Yellow Fever in Brazil: The Latest Global Health Security Threat

On January 13, 2017, Minas Gerais—Brazil’s second most populous state—declared a 180-day state of emergency following eight yellow fever-related deaths. Since then, the outbreak has swept across much of Brazil, with confirmed cases in rural villages spanning 130 municipalities in nine states. And while Brazil has responded aggressively to the outbreak, concerns remain that the virus could eventually reach one of its major cities or even move beyond its borders; thereby igniting a larger yellow fever epidemic. In late May, the Center for Strategic and International Studies (CSIS) Global Health Policy Center traveled to Brazil to look at the state of U.S.-Brazilian cooperation on matters of global health security. Throughout the course of our discussions, we came to learn more about the yellow fever outbreak and the risks it poses to the international community.

Yellow fever—a mosquito-borne virus that can cause fever, chills, muscle aches, headaches, and (in serious cases) bleeding, organ failure, and death—has long been endemic to Brazil. First brought to the Americas from Africa in the 1600s during the slave trade, yellow fever used to kill hundreds of thousands of people annually throughout the region. That changed in the early 1900s with aggressive regional Aedes aegypti mosquito elimination programs in cities (Aedes aegypti being the primary mosquito to transmit yellow fever in urban settings), the development of an effective yellow fever vaccine, and the implementation of large-scale urban immunization campaigns. These combined efforts caused the number of human fatalities in Latin America to drop exponentially and largely confined the virus to monkey populations living deep in the Amazon.  

However, the risk of yellow fever outbreaks in Brazil has remained. Several different mosquito species can transmit the virus from monkeys to humans who live or work near the jungle, resulting in localized yellow fever outbreaks. It’s of particular concern if an individual infected with the virus then travels to a major city. There, the presence of the Aedes aegypti mosquito increases the likelihood of a larger urban yellow fever outbreak. Aedes aegypti—the same mosquito that transmits dengue, chikungunya, and the Zika virus—is ubiquitous in large Brazilian cities and is uniquely suited to quickly spread yellow fever throughout a population. Unlike other mosquito species that only feed from a single person, Aedes aegypti feeds on several individuals throughout the course of its two- to four-week lifespan. This means that if an Aedes aegypti mosquito bites a person infected with yellow fever, it could potentially transmit the virus to dozens of individuals within a matter of days.

Such is the concern among health officials regarding the current situation in Brazil, where outbreaks in remote parts of the country are now threatening to spread to major urban centers for first time in the Americas since 1942. Accelerated urbanization along the Amazon’s border in recent years has concentrated a large number of non-immunized individuals in traditionally high virus transmission areas, resulting in the current spike of yellow fever cases in the country. As of May 31, there have been 3,240 suspected cases of yellow fever reported to the Brazilian Ministry of Health—792 of which have been confirmed and 519 that are still under investigation. Nearly all confirmed cases thus far have been concentrated in rural areas in just four states: São Paulo, Minas Gerais, Rio de Janeiro, and Espírito Santo. Three of those states—São Paulo, Minas Gerais, and Rio de Janeiro—are home to nearly 40 percent of Brazil’s 208 million citizens and boast some of Brazil’s largest cities. The cities of São Paulo and Rio de Janeiro alone are home to a combined 18.5 million people. The sheer size of these cities, plus their role as major regional and global transportation hubs, could pose a significant public health risk if a case of yellow fever were to reach them.

To Brazil’s credit, the government has responded aggressively to prevent such a scenario from occurring. It has worked closely with international actors like the Pan American Health Organization (PAHO)/World Health Organization (WHO) and has leveraged its considerable domestic health capacities—including the expertise of its world-renowned Oswaldo Cruz Foundation (Fiocruz) and its domestic yellow fever vaccine production capabilities—to avert a major urban outbreak. Since January, the Ministry of Health has dispatched over 26 million doses of yellow fever vaccine to affected states. Over 1,000 municipalities have conducted surge vaccination campaigns, while surveillance and case management capacities have been strengthened.

Additionally, Brazil’s experience combatting the 2015-2016 Zika epidemic has also helped stave off an urban outbreak. In the lead-up to the 2016 Olympics, the Brazilian government undertook an extensive vector-control campaign in the city of Rio de Janeiro. The Ministry of Health partnered with the Brazilian military to spray for Aedes aegypti throughout much of the city and surrounding countryside and the Rio city government spent months eliminating potential mosquito breeding grounds. That included cleaning up standing pools of water, removing trash from the streets, and properly discarding used tires (which serve as ideal mosquito breeding sites).

