Toward a Universal Definition of Post–COVID-19 Condition—How Do We Proceed?

The phrase "post-COVID-19 condition" (PCC) (sometimes called long COVID) refers to a large collection of symptoms and signs experienced by patients with COVID-19 following initial infection with SARS-CoV-2.With more than 651 million documented COVID-19 cases worldwide and a conservative estimated prevalence of 10% infected, 65 million individuals around the world are likely to have PCC. 1 Chaichana and colleagues 2 investigated how PCC was defined within the existing published scientific literature.The authors investigated whether included studies presented definitions of PCC that matched those used in guidance from the National Institute for Health and Care Excellence (NICE), World Health Organisation (WHO) and the Centers for Disease Control and Prevention (CDC).The authors found that only approximately a third of 295 studies used official PCC definitions.Instead, most studies used their own definitions for PCC, while 64 studies did not define PCC at all.How do these findings affect the PCC landscape?First, the lack of a consistent definition for PCC between organizations and studies makes assimilating and comparing findings from current studies difficult.There is also multiplicity of titles for what appears to be the same clinical problem-for example, long COVID, post-COVID symptoms, post-COVID syndrome, post-COVID condition, and post-acute COVID, among others.By using different definitions and titles, different studies will have focused on different people, symptoms, and time intervals and hence, any estimated prevalence or incidence of PCC will vary widely between studies, even if conducted in the same time period in the same population.Second, in the generation of their definitions, many studies will require the reporting of an initial infection episode; either self-reported or through molecular testing.However, initial lack of awareness of the range of symptoms associated with acute COVID-19 by the general population will mean that many people without access to testing may have not associated their symptoms with acute infection and therefore did not associate any enduring symptoms with PCC.Therefore, it is likely that current estimates are underestimating the true burden of those with PCC.Third, current studies differ substantially in the way that data are collected.The authors reported that 45% of studies investigating PCC were cohort studies, while 22% were cross-sectional and that this proportion of published studies remained roughly similar across the period in which the report was conducted.Cross-sectional studies would only provide a static prevalence statistic of signs or symptoms of PCC; since this is likely to vary over time, such studies will be inaccurate at estimating the true burden of PCC within populations.How do we tackle these problems?First, there must be one agreed universal definition of this condition; with one title that can be adopted by all studies in the future.We would suggest such a definition to be "signs and symptoms following initial SARS-CoV-2 infection, that persist for more than one month (in mild cases), and more than three months (in cases severe enough to warrant oxygen support), which have a disproportionately severe effect on a patient's quality of life, far beyond what is expected from their initial infection."The difference between this definition and existing ones from NICE, the CDC, and WHO is that it takes into account disease severity from initial COVID infection, therefore acknowledging that PCC is different than recovery from a straightforward pneumonia, which if severe enough, can also take a prolonged period for patients to fully recover.Our suggested definition also acknolwedges that PCC can be present even in those whose persistent symptoms can be explained, since the explanation may involve a long-term consequence of acute infection.