A yellow fever epidemic has hit central Africa. Is Asia next?

 作者:暴皓     |      日期:2019-03-06 11:19:05
Joost de Raeymaeker/EPA By Debora MacKenzie It’s getting scary. On Monday a yellow fever epidemic was declared in Kinshasa, the sprawling capital of the Democratic Republic of the Congo. The disease spread there from Angola, where it has been circulating since an outbreak began in December. There have been over 4000 known or suspected cases in the two countries, and the virus is thought to kill in some 5 per cent of cases. In an unprecedented move, last week the World Health Organization announced that experimental evidence shows the vaccine can be diluted fivefold to make stocks stretch further, in a bid to control the outbreak in Africa. Before this outbreak, Angola was not considered at risk so few people there had been vaccinated. Since February in Angola, and since March in Congo, 18 million doses of vaccine have been given to try and stop the virus spreading. There is not enough vaccine left to contain a major epidemic in Kinshasa without eating into stocks meant for child vaccinations, says Seth Berkley, head of GAVI, the international vaccine agency. It should never have come to this. Control of the mosquitoes that carry yellow fever drove the virus out of countries such as the US, where it used to cause deadly epidemics in places like Philadelphia. The vaccine for yellow fever, introduced in 1936, is cheap, safe and possibly the world’s most effective: one dose protects a person for life. By the 1970s mosquito control and vaccination had cut yellow fever to very low levels across tropical and subtropical Africa and South America. But “success caused failure”, says Duane Gubler of Duke-NUS Medical School in Singapore. As the disease disappeared countries stopped vaccinating, and financial crises plus rocketing urban populations hampered mosquito control. The virus cannot be eradicated from its tropical strongholds, as it normally lives harmlessly in forest-dwelling monkeys and the mosquitoes that bite them. But when a person infected by a forest mosquito travels to town, urban Aedes mosquitoes can pick up and spread the virus, causing an explosive outbreak when densities of susceptible people and mosquitoes are high. Aedes mosquitoes are the very ones that spread dengue, chikungunya and most recently Zika virus. “The rapid international spread of those viruses shows why we are worried about yellow fever,” says Gubler. An increase in urban outbreaks of yellow fever since the 1980s led to resumed vaccination of children in 2005 in West African countries most at risk, and a stockpile of 6 million doses of vaccine managed by the WHO. But “marketing considerations did not make it attractive for the manufacturers to make more than the usual stockpile”, says Jack Woodall of the Federal University of Rio de Janeiro, Brazil. That is why the world is out of vaccine. The stockpile was used up, then refilled and used up twice again, with vaccine procured from supply chains worldwide to battle the epidemic in Africa. More vaccine cannot be made quickly as it is grown in germ-free chicken eggs, which are scarce. Meanwhile, some 1.8 billion people in Asia live with Aedes mosquitoes, so would be vulnerable if yellow fever, carried by migrant workers in Africa for instance, took hold there. China has already detected the virus in 11 workers returning from Angola, fortunately to homes north of the country’s Aedes belt. Other countries may not detect the virus until it has spread. “Delhi has so much hepatitis that a fever with jaundice would never be suspected to be yellow fever,” says Woodall, and India’s diagnostic labs are not equipped to pick it up. Gubler hopes Asia’s ongoing dengue epidemic might help. Yellow fever is a related virus, and animal experiments in the 1970s suggested dengue antibodies might partially protect people from yellow fever, and perhaps slow its spread. Woodall, however, fears yellow fever could behave like many viral diseases when they invade new territory, with even higher death rates than in its native range. Right now, both are hoping we don’t have to find out. “Some of us have been warning for years that the question wasn’t whether this could happen, but when,” says Gubler. Now the world is facing an unknown, potentially severe risk from a disease we had under control in the 1960s. More on these topics: