The worst yellow fever outbreak Angola has experienced in 30 years is spreading through the capital city and other parts of the country, leaving nearly 300 people dead so far. The virus has also made its way to the Democratic Republic of Congo, Kenya, and China. The UN's World Health Organization decided this week that it is not yet a "public health emergency of international concern," but the UN agency stressed that the outbreak poses both national and international risks.

The WHO held an emergency committee meeting on Thursday afternoon at its headquarters in Geneva to discuss the ongoing outbreak and debate whether it should be deemed a "public health emergency of international concern" or PHEIC, a designation given to the current spread of the Zika virus and the most recent Ebola outbreak in West Africa.

There have been more than 2,400 suspected cases of the virus in the last five months in Angola, with more than 700 of those confirmed and a total of 298 deaths, as new cases continue to appear. At a press conference after the emergency meeting, the UN health agency highlighted the national and international risks the outbreak presents as it spreads through urban areas like Luanda in Angola and Kinshasa in the DRC. Despite the risk, however, experts concluded that the criteria for a PHEIC declaration had not been met.

"Urban yellow fever is a particularly dangerous and concerning situation because of both the potential for explosive spread in urban settings with high mortality and also the risk for international spread," Bruce Aylward, the WHO's executive director of outbreaks and health emergencies, said during the press conference announcing the decision.

The first case of yellow fever turned up in December in a municipality in Angola's Luanda province, the country's most populous province and home to the capital city of the same name. The outbreak has largely been centered in Luanda and three other provinces, with cases continuing to show up despite a nationwide vaccination program launched in February.

Even without designating the outbreak as a public health emergency, a move that would have urgently ramped up WHO's response to the situation, this was a positive sign that the agency was watching the outbreak, said University of Reading virologist Benjamin Neuman.

"It is encouraging that the WHO is monitoring the situation closely, and if local authorities are not able to get the infection under control in the next month or so, I would anticipate that the WHO will revisit this decision," he said.

Initially, Neuman said he did not think the outbreak would rise to the level of a major threat. But the latest figures and the rate of increase in the number of cases have started to changed his opinion. Still, he explained that a PHEIC designation requires an outbreak to be extraordinary, to present a risk of spreading widely, and for it to be a scenario that would benefit from WHO coordination.

"We are talking about an area that has never been free from yellow fever in human history," he said, explaining that outbreaks in the endemic area should be expected from time to time. "The fact that the outbreak has not spread to a country that did not have yellow fever virus before this year is evidence that this is not yet an international health concern."

The spread of the virus has still caught many Angolans by surprise. Lucia Kula, an Angolan doctoral researcher at the University of London currently working on a project in Luanda, has lost three family members in the capital city to yellow fever, including a toddler. Her teenage neighbor also died. From February to early April, she explained, there were funerals every other week in the neighborhood where she lives.

"It's an issue that is topic of conversation almost everywhere. It's concerning and very debilitating to the moral of the everyday Angolan," Kula said. "I think initially no one expected it to become such a big issue of concern."

Yellow fever is classified as a hemorrhagic fever, but it falls in a different family from the Ebola virus, another member of the hemorrhagic group of illnesses that spread through West Africa over the last two years. The virus is carried by the Aedes aegypti mosquito — also known to spread Dengue fever, Zika virus, and West Nile — and is transmitted from insect to human, with no risk of human to human transmission.

Endemic in central Africa and South America, yellow fever infects more than 80,000 people globally each year. While Aedes aegypti mosquitoes can be found all over the world, they do not always carry the virus.

The mosquitoes are commonly found living among humans, often thriving in trash and waste. An outbreak of yellow fever is connected with the number of mosquitoes, and as experts had predicted, the increase in rainy weather in Central Africa as a result of this year's El Niño contributed to a rise in mosquito populations and viruses like yellow fever and dengue.

In Angola, specifically, dropping oil prices have forced the government to divert money away from public services. Cuts to sanitation services caused a massive trash buildup in Luanda just in time for the rainy season, making it the perfect environment for mosquitoes carrying yellow fever to thrive.

"What makes it different and worse than we have seen in Angola and other countries, [is that] all the worst scenarios have taken place," she said, explaining all of the risk factors for an outbreak were present to spark a very quick expansion of the outbreak, "which is what happened."

While there is no medical treatment for yellow fever beyond basic medical care, the virus is unique compared to many other tropical diseases in that there is a highly effective vaccine on the market that can prevent infection, according to Neuman.

"We have the vaccine that's almost 100 percent effective, it's one of the best vaccines that's ever been made," he said, comparing it to the flu vaccine, which only has about a 40 percent efficacy rate.

Beyond just the vaccination itself, Neuman explained that we also have the ability to tackle the mosquitos as well.

"The real tragedy of a yellow fever outbreak like this is that with the right infrastructure, it is totally preventable," he said. "We have all the tools we need to control mosquito populations with insecticides and block infection with an excellent vaccine."

"A major outbreak like this is a (sure) sign that local health infrastructure may be strained to the point of fracturing, and the WHO was set up to deal with exactly this sort of situation," he said.

Confidence in the vaccine may have contributed to the initial expectations that the outbreak would come to a halt. When the vaccination program kicked off in February, Angola's ministry of health believed it had the capacity to handle the situation, according to Doctors Without Borders emergency unit deputy manager Llanos Ortiz.

Ortiz said that they may have been a bit too optimistic initially. According to their figures, the inoculation campaign achieved 95 percent vaccination coverage for the 7 million people in Luanda province. Despite these efforts, new cases have continued to appear.

"[Considering] it's still [seeing] a high number of daily cases, shows that somehow still a lot of people are vulnerable, exposed, and not protected, which is quite surprising," Ortiz said. "By now we should have seen a bigger drop."

The outbreak still remains at a manageable point, she said, and had not yet reached the point of being out of control. The fact that none of the cases exported to China managed to spark local transmission is also positive, Ortiz said.

Going forward, WHO called for action in various areas of the response efforts, specifically, pushing for the acceleration of surveillance, mass vaccination, risk communications, community mobilization, mosquito control and case management measures. While mass inoculation is highlighted as a key control measure, the unexpected outbreak has exacerbated a looming worldwide shortage of the yellow fever vaccine. Each year approximately 80 million vaccines are produced, and this year alone 6 million doses were taken from global stockpiles to administer in Angola.

For Kula, who has just recently relocated from the UK to Angola for a research project, one of the biggest frustrations so far during the outbreak has been the quality of healthcare available for Angolans. The spread of yellow fever coincided with the rainy season which also brought about the usual uptick in diseases like malaria. At the same time, the country's medical system was under pressure like other public services. Hospitals were running out of basic supplies, and earlier this year began taking private donations of medicine and equipment.

"During the peak of the outbreak, people were dying left and right because they couldn't get access to healthcare," she said. "Hospitals were more than full, no beds, lacking sanitation, understaffed, it all contributed to the epidemic. Mortuaries were beyond capacity and bodies were left to rot."