3.4 Estimating the severity and potential for spread

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    The initial evaluation should provide the information needed to assess the severity of the current outbreak and the risk that it will spread. The choice of response will depend on these data and the resources that can be mobilised. There are several elements to consider.

    3.4.1 Surveillance data

    • Number of cases, weekly incidence, number of deaths and case fatality rate
    • Epidemic curve trend
    • Laboratory confirmation
    • Surveillance data for the past 5 years
    • Information on previous epidemics: date, number of cases and deaths, area and age groups affected and immunisation activities conducted

    3.4.2 Population characteristics

    • Number of people living in the affected area
    • Population density: urban/rural area
    • Population movements (seasonal migration, social and religious events)
    • Vulnerability of the population: poor urban areas, refugees, internally displaced persons

    3.4.3 Size of the susceptible cohort

    Two major factors influence the size of the susceptible cohort:

    • the size of the birth cohort: this is estimated based on the total population, the expected number of pregnant women and the proportion of live births;
    • the vaccination coverage in previous years: routine, SIAs and other activities (e.g., outbreak response vaccination campaign). The coverage figures ordinarily used are based on routine data. Use survey results, if available, because they are often more reliable.

    3.4.4 Mortality rate

    The case fatality rate and measles-specific mortality rate will depend on the initial health status of exposed population (infant mortality rate, malnutrition) and on access to care (the health system, the drug supply, and the cost of services).


    These elements should be considered when organising patient care.

    3.4.5 Potential for spread

    The potential for spread of the outbreak is based on an evaluation and classification of epidemic risk factors, using a rating scale where + = low risk, ++ = high risk, and +++ = very high risk (Table 3.2).


    Table 3.2: Rating epidemic risk factors


    Major risk factors Risk

    Vaccination coverage

    ≤ 70% +++
    71-90% ++
    > 90% +

    Number of cases per week in a single geographic area

    Continuous (> 4 weeks) and rapid increase +++
    Increase ++
    Low and stable +

    Laboratory confirmation

    ≥ 3 confirmed cases +++
    < 3 confirmed cases ++
    No confirmed cases +

    Comparison of attack rates to the same period in previous years

    ≥ 10 x or the same as a previous epidemic year +++
    5 x to 10 x ++
    ≤ 5 x or the same as a non-epidemic year +
    Additional risk factors Risk

    Time since the last outbreak

    > 4 years +++
    ≥ 2 years ++
    ≤ 1 year +

    Population density

    Very high +++
    High ++
    Low +

    Birth rate

    Very high (≥ 4%) +++
    Intermediate (2-3.9%) ++
    Low (< 2%) +
    Existence of an outbreak in an adjacent area ++


    The greater the number of risk factors, the greater the threat of spread and a large-scale outbreak. The following table serves as a guide for analysing the potential for the outbreak to spread. A three-level classification scheme is proposed below:


    Table 3.3: Potential for a measles outbreak to spread


    Potential for occurrence/spread of a measles outbreak
    Low High Very high

    If the four major risk factors
    are + and the additional risk
    factors are low (++ or +).

    If one or more of the four
    major risk factors are ++.


    Risk increases with increasing
    number of additional risk

    If one or more of the four
    major risk factors are +++.


    Risk increases with increasing
    number of additional risk