In late summer of 2020, members of the European Burden of Disease Network convened to develop a consensus approach for estimating disability-adjusted life years (DALYs) due to COVID-19. DALYs combine time lost due to morbidity, using YLD, and time lost due to premature death using Years of Life Lost [4,5,6]. Particular considerations were given to data availability and how the quality of epidemiological data may have varied by country, to ensure that the model not only had a theoretical basis but was also applicable in different settings.

Although reports on the occurrence of long COVID had emerged when the European Burden of Disease Network’s consensus model was developed, epidemiological data were sparse. Given the gaps in knowledge at the time, the European Burden of Disease Network’s consensus models conservative recommendation was to include a single health state for long COVID. This involved using the disability weight for ‘infection, post-acute consequences’ to estimate YLD. This was similar to the approach the Global Burden of Disease (GBD) study have previously used to model the post-acute consequences of dengue and ebola [7]. The disability weight for this health state is a relatively high value of 0.219 [8]. Additionally, the value was fixed meaning that there was no scaling of degree of symptoms, for example from mild to severe. This presents a limiting factor when trying to match case definitions to the health state.

Although the mechanisms underlying the pathophysiology of long COVID are still debated, various hypotheses have been put forward, each of which can induce symptoms with different degrees of severity. On the one hand, long COVID may be related to organ damage following acute COVID-19 infection such as myocardial infection or renal failure, leading to persistent symptoms [9]. Another hypothesis is it is related to a prolonged pro-inflammatory response related to SARS-CoV-2, inducing an atypical response of the immune system and mast cells, and persistent symptoms [10]. In relation to the first hypothesis, this presents additional uncertainties in outcome-based YLD estimates as symptoms could plausibly present as cases of non-communicable disease, such as through increases in the prevalence of cardiovascular conditions, which could lead to double-counting in comprehensive burden of disease studies. Finally, the symptomatology of long COVID has been compared to that of fibromyalgia [11]. However, disability weights are not available for the latter as it is not included in the GBD study.