The two key epidemiologic indicators used during a cholera outbreak are the attack rate and the case fatality ratio or rate. These indicators are standard measures of epidemic intensity and impact. They allow for comparisons between different locations and previous outbreaks.
8.3.1 Attack rate
The attack rate (AR) is the cumulative incidence of cholera since the start of the outbreak.
As the AR is population-based, knowing the total number of people in the affected area is essential (Section 2.5.1).
The AR is more informative when using population figures that match the specific administrative areas reporting cholera cases. For example, the intensity of an outbreak affecting 3 neighbourhoods in a city is more accurately described when the total population for those 3 neighbourhoods is used in the calculation, rather than the population of the entire city.
AR is usually expressed as a percentage. The formula for calculating AR is as follows:
Number of cases during a time period
AR = –––––––––––––––––––––––––––––––––––––––––––––––– x 100
Population exposed to cholera during the same period
The AR will increase rapidly during initial phase of the outbreak and then reach a plateau as it ebbs. It never goes down.
8.3.2 Case fatality rate
The case fatality rate (CFR) is the proportion of cholera cases dying from cholera or its complications in treatment facilities and/or in the community1 .
CFR is expressed as a percentage. The formula for calculating CFR is as follows:
Number of deaths due to cholera during a time period
CFR = ––––––––––––––––––––––––––––––––––––––––––––––––– x 100
New cases of cholera during the same period
In treatment facilities, the CFR is calculated on a weekly and cumulative basis. It is used for assessing the quality of case management. The standard indicator for adequate case management is a CFR < 1%. All facilities must monitor the CFR and quality of care, particularly if the CFR is > 1%.
Global CFR which combines facility and community deaths is tracked for the duration of the epidemic and indicates the overall adequacy of the response in preventing avoidable deaths.
8.3.3 Examples of calculation
An outbreak of cholera has started in a city with a population of 300,000. Since the start of the outbreak, 455 cases total have been reported, including 150 new cases recorded during the last week.
Among these 150 cases, 6 die during the same reporting week.
AR = 455 ÷ 300,000 x 100 = 0.15% since the start of the outbreak
CFR = 6 ÷ 150 x 100 = 4% for the last week
By the end of the epidemic, a total of 3313 cholera cases and 25 deaths have been reported.
Population at risk is the same: 300,000 persons
Cumulative AR = 3313 ÷ 300,000 x 100 = 1.1%
Cumulative CFR = 25 ÷ 3313 x 100 = 0.75%
As the number of community cholera cases reported (denominator) is often less reliable than that reported in cholera facilities, it is difficult to calculate an accurate community CFR.