Measles outbreak reveals measles susceptibility among adults in Namibia, 2009 - 2011.

Abstract

The World Health Organization, African Region, set the goal of achieving measles elimination by 2020. Namibia was one ofseven African countries to implement an accelerated measles control strategy beginning in 1996. Following implementation of this strategy,measles incidence decreased; however, between 2009 and 2011 a major outbreak occurred in Namibia. Measles vaccination coverage data were analysed and a descriptive epidemiological analysis of the measles outbreak wasconducted using measles case-based surveillance and laboratory data. During 1989 - 2008, MCV1 (the first routine dose of measles vaccine) coverage increased from 56% to 73% and five supplementaryimmunisation activities were implemented. During the outbreak (August 2009 - February 2011), 4 605 suspected measles cases werereported; of these, 3 256 were confirmed by laboratory testing or epidemiological linkage. Opuwo, a largely rural district in north-westernNamibia with nomadic populations, had the highest confirmed measles incidence (16 427 cases per million). Infants aged ≤11 months hadthe highest cumulative age-specific incidence (9 252 cases per million) and comprised 22% of all confirmed cases; however, cases occurredacross a wide age range, including adults aged ≥30 years. Among confirmed cases, 85% were unvaccinated or had unknown vaccinationhistory. The predominantly detected measles virus genotype was B3, circulating in concurrent outbreaks in southern Africa, and B2,previously detected in Angola. A large-scale measles outbreak with sustained transmission over 18 months occurred in Namibia, probably caused by importation.The wide age distribution of cases indicated measles-susceptible individuals accumulated over several decades prior to the start of the outbreak.