Africa is in the grip of one of the worst outbreaks of yellow fever in 30 years. A wider disaster threatens
ONE of the mysteries of epidemiology is why Asia does not suffer from yellow fever. The disease is endemic in Africa, the continent where it evolved. It is widespread in South America, having been carried there by European slave ships. It was once found in both North America and southern Europe, but it was eradicated by the application of considerable effort and money—and both places shared the good fortune of lacking monkeys, which act as reservoirs for the yellow-fever virus. Yet, although much of Asia is plagued by Aedes aegypti, the mosquito that spreads yellow fever, it remains blessedly free of infections.
It may be that Asian strains of A. aegypti are poor transmitters of yellow-fever viruses (though they have no difficulty passing on those of dengue, a related illness). It may be that surviving and recovering from dengue, which is widespread in southern Asia, confers a degree of resistance to yellow fever. Or it may be that past trade between Africa and Asia was too modest for the virus to take root. The world may soon find out. For an unplanned experiment has just started that could, if it is not nipped in the bud, spread one of the world’s nastiest diseases to its most populous continent.
Eastward, ho
The centre of events is Angola. Like many other African countries, it has endemic yellow fever (reckoned to be responsible for 80,000 African deaths a year). But in December 2015 doctors there noticed their caseload was rising and realised that they had an epidemic on their hands (see article).
This epidemic has now spread to the Democratic Republic of Congo; Angola’s other neighbours are watching nervously. A distant country that has at least as much cause to worry is China. As a result of fast-growing trade between Asia and Africa, large numbers of Chinese live in Angola. They are there, though, not as emigrants but as expatriates who expect to return home. This return has, for the first time, brought 11 cases of yellow fever to China. More will surely follow.
Other Asian countries must also be alert to the threat. A. aegypti is rife in Cambodia, India, Myanmar and Thailand (see map). With more intercontinental flights carrying travellers to and fro, the risk of an outbreak in Asia has never been higher. And A. aegypti has recent form in spreading disease to new places. The Zika virus, which it also transmits, infected more than 1m people in South America in a little over a year, from what was more or less a standing start. Should yellow fever come to Asia, some experts reckon that over 100m people living in large, well-connected cities would need to be vaccinated. That would rapidly exhaust the world’s supply of vaccine, even if only a fifth of a dose (thought to be enough to confer immunity to adults) were administered to each person who needed it. In the long term, if the disease establishes itself in Asia’s jungles, over 1 billion more people could be at risk.
That would be an unconscionable outcome—doubly so because yellow-fever vaccine is cheap and confers a lifetime of immunity. Production must be stepped up. The World Health Organisation, which has been vaccinating Africans against the disease since 2007, must now accelerate its efforts to reach the remainder who are at risk of contracting it, using the reduced dose to stretch the supply if necessary. Asian countries need fumigation plans in place to eliminate the mosquito from their cities. They must also train medical staff to administer the vaccine widely in an emergency.
The world has already failed to thwart yellow fever effectively in Africa. That threatens to put millions more lives at risk. It is time to act.
This article appeared in the Leaders section of the print edition under the headline "A preventable tragedy"
