Yellow Fever Continues to Spread in Asia

Researchers have identified the first-ever reported cases of yellow fever in China, tracing their origins to travelers from Angola, where there is an ongoing outbreak. The findings were published online on May 5 in the International Journal of Infectious Diseases (IJID).

According to researchers, the emergence of yellow fever in the Asian nation—they have confirmed six cases in the Fujian province alone—is likely the result of several factors, including the large Chinese expatriate community in Angola, where the incidence of the disease is relatively high, and the fact that there is an extensive amount of travel between the two countries as a result. However, they believe there is significant risk for localized transmission of the virus in the Asia-Pacific region, particularly during the coming Dengue fever season, due to what they describe as “favorable ecology” (including climate and environment) and the large, unimmunized population. In fact, they note in their assessment that it is somewhat surprising that the region hasn’t experienced a yellow fever outbreak to date.

“What we have argued in our paper is that current conditions... together, potentially support the introduction and transmission of yellow fever in Asia for the first time,” study co-author Sean Wasserman, MD, a faculty member in the Division of Infectious Diseases and HIV Medicine at the University of Cape Town in South Africa told Contagion. “All the requirements seem to be in place: a receptive environment in terms of abundant and probably competent mosquito vectors, an unimmunized population living in close contact with the mosquitos, and, for the first time in history, a potentially sustained and large-scale introduction of virus by returning Chinese workers from the Angolan outbreak.”

Indeed, Dr. Wasserman and his colleagues write in IJID that there is already an abundant population of Aedes aegypti mosquitos, the vector linked with yellow fever, as well as other viruses such as Dengue and Zika, in Asia, with approximately two billion people living in regions infested with the mosquitos. In addition, the region is also characterized by forested areas with a population of monkeys susceptible to yellow fever, and they could potentially act as hosts for the virus.

Historically, of course, outbreaks of yellow fever have been reported in parts of Africa and South and Central America—with dramatic consequences. Between 1998 and 2012, for example, there were more than 4,000 reported cases of the virus in the Americas, with approximately 58 percent resulting in death. Between 1980 and 2012, there were at least 200,000 confirmed cases (research suggests the actual figures may be much higher) of the virus in Africa, with roughly 45 percent resulting in death.

To date, Dr. Wasserman says, the World Health Organization (WHO) has responded proactively to the Angolan outbreak, and distributed large quantities of emergency vaccine to the West African nation. Similarly, a vaccination campaign has been initiated in the neighboring Democratic Republic of the Congo. However, he cautions that “there is not nearly enough vaccine to immunize all at-risk people [there], and there would definitely not be sufficient vaccine in the event of an Asian outbreak.” He and his colleagues, therefore, are urging that measures such as vaccine dose reduction (to “stretch the current supply”) should be implemented immediately.

“Yellow fever needs to be taken seriously in the regions where it is endemic so that local outbreaks can be prevented and responded to effectively,” Dr. Wasserman explains. “The main tool for this is vaccination, but the other tools of surveillance and vector control need to be strengthened. The other message is that governments need to strictly enforce the International Health Regulations yellow fever vaccination requirements. If all Chinese workers and other travelers from Asia in Angola were vaccinated prior to departure there would be almost no risk of the virus spreading to Asia.”

Brian P. Dunleavy is a medical writer and editor based in New York. His work has appeared in numerous healthcare-related publications. He is the former editor of Infectious Disease Special Edition.