Cases, Clusters and Outbreaks - definitions
In the Superspreading Events database, we try to establish how many people were infected at the event itself, because this helps us to guage the risks involved in that situation. However, press reports often refer to the total number of cases linked to a superspreading event. Linked cases include those in the wider community arising from the superspreading event. For example, if 20 people were infected at a party, and some of those people infected 30 household members, an epidemiological investigation would conclude that 50 cases were linked to the party. But in terms of assessing the risks of a setting, there are important differences.
Index and Secondary Cases
An index case at an event is a person who brings the infection there. There may be more than one. Index cases are also known as 'Primary cases'.
Secondary cases are people who became infected in this setting.
Tertiary cases are people who later became infected by secondary cases, eg when they went home or to other gatherings.
The total linked cluster size, including tertiary cases, is important in assessing the overall impact of an event, but it doesn't tell us much about the risks of the event itself. For that, the secondary cases are more important.
Clusters and Outbreaks
The standard epidemiological definition of a cluster is : a number of cases linked in time and location, beyond that expected in the general population in that area. A cluster of cases goes beyond what you would expect to find by chance in that time and place. We aren't certain that the cases are linked, but a cluster alerts us that they might be.
An outbreak is a cluster where there is evidence that transmission occurred in that setting.
In an epidemiological investigation, the cluster/outbreak should be clearly defined in terms of time and place.
For example, Public Health England definitions are:
Cluster criteria
Two or more test-confirmed cases of COVID-19 among individuals associated with a specific non-residential setting with illness onset dates within a 14-day period.
(In the absence of detailed information about the type of contact between the cases).
End of cluster: No test-confirmed cases with illness onset dates in the last 14 days.
Outbreak criteria
Two or more test-confirmed cases of COVID-19 among individuals associated with a specific non-residential setting with illness onset dates within 14 days, and one of:
identified direct exposure between at least 2 of the test-confirmed cases in that setting (for example under one metre face to face, or spending more than 15 minutes within 2 metres) during the infectious period of one of the cases
when there is no sustained local community transmission - absence of an alternative source of infection outside the setting for the initially identified cases
End of outbreak: No test-confirmed cases with illness onset dates in the last 28 days in that setting.
References
CDC Epidemic Disease Occurrence
European CDC Surveillance Definitions for Covid-19
ECDC Outbreak and Cluster definitions (for Legionella but generally applicable)