Long-term effects of SARS-CoV-2 infection are often discussed in relation to previously healthy individuals. However, in clinical practice, the picture is significantly more nuanced: a significant proportion of those affected had pre-existing chronic conditions prior to their SARS-CoV-2 infection. These pre-existing conditions influence the risk of developing long Covid, its course, the severity of symptoms and diagnostic classification.
What research shows
Recent systematic reviews and meta-analyses have confirmed that there are several factors associated with an increased risk of long-term effects of the virus. The severity of the acute infection and the individual’s baseline health status are of particular relevance.
A large meta-analysis of prospective cohort studies involving over 168,000 people reveals that approximately one in five individuals infected with SARS-CoV-2 experience persistent symptoms. Furthermore, both female gender and severe acute illness were found to significantly increase the risk of long-term effects of the virus.
A large-scale systematic review published in Nature Communications analysed data from 50 studies involving a total of over 14 million people. The review found that SARS-CoV-2 infection is associated with a significantly increased risk of experiencing a wide range of persistent symptoms, particularly among hospitalised patients.
Chronic conditions as a contributing factor
Long-term effects of the virus are particularly relevant for people who had chronic conditions before becoming infected, including autoimmune, metabolic, cardiovascular s and chronic lung disorders.
These conditions often have the following features in common: they are associated with altered immune regulation, increased inflammatory activity and/or reduced physiological reserve. When these systems encounter an acute viral infection, it can impair the body’s ability to recover. In some cases, this results not in a clearly defined recovery process, but in a prolonged state of biological dysregulation — that is, persistent disruption to the body’s regulatory mechanisms.
Those with pre-existing neurological conditions are particularly vulnerable
A particularly relevant group consists of people with neuroimmunological disorders, such as multiple sclerosis (MS). This condition involves two complex disease processes that affect the central nervous system overlapping.
A study by the Swiss MS Registry shows that people with MS face significant medical and psychosocial challenges. In particular, the study highlighted their vulnerability to infections and their indirect consequences.
Clinical practice also shows that SARS-CoV-2 infection in people with MS can lead to an exacerbation of non-specific symptoms, such as fatigue, cognitive impairment and reduced exercise tolerance. However, it can be difficult to distinguish between a classic MS relapse, a temporary worsening of existing symptoms (pseudo-relapse) and post-infectious Long COVID-like symptoms.
This diagnostic uncertainty is clinically significant as it can directly affect treatment decisions and disease management.
Classification is difficult due to overlapping symptoms
A key issue regarding Long Covid and chronic conditions is the significant overlap in symptoms. Symptoms such as fatigue, sleep disturbances, cognitive impairment, pain and exercise intolerance are common to both Long Covid and many existing chronic conditions.
This creates a diagnostic gray area in which new post-infectious symptoms cannot always be clearly distinguished from the underlying condition. In clinical practice, this often results in worsening symptoms being attributed to existing diagnoses while long COVID is recognized as a distinct clinical entity only after a delay.
A systemic challenge in the healthcare system
This complexity collides with a healthcare system that has traditionally focused on treating individual conditions. For people with multiple conditions, this creates structural challenges. Symptoms are often viewed in isolation, responsibilities are fragmented, and interdisciplinary assessments are often delayed.
There is a particular gap between clinical classification and the actual reality of post-infectious diseases.
Conclusion
Long COVID does not present with a uniform clinical picture, nor does it affect only individuals who were previously healthy. Rather, current data show that chronic pre-existing conditions can significantly influence the course of the disease. This underscores the extent to which existing disease processes and post-infectious mechanisms can overlap.
This perspective is crucial for understanding Long Covid not merely as an isolated post-viral syndrome, but as part of a broader spectrum of complex, systemic diseases.
