Case Definition Explained

by Ximena Bonilla -- Apr 16, 2020

When a disease is being monitored, epidemiologists follow its progression by counting cases, calculating infection rates in the population, and then tracking the changes in these numbers over time. However, in order to count cases, a case definition needs to be established.

By following standard guidelines, it is possible to compare the number of cases, or the affected rates at different times or with other countries (providing they use the same guidelines to define their cases)1. We could compare the number of COVID-19 cases occurring in the Canton of Bern today, with the number of cases a year from now, or the disease rates for Vaud with those of Zürich, or with those of the whole of Switzerland. These types of comparisons allow for example, the identification of areas that are more susceptible to COVID-19 in order to allocate more resources, or areas where the disease has been controlled where it is OK to relax restrictions. When a difference is observed using the same case definition, the difference is likely to be a real one rather than a consequence of using distinct ways of classifying cases.

  1. COVID-19 suspected case: Individuals with acute respiratory illness (fever AND at least one more sign or symptom of disease, such as cough and shortness of breath), living in a location with COVID-19 community transmission (such as Switzerland) OR with a history of travel to areas with community transmission of the disease, OR in contact with a confirmed or probable COVID-19 case.
  2. COVID-19 probable case:A suspected case where the test is inconclusive or could not be performed.
  3. COVID-19 confirmed case:A person with laboratory confirmation of COVID-19 infection, irrespective of clinical signs and symptoms.

Only those with a positive laboratory test (whether they have symptoms or not) are considered confirmed cases. At this time, Switzerland is testing mostly severe cases and thus, the number of confirmed cases is not necessarily indicative of the number of people with COVID-19 in the country, since many have very mild symptoms, or only moderate ones that do not require hospitalization. Therefore, we don't currently show confirmed cases on the map, as this number is small in the current situation. We have decided to show COVID-19 suspected cases, a definition better suited to inform on the prevalence of disease symptoms in our community.

As the disease definition is refined by health authorities, we may adapt to reflect other variables (e.g. additional risk factors, comorbidities, new symptoms). Please tune in again for updates, we will post them in this blog when they occur.

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