Global stockpiles of the yellow fever vaccine are diminishing as fears mount over a global epidemic.

Currently, people are being immunized through Kinshasa in the Democratic Republic of the Congo in a bid to prevent the disease spreading – however the vaccine contains a fifth of the normal dose because the global stockpile is so low.

Yellow fever is transmitted through the bite of the Aedes aegypti mosquito. It is an acute viral haemorrhagic disease transmitted by infected mosquitoes. The "yellow" in the name refers to the jaundice that affects some patients.

Symptoms of yellow fever include fever, headache, jaundice, muscle pain, nausea, vomiting and fatigue.

A small proportion of patients who contract the virus develop severe symptoms and approximately half of those die within 7 to 10 days.

However few adults had been immunised in Angola when yellow fever broke out there in December last year, and in the DRC, to where it has spread.

The yellow fever outbreak has found its way to dense, urban areas and hard-to-reach border regions, making planning for the vaccination campaign especially complex, WHO said.

Emergency yellow fever vaccination campaigns have already reached more than 13 million people in Angola and more than 3 million in Democratic Republic of the Congo.

Kinshasa has more than 10 million people, with only 2 million already vaccinated against yellow fever.

With local transmission of the virus and low immunity in the population, there is a potential risk that the deadly outbreak could spread to other urban areas.

The WHO is also faced with limited supplies of the vaccine, and a 6-month minimum manufacturing process.

It said it is planning the mass vaccination campaign that uses one-fifth of the standard vaccine dose as a short-term emergency measure to reach as many people as possible.

″Protecting as many people as possible is at the heart of this strategy. With a limited supply we need to use these vaccines very carefully,″ says William Perea, Coordinator for the Control of Epidemic Diseases Unit at WHO.

There are fears that if it takes hold in Kinshasa, a densely packed city of more than 10 million people, the disease will travel beyond the central African region,.

The UK Guardian reports that in Angola, there have been nearly 4,000 suspected cases of yellow fever of which 879 have been confirmed, the DRC has had more than 2,200 suspected cases, 68 confirmed.

There have been about 400 reported deaths in the two countries, mostly in Angola.

To combat the spread, almost 19m doses of vaccine have been administered since January, but there are only 5m left in the emergency stockpile.

The emergency vaccination campaign beginning on Wednesday was one of the largest ever attempted, WHO said.

Since the launch of the Yellow Fever Initiative in 2006, significant progress in combating the disease has been made in West Africa and more than 105 million people have been vaccinated in mass campaigns. No outbreaks of yellow fever were reported in West Africa during 2015, the WHO reports.

In June, the Ghana Health Service said it was stepping up efforts to combat yellow fever which had resurfaced in some parts of the country.

Although the cases were relatively few, the service has said it is going to increase vaccinations against the disease to prevent a flare up.