Preplanned National Measles Vaccination Campaign at the Beginning of a Measles Outbreak--Sierra Leone, 2009-2010

Background: Large-scale measles outbreaks occurred throughout Africa from 2008-2010. In Sierra Leone, in November 2009, preceding a measles supplemental immunization activity (SIA), the largest measles outbreak in a decade started.

Methods: We analyzed data from the national measles case-based surveillance system, developed a susceptibility profile of the population, and calculated vaccine effectiveness (VE) among children 12-59 months of age.

Results: From November 1, 2009 to July 13, 2010, 1,094 confirmed cases, including 9 deaths, were reported; 716 (66%) were <5 years of age. B3 genotype was identified. Measles attack rates per 100,000 population were highest among infants aged 6-8 months (56.4) and in Bo district (49.4). Districts with higher estimated SIA coverage tended to have lower attack rates (Spearman Correlation Coefficient=-0.63), p=0.07. Among 473 cases with information on vaccination status, 222 (47%) were unvaccinated; estimated VE was 74%. The 2009 measles SIA led to 165,000 fewer estimated susceptible individuals.

Conclusions: The 2009 measles SIA reduced the overall magnitude of the outbreak, though routine and SIA coverage was insufficient to prevent it entirely. Maintaining high coverage through routine services and SIAs in all districts and conducting follow-up SIAs prior to the end of the low transmission season may prevent future outbreaks.