HomeAn outbreak is the occurrence of more cases than expected over a given time period, with ongoing transmission. Outbreaks may be suspected due to clustering of cases by location, time, behavioral factors or strain genotype.113,114 Note that cases may increase without ongoing transmission, for example due to changes in migration patterns.
The following is a working definition of outbreaks,14 intended to help identify and contain rapidly evolving clusters:
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During and because of a contact investigation, 2 or more of the identified contacts are diagnosed as secondary cases of active TB (confirmed by genotyping/WGS if available); or
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any 2 or more cases in TB patients occurring within 1 year of each other are discovered to be epidemiologically linked (and matched by genotyping/WGS if available), but the linkage is recognized outside of a direct contact investigation.
Most situations that have been recognized as TB outbreaks involve chains of many more than 2 secondary cases, or one previously unrecognized link to a secondary case, and extend over several years. A slower cluster of linked cases that spans several years may still require heightened TB program response for an identifiable population group yet not be an “outbreak” by the aforementioned definition.
By definition, once an outbreak is declared, additional cases are usually restricted to those who have the outbreak strain, as confirmed by genotyping/WGS of TB isolates. Note that additional unrelated cases may also be diagnosed within the same population during the outbreak period.
While there is no standard for declaring a TB outbreak over, we suggest a functional timeline of 2 years past the last case, consistent with the highest risk period for close contacts to develop active TB.115
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