Yellow Fever now risks Spreading to Asia where it has never before taken hold
In about 15% of cases, however, the disease
causes severe abdominal pain, jaundice
and the infected bleed internally and from their eyes, mouth and nose.
About half of these people die.
IT OCCUPIES a strange place on the spectrum of infectious tropical
diseases. Not as important as malaria. Not as terrifying as Ebola. Not as
revolting as elephantiasis. Yet yellow fever is a grave illness, incurable once
contracted. It kills 80,000 Africans a year. And that is a scandal, both
because it can be prevented by a single inoculation and also because yellow
fever now risks spreading to Asia, where it has never before taken hold.
This is the background to the latest epidemic of the disease, in Angola.
Since December, around 2,300 suspected cases have been reported there, with
nearly 300 deaths. Set against 80,000 deaths, this may not sound like many. But
experience suggests that, for each case brought to the authorities’ attention
in a country where health care is as fragmentary as it is in Angola, between 50
and 500 probably go unreported.
Yellow fever is spread by Aedes
aegypti, the mosquito that also carries dengue and Zika. Its early
symptoms—a high temperature, nausea, vomiting and muscle pain—are reasonably
mild and usually last only a few days. In about 15% of cases, however, the
disease later returns with a vengeance. Patients experience severe abdominal
pain, become jaundiced and bleed internally and from their eyes, mouth and
nose. About half of these people die.
The UN and the World Health Organisation (WHO) have shipped 9m doses of
vaccine to Angola, enough for about a third of the population. But that is
around a fifth of all the vaccine held worldwide at any one time. If the
epidemic spreads, stocks will rapidly run out.
And spread it might. Almost 6m people in Luanda, Angola’s capital, should
now be immune, and the number of Angolan cases being reported to the WHO has
indeed dipped in recent weeks. Yet vaccination rates outside Luanda remain low,
and the efforts have not stopped the disease from crossing borders.
Laboratory analyses have linked a few cases in Kenya to the Angolan
outbreak. More worrying is the Democratic Republic of Congo (DRC). On May 2nd
the WHO reported 453 suspected cases of the disease there, including some in
the capital, Kinshasa. Less than 30% of the country’s population was thought to
have been vaccinated before today’s outbreak. A booming trade in forged
vaccination certificates could also let infected people slip past border
checkpoints from Angola into Zambia and Namibia, which reported its first case
on April 28th.
The best way to contain the disease now is to vaccinate all those at risk
as soon as possible. Every day increases the chance that one of the thousands
of Asian workers in Angola will carry the disease home, sparking a full-scale
outbreak on a continent that has yet to experience one.
Deployment of the vaccine in all African countries where yellow fever is
endemic could slash the number of cases. The Yellow Fever Initiative, which is
led by the WHO and UNICEF and funded by GAVI, an international public-private
alliance that provides vaccines to poor countries, aims to cover the continent
by 2020, at a cost of $300m. More than 100m people have been vaccinated since
it started in 2007. With more funding, it might have averted this outbreak:
Angola was not among the 12 countries that were considered most susceptible to
Production of yellow-fever vaccine has increased in the past five years,
but it would be difficult to raise further. It has only four sources: Sanofi
Pasteur, a French drug company, and institutes in Brazil, Senegal and Russia.
“That leaves us in a very vulnerable position,” says Peter Piot, the director
of the London School of Hygiene and Tropical Medicine. If yellow fever did take
hold in Asia, he says, then the numbers at immediate risk would rise from tens
of millions to 100m or more.
The world’s emergency stockpile of 11m doses, which is held on top of
normal supply to enable a rapid response to outbreaks, is already being
depleted to control the one in Africa. If the disease takes hold in Asia, says
William Perea of the WHO, there would be little choice but to limit
inoculations to a fifth of a standard dose so as to make supplies of the
vaccine stretch further. Small studies give reason to hope that this would
protect adults, but the efficacy of a low dose for children is unknown. The
International trade and migration mean that the chances of yellow fever
spreading to Asia are higher than ever before, warns John Woodall of the
Programme for Monitoring Emerging Diseases, an online-alert service. Cool
weather has meant that up till now there have been few mosquitoes in China to
spread the disease. Even so, the country has already reported its first 11
cases, and summer is approaching. All those diagnosed had returned from Angola,
home to an estimated 100,000 Chinese workers.
Once yellow fever is established in a tropical country, it is almost
impossible to eradicate. Monkeys infected by the virus act as a reservoir for
the disease. People who travel to the jungle can carry it back to towns and
cities, where mosquitoes quickly breed— A. aegypti lays its eggs in
standing water, meaning that even a discarded food tin could be a breeding
Why Asia has never had a large outbreak of yellow fever is something of a
mystery. A. aegypti is found across much of southern Asia (see map),
and the continent’s jungles have monkeys that would seem an ideal reservoir for
the disease. One possibility is that antibodies against dengue, a related
disease, partially protect survivors against yellow fever. A second is that the
Asian type of A. aegypti may be less able to carry the virus than its
African cousin. But it is not immune. The fear is that a traveller who has returned
from Africa with yellow fever will be bitten by an indigenous mosquito, which
then spreads the disease.
America, which has not had an outbreak in more than a century, is at risk,
too. Yellow fever used to be common there: Philadelphia suffered one of the
country’s worst outbreaks in 1793, when the disease killed 5,000 people, then
about a tenth of the city’s population. In New Orleans in 1853, 9,000 died. The
port cities of Europe also suffered outbreaks: one in Barcelona in 1821 killed
But by the middle of the 20th century yellow fever was gone from the
northern hemisphere, as fumigation was used to beat the mosquito back. In Cuba
the same remedy, and more effective sanitation, also removed the source of many
of America’s epidemics. Vaccination campaigns in France’s west African colonies
between 1933 and 1961 caused yellow fever virtually to disappear from the
continent—until decolonisation, when vaccination rates plummeted and the
In South American cities yellow fever was once kept at bay by
mosquito-control measures. But international arrivals add to the threat from
travellers who have visited remote jungle areas, in some of which the disease
is endemic. For many places now free of yellow fever, a few infected visitors
at the height of summer, and some bad luck, could mean its unwelcome return. The Economist
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