Key Findings: No Single Biological Signature

Participants with long Covid were more frequently female, had a higher BMI, and were infected earlier in the pandemic than those who recovered. The median number of persistent symptoms was three per person, with fatigue, dyspnea (shortness of breath), cough, and sleep disturbances being the most common. Scores for depression, anxiety, fatigue, and cognitive impairment were also significantly higher in the PASC group.

Network analysis of symptoms revealed a low overall correlation between different symptoms, suggesting they don’t arise from a single mechanism. However, four major symptom groups were identified: a thoracic syndrome, a cognitive syndrome, a general syndrome, and an arthromyalgic syndrome. Anosmia (loss of smell) was notably not associated with other persistent symptoms.

Biologically, no single biomarker was significantly associated with overall long Covid status. However, when analyses were stratified by symptoms and time since infection, specific associations emerged. In those infected for less than a year, some symptoms – particularly anosmia/ageusia (loss of taste) and cough – were associated with markers of immune and viral activation (IFN-γ, IP-10, PD-L1), as well as markers of endothelial activation (ICAM-1, VCAM-1). These associations weakened or disappeared in participants infected for more than a year.

Symptom Resolution and Biomarker Changes

The six-month follow-up showed that 22% of PASC participants experienced complete symptom resolution, particularly those infected less than a year prior (38% vs. 20%). Changes in biomarkers over time were only weakly correlated with clinical outcome, except for anosmia/ageusia and fatigue, suggesting a closer link between these specific symptoms and underlying biological processes.

Implications for Diagnosis and Treatment

The authors of the COPER cohort study emphasize that long Covid is not a homogeneous entity, either clinically or biologically. The absence of a global biological signature highlights the limitations of approaches treating long Covid as a disease with a single pathophysiological mechanism. The observed associations between specific symptoms and biomarkers, particularly within the first year of infection, suggest dynamic and evolving mechanisms. Persistent immune or viral activation may play a role in early manifestations, while other mechanisms – potentially neurovascular, autoimmune, or functional – may take over later.

These findings support an individualized approach to long Covid, considering the symptomatic profile and the evolving context of the patient, rather than a uniform interpretation of biomarkers. Therapeutic strategies should be tailored to symptom profiles and time since infection.

Looking Ahead

This large-scale study in the general population sheds light on the complexity of long Covid. It demonstrates that the biological mechanisms associated with persistent symptoms vary depending on symptom type and duration, and no single biomarker currently defines or diagnoses PASC. As long Covid continues to burden health systems, these findings underscore the need for personalized approaches in both research and clinical care, and pave the way for longitudinal studies integrating broader biological analyses to better understand this enigmatic condition. Harvard Gazette

Reference: Robineau O. Et al. Symptoms and pathophysiology of post-acute sequelae following COVID-19. eBioMedicine, Volume 117, 105792.