The world's worst yellow fever outbreak in decades took hold in an Angolan slum because its early victims were Eritrean migrants whose false vaccination papers sent doctors off on the wrong path for weeks, international health officials say.

Key points: First deaths baffled doctors over "mystery disease"

First deaths baffled doctors over "mystery disease" Time-consuming vaccine production and increasing demand raising fears of global shortage

Time-consuming vaccine production and increasing demand raising fears of global shortage Health officials may be forced to stretch stockpile by giving one-fifth doses

The flare-up of the mosquito-borne disease has killed 325 people in Angola, and spread as far as China — which has close commercial links with oil-rich Angola — raising fears of the world running out of vaccine. But it might have been stopped in its tracks if it had been identified quickly in Luanda.

Luanda World Health Organisation (WHO) representative Hernando Agudelo said he and government experts thought they were dealing with a mystery disease when unexplained deaths first surfaced in mid-December.

"The first people that we found with this strange way of dying, this syndrome, they had vaccination cards," Mr Agudelo said.

"We were analysing 'What the hell is it?'"

Yellow fever is transmitted by the same mosquitoes that spread the Zika and dengue viruses, although it is much more serious, with death rates as high as 75 per cent in severe cases, requiring admission to hospital.

The condition takes its name from the jaundiced colour of some patients.

With the devastating West African Ebola outbreak in the back of their minds, Angolan and WHO investigators quickly nailed down a restaurant in a Luanda market district as the link between the victims.

"We started to look at what they had in common and they were from Eritrea," Mr Agudelo said.

"All of them had passed through the same restaurant."

When the restaurant owner also died, samples were sent to South Africa's National Institute of Communicable Diseases, where technicians who had been told about the vaccination papers only ran a yellow fever test "by chance", Mr Agudelo said.

Angola is Africa's biggest oil producer and attracts a sizeable share of Eritreans fleeing the rigidly-controlled Horn of Africa country.

However, Angola denies entry to anybody without vaccination documents.

Since the outbreak was identified in January, 10.5 million Angolans — 40 per cent of the population — have been vaccinated and the WHO plans to cover the rest of the country by the end of the year.

But with the disease jumping via a mosquito from one person to another in Kinshasa, a city of over 12 million in neighbouring Democratic Republic of Congo, there are concerns about global vaccine supplies running out.

There is no quick way to boost production.

Manufacturers, including the Institut Pasteur, government factories in Brazil and Russia, and French drugmaker Sanofi, use a time-consuming method involving sterile chicken eggs.

Should the disease take hold in Kinshasa, one of Africa's biggest cities, health officials may have little option but to stretch out the global stockpile of 6 million shots by administering one-fifth doses.

Reuters