General introduction 21 1 ability to resume normal activities. In addition, patient-researchers (often healthcare workers who had occupational exposure to SARS-CoV-2) such as those of the Patient-Led Research Collaborative[61] aimed to ensure long COVID featured on the COVID-19 research agenda and that patients’ lived experience were reflected in research. Crucially, it became clear that individuals with long COVID were not only patients with severe or critical disease requiring mechanical ventilation in hospital (for whom persisting symptoms could be explained by post-intensive care fatigue syndrome [PICS]). Long COVID seemed to also affect young individuals with mild COVID-19 and without any pre-existing comorbidities. Although some parallels were drawn with other post-infectious fatigue syndromes (such as Lyme disease[62] or infectious mononucleosis[63]), it soon became apparent that the full constellation of symptoms experienced by long COVID patients was perhaps more complex. Definitions In December 2020, the National Institute of Health and Care Excellence (NICE) proposed one of the first clinical definitions of long COVID. NICE considered COVID-19 patients with symptoms that persisted more than 12 weeks after illness onset, which were not explained by an alternative cause, to have developed the condition[64]. The WHO soon followed, using a Delphi consensus method to characterise the condition according to a minimum duration of symptoms and the impact of the condition on quality of life[65]. Although alternative terms such as post-COVID syndrome and post-acute sequelae of COVID-19 (PASC) were introduced by researchers, the broader term ‘long COVID’ remained the preferred term for many individuals due to its origins in the patient-led research community. Challenges in researching long COVID A lack of consensus on the definition of long COVID continues to create several challenges when researching the condition. These hurdles will be discussed in more detail in the general discussion. However, it is important to keep several key concepts in mind when reading Chapter 4-8 of this thesis. Firstly, the absence of a universally adopted definition of long COVID hinders the extent to which results from different studies can be compared and consolidated. Secondly, studies differ as to which symptoms the authors consider relevant to the condition, leading to information bias between studies. The probability of a study participant reporting one or more symptoms is clearly higher when including more than 50 symptoms[66] as opposed to fewer than 10[67]. Thirdly, several different long COVID phenotypes have been described. This has led researchers to question whether long COVID represents one pathophysiological process or several different, possibly co-existing, conditions. This may account for inconsistent findings in basic science studies attempting to identify signature
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