Definitions of new diseases evolve with understanding

Data revisions are inevitable during the epidemic of a new infection. Throughout the epidemics of vCJD (“mad cow disease”) and HIV/AIDS, for example, the definition of a case got revised to more accurately reflect the disease. This is inevitable, because we do not know the full spectrum of a disease when its new.

Throughout January and February the first clinical descriptions of the consequences of infection with the virus, SARS-CoV-2, were being published. When the epidemic in the UK started, we still only knew relatively little about the spectrum of disease that the virus could cause. We name the disease COVID-19, but what is COVID-19? We are fortunate that we have a test (the test for HIV came several years after the first disease, and we still do not have a definitive pre-mortem test for vCJD). So the sensible definition of COVID-19 is “sick and positive test”. There is no other option at the start of an epidemic.

This was the definition that the NHS used for admissions. If somebody was admitted to hospital (i.e. they are sick), and they have had a positive test, then they are recorded as a “COVID admission”. But not all sick people with a positive test are made sick by having had the virus. The extreme example of somebody hit by a bus six months after having had the virus shows that the first definition has to, at some point, be revised. Again, there is no other option. But at this point the clinical understanding of the consequences of infection is much better – 10,000s of people had been admitted with a positive test.

This revision means that somebody recorded as a COVID admission the day before the revision would recorded as a non-COVID admission the day after. Again, it’s inevitable, and does not reflect mismanagement. Epidemiologists expect this revision of definitions and accommodate them. At no point does it change clinical care – “COVID” and “non-COVID” are just labels not diagnoses. Clinical diagnosis and treatment is made on the basis of symptoms, not the cause of the symptoms.

This process will continue and definitions will continue to be revised. This does not mean that reported data are “inflated” or untrustworthy, but rather reflects the improved understanding of the disease caused by SARS-CoV-2.

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