3.3 Confirming the outbreak

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    An outbreak is suspected when the number of suspected measles cases reported by a geographic unit a Citation a. A geographic unit may be the area covered by a health post, an administrative region (canton), or a city neighbourhood.  is higher than expected. It must always be confirmed by the laboratory.

     

    In countries that conduct supplementary immunisation activities (SIAs), an outbreak is suspected as soon as the first cases appear. The vaccination coverage and time since the last SIA are taken into account, as shown in the table below.

     

    Table 3.1: Definition of a measles outbreak

     

     

    Catch-up campaign completed
    less than
    4 years ago
    AND vaccination coverage ≥ 90%

    Catch-up campaign not done
    OR done 4 or more years ago
    OR vaccination coverage < 90%

    Suspected
    outbreak

    5 suspected cases reported by a single geographic unit in a one month period

    Within a geographic unit:
    – Number of cases or weekly incidence higher than in previous (nonepidemic) years, or the same as in an epidemic year
    OR
    – If no data from previous years: increase in the number of cases in the last 3 or 4 weeks

    Confirmed
    outbreak

    > 2 confirmed cases (IgM+) in a one-month period

     

    In all countries (whether they have SIAs or not), the onset of an outbreak can be detected by comparing the weekly attack rate to the same period in previous years (epidemic or nonepidemic).

     

    Example: Measles cases and attack rates, 2002-2004, urban area, Niamey, Niger.

     


    Year
    Week 1 Week 12
    Cases AR/100,000 Cases AR/100,000
    2002 2 0.3 23 3.2
    2003 1 0.1 53 6.7
    2004 60 7.2 3,832 462.4

     

    Source: SNIS [Système National d'Information Sanitaire du Niger]

     

    In 2004, the number of cases and the attack rate for Week 1 were markedly higher than those in previous years. In Week 12, the number of cases and the attack rate show that there was indeed an outbreak of measles.

     

    In refugee camps and other closed setting (orphanages, feeding centres, prisons, etc.), a single case of measles constitutes an outbreak.

     

    The risk in poor urban areas with low vaccination coverage is similar to that in refugee camps.

     

    Deciding to intervene is always a guess that things will probably progress. If in doubt, however, deciding too soon is better than deciding too late.

     

    Footnotes
    • (a)A geographic unit may be the area covered by a health post, an administrative region (canton), or a city neighbourhood.