3.3 Confirming the outbreak

An outbreak is suspected when the number of suspected measles cases reported by a geographic unit1 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 1Week 12
CasesAR/100,000CasesAR/100,000
200220.3233.2
200310.1536.7
2004607.23,832462.4

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