Understanding case definitions and COVID-19

As the COVID-19 pandemic continues to evolve, we have been able to develop a greater understanding of the infection as a result of having more data generated by the rapid introduction of disease specific surveillance programmes.

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All of this information can, however, be a bit overwhelming and challenging to understand. There are a range of terms, used by epidemiologists and public health specialists as part of their daily work, that are now being frequently reported in relation to COVID-19 and we are rapidly trying to get to grips with this and understand what it means for us, our families and communities.

In this blog, we explain important definitions of COVID-19 infection, and why this level of clarity is important in the management of the current pandemic, for our future monitoring and learning.

When we talk about infectious diseases, we need to ensure that everyone’s understanding of a ‘case’ is the same. This is guided by what is known as the case definition. The case definition is a standardised set of criteria used to classify whether someone may have a particular infection, such as COVID-19, and is often agreed nationally by public health authorities.

There can be several levels to a case definition which will be based on levels of certainty about whether an individual has a particular disease. For example, during the early stages of the pandemic we were identifying individuals who were ‘possible’ cases based on a particular set of symptoms and travel history.

If these individuals then had a laboratory confirmed test result for SARS-CoV2 they would be categorised as a ‘confirmed’ case of COVID-19.  

While this appears straightforward, within each category over time definitions are regularly amended to reflect increasing knowledge of the condition or infection. With COVID-19, the list of symptoms to identify possible cases that should be tested was expanded to include ‘anosmia’, which is the loss of taste or smell.

This occurred because of an increasing knowledge about symptoms present in confirmed cases, acquired through surveillance and other methods such as the Zoe app (https://covid.joinzoe.com/). Updates to the case definition will be helpful in facilitating increased recognition of cases, but we must remember to be cautious in making comparisons, including comparing trends over time and between different countries or places. For example, at the early stages of the pandemic a case was someone admitted to hospital with certain symptoms.

This has now evolved, to be anyone in the community who has a fever, a new continuous cough or a loss of taste or smell. The definition now is therefore capturing a greater spectrum of the disease compared to the more severe disease we would have been capturing in the early phase. It is therefore important to understand how the case definition has changed over time to allow us to accurately interpret the trends.

With changes in testing patterns over time, another important consideration when comparing trends is the ‘test positivity’. This helps us to distinguish whether changing case numbers indicates an actual change in the case numbers, or if this is an artefact associated with changes in the pattern of testing.

Again, in the early stages of the pandemic when we were testing hospitalised patients with respiratory symptoms the positivity rate was high at around 10-20% in epidemiological weeks 13 – 19. With the expansion of testing in the community this declined to below 1%, but we have seen in recent weeks that this has started to climb again to above 3%. As this has occurred with a sustained level of increased testing, this may represent a real increase in disease incidence.

From a public health perspective, controlling the current pandemic and managing COVID-19 transmission is facilitated by ‘possible’ cases self-isolating when symptoms develop (to break chains of transmission) and early testing to identify ‘confirmed’ cases. A UK report estimated that manual contact tracing of non-household members would reduce the number of new infections occurring by 5–15% (DELVE Report).

For confirmed cases, contact tracing, which is a well-established public health method that aims to break the chain of viral transmission, is then used to identify any close or high-risk contacts. Isolation of both ‘possible’ and ‘confirmed’ cases AND any high-risk or close contacts is an important part of the process to break transmission of COVID-19 and reduce the incidence of the disease in the community. 

We all have a part to play in this process and hopefully understanding this information will help.  For further information on symptoms, testing or contact tracing, please visit our website at: www.publichealth.hscni.net/covid-19-coronavirus.

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