12. DETECTION OF AN OUTBREAK
Outbreaks tend to generate high levels of public alarm, especially as these are unpredictable and can develop quickly. Recognition of an outbreak of meningococcal disease particularly in the community can be challenging. Careful but rapid epidemiological investigation and calculation of attack rates is essential in determining whether an outbreak exists and its extent.
Look out for:
- An increased rate of disease in defined populations and/or an absolute increase in cases
- A cluster of patients in a particular age group
- A shift in the age distribution of cases
12.1 Classification of cases for determining incidence/attack rates
Reported cases should be classified as follows, to allow accurate determination of rates of disease within the population concerned:
Sporadic case
A single case with no known history of close contact with another case
Primary case
A case with no known close contact with another case
Co-primary case
A close contact in whom disease develops within 24 hours of onset of illness in the primary case
Secondary case
A close contact of a primary case who becomes ill more than 24 hours after onset of illness in primary case.
12.2 Definition of an outbreak
12.2.1 Organisation/institutional outbreak
- Two or more probable or confirmed cases during a 4 week interval in a group which makes sense epidemiologically (if cases are laboratory confirmed – serogrouping should be the same).
OR
- Three cases of confirmed or probable meningococcal disease in ≤ 3 months of the same serogroup (if available) with a history of a common affiliation but no close contact giving a primary disease attack rate of ≥ 10 cases/100 000 persons.
(Reference: Guidelines for the public health management of meningococcal disease in the UK PHLS September 2002, Vol. 5 No 3 reprint 187 - 204 plus appendices)
12.2.2 Community-based outbreak
- Three cases of confirmed or probable meningococcal disease within a three month interval of the same serogroup (if available) in persons who live in the same area AND who have not had close contact with each other and do not share a common affiliation. Giving a primary disease attack rate of ≥ 10 cases/100 000 total community population. The population should include recognised political boundaries most closely related to the residences of these cases.
- The numerator is the number of confirmed cases in the population at risk caused by strains of the same serogroup and that are not distinguishable. Count primary cases together with related co-primary and secondary cases as a single case.
- The denominator is the population at risk. This population should be clearly defined and make sense to the people who live within and without the selected boundaries. It may not be easy to define such a population. Examples are a rural town/village or a secondary school with its feeder schools.