(CNN) Angola and the Democratic Republic of Congo began an emergency vaccination campaign this week to curtail a yellow fever outbreak that has sickened thousands of people and killed more than 400, the World Health Organization said. The two countries, which together reported more than 6,136 suspected cases and 953 confirmed cases since the outbreak began in December, plan to vaccinate more than 14 million people in more than 8,000 locations.
Emergency measures using just one-fifth the standard dose will be implemented in order to reach as many people as possible. This "fractional dosing" method was recommended by an advisory group of experts as the best way to maximize the limited supply of vaccines, which require at least six months to manufacture. The WHO approved 21 million vaccine doses for Angola and 11.5 million doses for Congo.
Yellow fever is a viral hemorrhagic disease transmitted by infected mosquitoes. Symptoms include fever, headache, jaundice, muscle pain, nausea, vomiting and fatigue. A small proportion of patients develop severe symptoms, and nearly half of these die within 10 days.
In Angola, the outbreak began in late December in the capital city, Luanda, and spread to 16 of the country's 18 provinces. As of August 4, the date of the most recent report for the country, a total of 3,867 suspected cases have been recorded, though only 879 of these cases have been laboratory confirmed. Though deaths now number 369, no new cases have been confirmed during July and early August. To prevent any renewed outbreaks, the Angola vaccination program began on Monday.
In Congo, the outbreak was declared on April 23, with health officials reporting a total of 2,269 suspected cases and 74 confirmed as of August 8, the most recent reporting date for the country. Seven of the country's 26 provinces have confirmed cases, and 56 of these are known to have been imported from Angola. The health ministry scheduled the vaccination campaign to begin Wednesday, focusing first on the capital province of Kinshasa and border areas with Angola.
Two million of Kinshasa's 10 million residents have been vaccinated, according to the WHO, and more than 16 million people have been vaccinated in Congo and Angola combined.
The virus is endemic in tropical areas of 47 countries in Africa and Central and South America. Since 2006, more than 105 million people have been vaccinated in mass campaigns.
Usually, the planning stage of an emergency vaccination campaign can take between three and six months. However, even though the current campaign is one of the largest ever attempted in Africa, it must begin as soon as possible to end transmission before the rainy season in September.
The time crunch is necessary to get ahead of the peak transmission season that follows the rain, according to Dawn Wesson, associate professor of the vector-borne infectious diseases initiative at Tulane University School of Public Health and Tropical Medicine. "Basically the rainy season creates many, many, many more breeding spots, which leads to more mosquitoes and more disease transmission," she said.
According to the WHO, some areas also become inaccessible during the rainy season.
To accomplish the emergency vaccination campaign, the health ministries in Angola and the Congo and the WHO are coordinating with 56 global partners, including Médecins sans Frontières, the International Federation of the Red Cross and UNICEF. Approximately 41,000 health workers and volunteers are needed for the campaign, the WHO noted. Supplies include 500 vehicles and 17.3 million syringes.