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Clinical Case Definitions

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Terms used in the guideline

When developing the terms used in this guideline, many different factors were taken into account.  The aim was to reduce the existing confusion about how to define the disease for clinical guidance.

Long COVID and other terms in use

The expert advisory panel recognised the significant progress made by patient groups using the term ‘long COVID.’ However, the term ‘long COVID’ has been used in multiple ways across the literature. Other terms have also been used. Greenhalgh et al (2020)1 use the terms ‘post-acute COVID-19’ (from 3–12 weeks) and ‘chronic COVID-19’ for symptoms extending beyond 12 weeks. The National Institute for Health Research themed review2 notes the possibility of a number of different syndromes.

The evidence on and pros and cons of different terms was reviewed. Specific clinical diagnostic criteria were needed to facilitate access to support, provide the basis for planning services and to enable formal codes to be developed for clinical datasets. Three definitions were developed: acute COVID-19 (0–4 weeks), ongoing symptomatic COVID-19 (4–12 weeks) and post-COVID-19 syndrome (12 weeks or longer).

Time periods

In deciding these time periods, the panel was aware of evidence showing that most people’s symptoms will resolve before 12 weeks from the start of acute COVID-19, while for a smaller proportion of people they will continue for longer. People may also develop signs or symptoms of a life-threatening complication at any time and these need to be investigated urgently.
The panel concluded that most people who have symptoms or had a positive COVID-19 test would no longer be self isolating after four weeks and could be investigated for ongoing symptomatic COVID-19 (4–12 weeks) with the possibility of later being diagnosed with post-COVID-19 syndrome (12 or more weeks).

There is currently no long-term evidence base to help determine how long the ongoing effects currently seen after a SARS-CoV-2 infection will last. The term ‘post’ COVID-19 syndrome was agreed to reflect that the acute phase of the illness has ended, not that the person has recovered.  Because it is not clear how long symptoms may last the panel agreed that time-specific terms such as ‘chronic’ or ‘persistent’ were not appropriate. ‘Syndrome’ was agreed to reflect the ‘running together’ or concurrence of the multisystem, fluctuating and often overlapping clusters of symptoms that people present with.

Acute COVID-19
Signs and symptoms of COVID-19 for up to 4 weeks.
Ongoing symptomatic COVID-19
Signs and symptoms of COVID-19 from 4 weeks up to 12 weeks.
Post-COVID-19 syndrome
Signs and symptoms that develop during or after an infection consistent with COVID-19, continue for more than 12 weeks and are not explained by an alternative diagnosis. It usually presents with clusters of symptoms, often overlapping, which can fluctuate and change over time and can affect any system in the body. Post-COVID-19 syndrome may be considered before 12 weeks while the possibility of an alternative underlying disease is also being assessed.


In addition to the clinical case definitions, the term 'long COVID' is commonly used to describe signs and symptoms that continue or develop after acute COVID-19.  It includes both ongoing symptomatic COVID-19 (from 4 to 12 weeks) and post COVID-19 syndrome (12 weeks or more).

1.  Greenhalgh T, Knight M, A’Court C, Buxton M, Husain L. Management of post-acute covid-19 in primary care BMJ 2020; 370:m3026

2.  National Institute for Health Research. Living with COVID19. Themed review. [cited 15 Dec 2020]. Available at URL: