A yellow fever outbreak in Angola — which has completely depleted the world's emergency vaccine supply — has spread to neighbouring Congo, amid warnings millions could die if the disease reaches Asia.

The outbreak is the worst in Angola for 30 years, according to the World Health Organisation (WHO).

The potentially fatal mosquito-borne disease has killed at least 225 people and infected about 1,600 since the outbreak began in Angola's most populated province, Luanda, in December.

The disease has since spread to 16 of Angola's 18 provinces, with Angola one of more than 30 countries in Africa where yellow fever occurs.

The outbreak has now killed 21 people in the Democratic Republic of Congo (DRC), many from infections contracted in Angola.

The WHO said there was a serious risk of the further spread of yellow fever in the Congo given the number of people who travel regularly between the two countries.

Health officials in Uganda have also reported a yellow fever outbreak in Masaka and at least three people have died, although it is not clear if these cases are linked to the outbreak in Angola, given the distance between the two countries.

There have also been nine cases in China, two in Kenya, and others in Mauritania and Morocco, linked to the current outbreak in Angola.

Transmitted by same mosquito that spreads Zika, dengue

Yellow fever is an acute viral haemorrhagic disease, so named because of the jaundice that affects some patients.

It is transmitted by infected mosquitoes, the most common species being Aedes aegypti — the same mosquito that spreads the Zika virus, dengue fever and chikungunya virus.

Vaccination can protect against the disease but there is no cure for those already infected. Symptoms include fever, nausea and vomiting.

Most patients recover within a few days, but about 15 per cent develop a more "toxic" phase of the disease, with symptoms including bleeding and organ failure.

Yellow fever is transmitted by infected mosquitoes, the most common being Aedes aegypti. ( Center for Disease Control )

Up to half of these patients die without treatment.

A WHO campaign to stop the outbreak has now exhausted the world's emergency supply of yellow fever vaccine.

By late March nearly 6 million people in Luanda province had been vaccinated against yellow fever, using vaccines from the International Coordination Group emergency stockpile.

But the WHO said an additional 1.4 million doses were needed just to vaccinate the population at risk in Luanda province alone — Angola's total population is about 20 million.

Vaccine demand outstrips supply by 42 per cent

A UNICEF report last year listed only four manufacturers worldwide that made "WHO prequalified" yellow fever vaccine: one each in France, Brazil, Senegal and Russia.

World demand for the vaccine outstripped availability by 42 per cent, while a 2013 report estimated global production of the vaccine at 75 million doses, against demand for 105 million doses that year.

The WHO has urged non-affected countries to prioritise the vaccination of those at highest risk.

The organisation was in discussion with manufacturers and partners to divert shipments of vaccines from other countries to cope with the outbreak in Africa.

One virologist recently sounded a warning that the lack of vaccine supplies could put millions at risk.

A young girl receives a yellow fever vaccine in Monrovia in 2009. ( AFP: Glenna Gordon )

"If YF were to spread in Asia, where 2 billion are at risk in 18 dengue-affected countries, hundreds of thousands could die before YF vaccine stocks could be boosted and delivered," John Woodall, who co-founded online disease alert service Program for Monitoring Emerging Diseases, wrote on the Pro-Med website.

"Apocalyptic forecasts of the numbers of fatalities from Ebola turned out to be wildly wrong, and we can hope that will again be the case here, but given the way Zika has exploded in the Western hemisphere, we can't count on it.

"World stocks of 7 million doses have been exhausted just to protect the population of Luanda, Angola's capital.

"18 million more are needed for the rest of the country, but only half that number will be available by the end of [April 2016]; 19 million are on contracts to UNICEF and could be repurposed.

"Around 40 million more may be available by the end of the year, but China will need 300 million if it spreads across its dengue-endemic provinces."

'We may have time if we start now'

Dr Woodall said recent studies suggested the yellow fever vaccine could be stretched five or 10-fold, without loss of potency, but field trials would be needed first, which would take months.

"But we may have time if we start now. Brazil has the capacity to carry out a field trial if given the funding," Dr Woohall said.

"WHO and other international agencies such as EMA (the European Medicines Agency) could help with that."

WHO estimates since the early 1990s suggest at least 30,000-60,000 people die globally each year from yellow fever, with 90 per cent of deaths in Africa.

"Without vaccination, the burden figures would be much higher," the organisation said.