Impact of an Outbreak Response Immunization campaign during a large measles outbreak in Dar es Salaam, Tanzania, 2006

Learning Objectives for this Presentation:
By the end of the presentation participants will be able to:
1. Define Outbreak Response Immunization campaign (ORI)
2. Describe possible impact of ORIs in an outbreak

Background:
Estimated coverage for the first dose of measles-containing vaccine in Tanzania increased from 78% in 2000 to 91% in 2005. The number of measles cases decreased from 14,649 in 2000 to 713 in 2005. In 2005 a nation-wide supplemental immunization activity reached 93% of children aged 9 - 59 months. In 2006, Dar es Salaam (DSM) experienced a large measles outbreak of 1533 confirmed cases. An Outbreak Response Immunization campaign (ORI) was conducted September 22-24 targeting children aged 6 months to 14 years 11 months in DSM.

Objectives:
We analyzed the impact of the ORI, and identified the genotype of the circulating measles virus.

Methods:
We analyzed surveillance data for two periods: 2 months prior to and 3 months following the ORI. Nasopharyngeal and oral specimens were tested at the regional reference laboratory (Uganda) and global reference laboratory (CDC/Atlanta) by RT-PCR for genotyping.

Results:
Prior to ORI, 67% (696 of 1037) of cases were in the ORI target age group, compared to 46% (230 of 496) afterwards. The proportion of patients in the targeted age group that reported prior vaccination increased from 38% (246 of 650) prior to the ORI to 48% (109 of 229) after the ORI. The genotype was identified as B3; genomic sequencing indicated it to be closely related to virus circulating in Kenya.

Conclusions:
This ORI led to additional protection among the targeted population as evidenced by the shift in age distribution and cases vaccinated. Viral genotyping confirmed that the measles virus was closely related to the virus in Kenya that was imported into Europe and North America.