Background: Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is a poorly understood, complex, multisystem disorder, with severe fatigue not alleviated by rest, and other symptoms, which lead to substantial reductions in functional activity and quality of life. Due to the unclear aetiology, treatment of patients is complicated, but one of the initial problems is the insufficient diagnostic process. The increase in the number of undiagnosed ME/CFS patients became specifically relevant in the light of the COVID-19 pandemic. The aim of this research was to investigate the issues of undiagnosed potential ME/CFS patients, with a hypothetical forecast of the expansion of post-viral CFS as a consequence of COVID-19 and its burden on society. Methods: The theoretical research was founded on the estimation of classic factors presumably affecting the diagnostic scope of ME/CFS and their ascription to Latvian circumstances, as well as a literature review to assess the potential interaction between ME/CFS and COVID-19 as a new contributing agent. The empirical study design consisted of two parts: The first part was dedicated to a comparison of the self-reported data of ME/CFS patients with those of persons experiencing symptoms similar to ME/CFS, but without a diagnosis. This part envisaged the creation of an assumption of the ME/CFS shadow burden “status quo”, not addressing the impact of COVID-19. The second part aimed to investigate data from former COVID-19 patients’ surveys on the presence of ME/CFS symptoms, 6 months after being affected by COVID-19. Descriptive and analytical statistical methods were used to analyse the obtained data. Results: The received data assumed that the previously obtained data on the ME/CFS prevalence of 0.8% in the Latvian population are appropriate, and the literature review reports a prevalence of 0.2–1.0% in developed countries. Regarding the reciprocity of ME/CFS and COVID-19, the literature review showed a lack of research in this field. The empirical results show quite similar self-esteem among ME/CFS patients and undiagnosed patients with longstanding disease experience, while former COVID-19 patients show a significantly lower severity of these problems. Notably, “psychological distress (anxiety)” and “episodic fatigue” are significantly predominant symptoms reported by former COVID-19 patients in comparison with ME/CFS patients and undiagnosed patients prior to the COVID-19 pandemic. The results of our analysis predict that the total amount of direct medical costs for undiagnosed patients (out-of-pocket payments) is more than EUR 15 million p.a. (in Latvia), and this may increase by at least 15% due to the consequences of COVID-19. Conclusions: ME/CFS creates a significant shadow burden on society, even considering only the direct medical costs of undiagnosed patients—the number of whom in Latvia is probably at least five times higher than the number of discerned patients. Simultaneously, COVID-19 can induce long-lasting complications and chronic conditions, such as post-viral CFS, and increase this burden. The Latvian research data assume that ME/CFS patients are not a high-risk group for COVID-19; however, COVID-19 causes ME/CFS-relevant symptoms in patients. This increases the need for monitoring of patients for even longer after recovering from COVID-19′s symptoms, in order to prevent complications and the progression of chronic diseases. In the context of further epidemiological uncertainty, and the possibility of severe post-viral consequences, preventive measures are becoming significantly more important; an integrated diagnostic approach and appropriate treatment could reduce this burden in the future.