Such efforts, coupled with the fortuitous arrival of the cooler, drier winter months which typically experience a decrease in mosquito populations, might prove enough to curb the current outbreak. However, concerns remain.  

First, the outbreak response to-date has severely depleted the global supply of yellow fever vaccine, thereby calling into question whether the international community has the means to effectively respond to a potential outbreak in a major Brazilian city or elsewhere in the world. Brazil is one of only four manufacturers of yellow fever vaccine worldwide and is a key contributor to global yellow fever vaccine emergency stockpiles like that of the International Coordinating Group (ICG) on Vaccine Provision or the PAHO Revolving Fund. As Brazil has been forced to use more of its domestically-produced yellow fever vaccine to deal with its own outbreak, less is available to contribute to such global reserves. Additionally, the outbreak response has been so taxing on Brazil’s national yellow fever vaccine stockpile that it’s had to request an additional 3.5 million vaccine doses from the ICG’s 6 million dose vaccine inventory. With global vaccine stockpiles already low from the Angolan outbreak last year and the long time needed to produce additional yellow fever vaccine, this additional strain has severely weakened global capacities to respond to a broader epidemic.  

Second, while the number of yellow fever cases in Brazil are currently declining, risks remain that the virus could still reach other parts of the Americas. As occurred with Zika, it’s possible that the yellow fever outbreak in Brazil could spread to neighboring parts of Latin America and the Caribbean through travel-related case importation or vector-borne transmission. Granted, there are procedures currently in place meant to address travel-related case importation, including provisions in the International Health Regulations (IHR) and country requirements to show a WHO vaccination yellow card upon entry. However, with over one hundred million airline passengers traveling to and from Brazil per year, countless overland crossings with neighboring countries, and Aedes aegypti pervasive throughout much of the region, it is conceivable that isolated cases could still slip by, potentially leading to similar outbreaks with local transmission of the virus in neighboring countries.

While a significant yellow fever outbreak in the United States in unlikely, the situation in Brazil still poses some risk to the homeland. A combination of low yellow-fever vaccination rates—citizens of and travelers to the U.S. aren’t required to have a yellow fever shot—and the concentration of Aedes aegypti throughout a large portion of the South means that it is plausible that a limited outbreak with some localized transmission of the virus could occur if a travel-related case of yellow fever were to reach the southern U.S. Effectively responding to such a scenario could prove particularly challenging right now, as manufacturing problems at the only U.S.-licensed yellow fever vaccine production facility have caused a significant vaccine supply shortage, with the U.S. Centers for Disease Control and Prevention (CDC) estimating that the U.S. stockpile could run out by mid-July.  

Finally, there is a chance—albeit minimal—that yellow fever could ultimately migrate beyond the Americas and lead to localized outbreaks in Asia. Using the Angolan outbreak last year as an example, ten travel-related cases of yellow fever were confirmed in China, including six in the Fujian Province. These cases suggest that even though China requires travelers from yellow fever endemic countries to produce a WHO vaccination card upon entry, unvaccinated individuals were still able to enter. Considering China’s large expatriate community in Angola, it’s possible that additional cases may have been imported into China as well yet gone undetected. The deepening of economic and commercial ties between Brazil and Asia in recent years, large Chinese and Japanese expatriate communities in Brazil, the abundance of Aedes aegypti throughout much of China and Southeast Asia, and the potential for a similar public health surveillance breakdown with the current outbreak at least raises the possibility of travel-related yellow fever cases with some degree of local transmission arising in Asia; a prospect that must be taken quite seriously.

The yellow fever outbreak in Brazil is simply the latest in a long line of health security threats the world has faced in recent years. While Brazil has responded admirably to the outbreak, there are still concerns of a broader public health crisis unfolding should the virus reach a major urban center or be exported out of the country. Ensuring such scenarios don’t come to pass will require continued efforts by the Brazilian government at boosting yellow fever vaccination rates in rural areas, implementing mosquito-control programs in major cities, and bolstering surveillance capacities throughout the country. It will also require a concerted effort by the international community to strengthen global health security capacities in the region and beyond, including strengthening detect and response capacities throughout Latin America and the Caribbean, rectifying shortcomings in the current global vaccine stockpile system, and shoring up potential gaps in countries’ travel-related case detection protocols. 

As recent history has demonstrated time and time again, a health threat anywhere is a health threat everywhere. As Brazil continues its tireless efforts to prevent a yellow fever epidemic from taking hold, the world would be wise to take heed.