An analysis of the data collected at this point will be used to determine if a cholera outbreak is underway, to estimate the risk of a severe outbreak, to determine the potential benefit of vaccination against cholera, and to evaluate the treatment resource needs.
2.6.1 Defining an outbreak
In general, a cholera outbreak is defined as any increase in the number of cholera cases compared to the expected number for a given place over a particular period of time.
In areas with no history of cholera
The appearance of any case or cluster of cases of acute watery diarrhoea meeting the cholera case definition, and subsequently confirmed by culture, can be considered an outbreak.
In areas where cholera is known to occur
The Ministry of Health usually has a national definition of a cholera outbreak.
If not, a definition can be established using current and/or historical data:
– If there are no historical data: for a given location, a doubling of the number of cases meeting the clinical case definition of cholera over 2 to 3 consecutive weeks can be considered an outbreak.
– If data from previous years are available (same calendar period and location): calculate the average number of expected cases per week in non-epidemic periods. An increase in the weekly average number of cases above this non-epidemic average indicates a developing outbreak.
In all cases, the definitive declaration1 of an outbreak depends on culture confirmation.
2.6.2 Estimating outbreak severity
Determine the level of risk an outbreak poses to the population in terms of morbidity, mortality, probability of extension.
Risk factors2 to consider are:
– History of outbreaks with high attack rates, high case fatality rates, or large geographical extension.
– No outbreak in the preceding 2-3 years (loss of innate immunity from prior infection).
– Divergence from the pattern typical of previous outbreaks: onset prior to the usual season, location in an area where outbreaks were previously unknown, early involvement of a large number of individuals affected, early geographic extension, or outbreaks in multiple locations.
– Emergence in population-dense communities (e.g. slums, refugee camps) or in mobile populations, either coming from an area with no prior history of cholera (no natural immunity) or from a cholera-endemic area (importation of Vibrio cholerae by asymptomatic carriers).
– Current meteorological conditions outside the norm (either very rainy or very dry).
– Disruption of water and sanitation systems or access to care or lack of human resources to manage the outbreak due to economic collapse, natural disaster, conflict, etc.
The level of risk for a severe outbreak grows as the number of risk factors increases.
2.6.3 Evaluating the potential benefit of cholera vaccination
The use of cholera vaccines in response to an epidemic requires a rapid analysis of the current and historical data. The aim is to assess the likely benefit of a reactive vaccination campaign and to make a request to the International Coordination Group on vaccine provision if the campaign is presumed beneficial and feasible (Section 3.6 and Section 4.6).
In principle, it is the national authorities who officially declare an epidemic (or its end) in their country.