Infections with the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) have
been observed in Germany since January 2020 and can result in acute coronavirus disease
2019 (COVID-19) as well as in long-term sequela, referred to as Long COVID. The umbrella
term Long COVID comprises ongoing new SARS-CoV-2-associated symptoms later than four
weeks from the onset of acute COVID-19. The term Post-COVID-Condition (PCC) was
introduced by the WHO and stands for ongoing or new SARS-CoV-2-associated symptoms, which
persisted for at least two months. These symptoms often include fatigue, brain fog,
dyspnea, orthostatic intolerance, as well as exercise intolerance, with a worsening of
symptoms after mild daily activities (post-exertional malaise, PEM). PCC can severely
impair daily function, social participation, and health-related quality of life. The
prevalence was estimated as up to 15% in infected people with pre-Omicron variants and
lower with Omicron variants. Risk factors include female gender and pre-existing
morbidity, with low risk in young children. The most severe form of PCC in people with
initially mild or moderate COVID-19 is myalgic encephalomyelitis/chronic fatigue syndrome
(ME/CFS). The prevalence of ME/CFS was shown to increase in the context of the pandemic.
While progress has been made in deciphering the pathogenesis of acute COVID-19, the
mechanisms of Long COVID/PCC are poorly understood. Increasing evidence indicates that
remaining viral particles, as well as inflammation, altered microcirculation, dysfunction
of the autonomic nervous system, and/or autoimmunity, may contribute.

With the web-based German MLC-R, we aim at deep phenotyping of PCC, identification of
subtypes and risk factors, describing trajectories of the disease and patient journeys,
and providing clinical data for future clinical trials. Patients are also invited to
contribute biosamples for future translational research.