Yellow Fever Outbreaks in Unvaccinated Populations, Brazil, 2008–2009

Due to the risk of severe vaccine-associated adverse events, yellow fever vaccination in Brazil is only recommended in areas considered at risk for disease. From September 2008 through June 2009, two outbreaks of yellow fever in previously unvaccinated populations resulted in 21 confirmed cases with 9 deaths (case-fatality, 43%) in the southern state of Rio Grande do Sul and 28 cases with 11 deaths (39%) in Sao Paulo state. Epizootic deaths of non-human primates were reported before and during the outbreak. Over 5.5 million doses of yellow fever vaccine were administered in the two most affected states. Vaccine-associated adverse events were associated with six deaths due to acute viscerotropic disease (0.8 deaths per million doses administered) and 45 cases of acute neurotropic disease (5.6 per million doses administered). Yellow fever vaccine recommendations were revised to include areas in Brazil previously not considered at risk for yellow fever.Author Summary: Yellow fever is a viral hemorrhagic disease transmitted by mosquitos, endemic in tropical regions of Africa and South America. Large urban outbreaks of yellow fever have been eliminated in the Americas, where most yellow fever cases result from human exposure to jungle or forested environments. Vaccination is effective but carries a risk of potentially fatal adverse events in a small number of vaccinees. In a large country such as Brazil, vaccination is recommended only in areas where there is a risk of exposure to yellow fever virus. We describe two outbreaks of yellow fever in areas without yellow fever vaccine recommendations. Numerous epizootics, or die-offs of non-human primates, were reported from areas with human cases. In response to the outbreaks and epizootic activity, over five million doses of vaccine were administered in previously unvaccinated populations, resulting in vaccine associated adverse events, six of which were fatal. The outbreaks resulted in expansion of areas with yellow fever vaccine recommendations, and highlight the need for safer yellow fever vaccines.