[This article is written by Adelaida Sarukhan (ISGlobal) and Jose Muñoz (ISGlobal-Hospital Clínic)]
In March 2016 the first ever cases of yellow fever were reported in Asia
While worldwide attention was drawn during the first months of 2016 towards the Zika epidemics and the up to then unknown effects of the virus on the developing foetus, another epidemic caused by a related virus was threatening to explode. In March 2016 the first ever cases of yellow fever were reported in Asia. All cases had been imported to China from Angola, where an ongoing outbreak has already caused more than 3800 cases and 360 deaths.
The good news with yellow fever is that, in contrast to Zika or Ebola, there is a vaccine. The bad news is that its production is complicated and therefore limited, and there would not be sufficient doses in case of a full-blown epidemic. And herein lies the concern of a possible introduction of the virus into Asia.
Why hasn’t there been yellow fever in Asia?
Yellow fever is endemic in tropical regions in Africa and South America, where it arrived more than three centuries ago as a result of the slave trade. In large cities it is transmitted by the Aedes aegypti mosquito (the same mosquito that transmits Zika) via an urban transmission cycle (i.e. it does not involve other animals).
The limited slave trade between Africa and Asia in the past may in part explain this
Why yellow fever has not become endemic in Asia, despite the broad distribution of the mosquito, remains a mystery. The limited slave trade between Africa and Asia in the past may in part explain this. In addition, it has been proposed that Asiatic mosquitoes are less competent in transmitting the virus, that mosquitoes infected by dengue virus are not easily infected by yellow fever, and/or that antibodies against dengue or Japanese encephalitis protect against yellow fever. However, the simultaneous presence of yellow fever and dengue in Angola and other African regions argues against these hypotheses, for which there is little scientific evidence in favor. The current expansion of Zika virus in Latin America is further proof that a common vector and previous exposure to other viruses are not an obstacle to the introduction of new virus in dengue-endemic regions.
Yellow fever has a higher mortality rate than other mosquito-borne diseases
The explanation is most probably a simple matter of luck: the number of sick people that has arrived to Asia has been too small and the mosquito has not had the chance of picking up the virus. In fact, yellow fever has a higher mortality rate than other mosquito-borne diseases (15 to 20% of people develop severe and potentially lethal symptoms) and infected people are more likely to feel sick and less likely to get on an airplane (the virus only lasts around 5 days in the blood after symptom onset).
Similar situations have already been observed with other viruses. For instance, chikingunya did not reach Latin America until 2013, even though it caused a big outbreak in Asia in 2004 as a result of imported cases from Kenya.
The Threat of a Yellow Fever Epidemic in Asia
The introduction of a new virus in an unvaccinated population and with no natural immunity to the virus represents a global health threat
Curently, 2 billion people in Asia live in regions infested with Aedes aegypti. The introduction of a new virus in an unvaccinated population and with no natural immunity to the virus represents a global health threat. Fortunately, the regions in China where the 11 imported cases were reported (Beijing, Shangai, Fuhian) but this is not the case of Southern China, where there is ongoing dengue transmission.
Asiatic countries will need to establish strict mechanisms of disease surveillance, traveler vaccination and vector control
Thus, the risk of yellow fever introduction in Asia will be at its highest this summer due to the increase in the number of mosquitoes, the constant flow of Chinese workers that travel to and from Angola (where an outbreak of yellow fever persists, even if it seems to be under control) and the scarcity of vaccines due to the outbreaks in Angola and the Democratic Republic of Congo, that threaten to expand to other neighboring countries (WHO launched some days ago the largest yellow fever vaccination campaign in the region).
Asiatic countries –China in particular– will need to establish strict mechanisms of disease surveillance, traveler vaccination and vector control in order to avoid the arrival and spread of the virus. Even though an effective yellow fever vaccine exists, there is a problem of quantity (the global production would not be enough to cover an outbreak in a large Asiatic city) and of control (in Angola and DRC false vaccination certificates have been circulating).
Even though an effective yellow fever vaccine exists, there is a problem of quantity and of control
Luck does not last forever, and the arrival of yellow fever to Asia is a ticking time bomb. The Zika and chikungunya outbreaks in Latin America have shown us that when a large part of the population is not immune and the vector is abundant, outbreaks can be explosive and difficult to control.
Adelaida Sarukhan is PhD in Immunology and a scientific writer at the Barcelona Institute for Global Health (ISGlobal).
Jose Muñoz is Assistant Research Professor at ISGlobal and head of the Tropical Medicine and International Health Service at the Hospital Clínic.