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Yellow fever in Angola and the DRC: a stark warning from an old enemy

As yellow fever spreads through Kinshasa in the Democratic Republic of the Congo, it is worth remembering how long people have been battling this disease. It is said that the capital city of the US might well have been Philadelphia if not for an epidemic of yellow fever in 1793 which drove members of the federal government, including George Washington, out of the city and towards the arguably even more pestilential climes of Washington DC.

This was not the first time that Philadelphia had experienced an epidemic of yellow fever. The high temperatures and humidity of many North American and southern European cities supported outbreaks of this mosquito-borne disease until the early 20th century, but one could rely on the winter frosts to put an end to the devastation caused by the disease. Harder to tackle was the inevitable death toll among susceptible troops and workers who were moved into an endemic area. Efforts to construct the Panama canal were compromised by mortality from yellow fever until severe measures were taken to reduce the mosquito population.

Trying to eliminate an infectious agent by eradicating its carrier is a tall order unless the insect population is actually only hanging on by its fingertips within an essentially hostile or variable climate, or if there is a reasonable infrastructure to, for example, keep marshes continuously drained. Sadly yellow fever can happily exist without a human host, completing its life-cycle in monkeys in tropical forests.

A much simpler solution, therefore, is to protect the human population by conferring immunity through vaccination. Happily a vaccine was developed by Max Theiler and colleagues in the 1930s.

It was probably the first “exotic” vaccine I received, at the age of 18 months, when I was to set sail for Ethiopia with my parents in 1966. My mother tells me that after I had received the shot, I offered my other arm; this was hardly the case 10 years later when I recall running as fast as I could away from the surgery to avoid vaccination.

In those days, we were obliged to submit ourselves to re-vaccination every 10 years if we wished to preserve the right to enter certain endemic areas. The current edict is that protection offered by a single dose is actually lifelong: a very good thing indeed.

Why is yellow fever then still a problem? In particular, why has there been such a serious resurgence in Angola and the DRC, stretching vaccine supplies to the point that consideration is being given to reducing the dose in order to cover more people – or even to offering doses of lower efficacy to larger numbers of individuals in an effort to halt transmission through an overall increase in protection?

The usual suspects – climate change, deforestation – are partly to blame, but somewhat paradoxically so is the lack of yellow fever in these areas, due to previous intervention efforts. This has led to large numbers of non-immune adults, who never had the chance to build up any natural resistance to the disease, and are therefore ripe to fuel a new epidemic.

The worst situation that we can create is a sort of halfway house where the risk of infection is reduced so that many people remain unexposed to yellow fever, and yet the likelihood of recreating the conditions for the disease to prevail remains high. That this can happen despite the best efforts of the World Health Organisation and other partners in the otherwise highly successful yellow fever initiative highlights how formidable a task it can be to deliver simple, effective healthcare in particular economic settings.

The yellow fever vaccine has now been introduced in routine infant immunisation programmes in the majority of regions at risk in Africa and the Americas, and emergency campaigns in response to epidemic threats have been largely successful. But pockets of vulnerability remain, and it appears that the current stockpile is not sufficient to deal with possible outbreaks occurring in these areas.

One can only hope that the issues raised – namely what is actually the minimum effective dose, how best to administer possibly partially efficacious doses to reduce the burden of disease, and how to improve vaccine yield – will better prepare us in our ongoing battle with this very old enemy.

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