Yellow fever: Status of research, development, and access to countermeasures against an emerging epidemic
Just one case of yellow fever in an unvaccinated population is considered an outbreak, according to the World Health Organization (WHO). And now, an outbreak in Angola has spread to Kinshasa, a city of 12 million in the Democratic Republic of Congo. Health experts are calling it the worst yellow fever outbreak in Africa in decades.
The outbreak in Angola has resulted in nearly 350 deaths, and unrelated outbreaks are ongoing in Uganda and Peru. Meanwhile, the WHO’s emergency stockpile of the yellow fever vaccine—the only available countermeasure against the virus—has been depleted twice this year, and is currently running low at only 6 million doses, raising concerns about global ability to respond to growing outbreaks.
Background: The illness and impact of a historic virus
Yellow fever is a mosquito-borne disease—spread by both the Aedes and Haemogogus species—found in both urban and rural settings. The disease is endemic in forty-seven countries across Africa and Latin America, and each year results in anywhere from 84,000 to 170,000 severe cases and 29,000 to 60,000 deaths.
Many cases of yellow fever are asymptomatic, and when they manifest, initial symptoms are nonspecific: fever, fatigue, aches and pains, nausea, and vomiting. Fifteen percent of cases progress to a more severe illness characterized by high fevers, liver and kidney damage, jaundice—thus the name yellow fever—and even hemorrhaging. Half of those who enter this toxic stage of infection die.
While the yellow fever virus has plagued the world since the 17th Century, it was first isolated in 1900 by American pathologist Walter Reed, earning it the distinction of being the first human virus ever to be discovered. A safe and effective vaccine was first introduced in 1936; however, in the eight decades since, few new tools against the virus have been developed. Below, we break down the status of research and development (R&D) for vaccines, drugs, and diagnostics for yellow fever.
Status of R&D and access to yellow fever countermeasures
one dose provides lifetime protection from the virus for 99 percent of those vaccinated. However, the vaccine contains live virus, and in extremely rare cases, can result in illness.The yellow fever vaccine is effective, safe, and affordable. Just
Since 2000, the price of the vaccine has risen 30 percent annually. However, this increase has not incentivized manufacturers to scale up production. There are only four manufacturers of the vaccine, and it’s notoriously difficult to produce. Consequently, the global demand for the vaccine surpasses availability by 42 percent.
Yellow fever vaccine production relies on eggs from pathogen-free chickens flocks, which are hard to come by. The embryos are infected with the yellow fever virus, and after three to four days, the embryos are harvested under sterile conditions. Each egg can provide anywhere from 100 to 300 doses of the vaccine. The entire production process takes about one year.
A vaccine candidate with an inactivated version of the virus completed successful phase 1 clinical trials in 2011. However, since GE Healthcare acquired the intellectual property for the vaccine in 2012, no additional clinical trials have been undertaken. Fortunately, last month PnuVax Incorporated entered into an agreement with GE Healthcare, under which PnuVax will advance the vaccine through clinical trials and will partner with GE Healthcare for manufacturing.
In the meantime, the WHO is recommending that the vaccine be administered in fractional doses, using just one-fifth of the standard amount. Studies suggest that this approach still confers at least 12 months of protection from the virus, however, booster shots would be required to sustain immunity when a global shortage is no longer imminent. Additionally, fractional dosing has not been tested in children, raising concerns that this approach might not be sufficient for kids, whose immune responses, including to vaccines, are typically weaker.
There are no specific therapies for yellow fever. Current treatment involves supportive care (i.e., addressing the symptoms). Animal studies suggest that the widely-used antiviral ribavirin and an active carboxamide drug reduce mortality in hamsters, and the molecule interferon-α, used in some cancer therapies, effectively treats the disease in monkeys. However, these treatments were only effective when administered within a few days of infection. Thus, even if they were to prove effective in humans, their application would be limited. The yellow fever virus incubates for three to four days in the body before symptoms appear, and as aforementioned, initial symptoms are nonspecific and easily confused with anything from malaria to dengue fever or leptospirosis to poisoning.
As yellow fever is hard to distinguish from other illnesses based on initial symptoms alone, the development of rapid, point-of-care diagnostics is critically needed. Currently, cases can only be confirmed in laboratory settings. Diagnosis requires examining blood or serum for either the virus itself or antibodies against the virus.
However, researchers at the Massachusetts Institutes of Technology are currently developing a paper-based diagnostic that simultaneously tests for Ebola, dengue fever, and yellow fever. The test is designed for use in low-resource settings, requiring no electricity or specialized lab equipment and the results are easy to interpret. Testing requires a small amount of blood, and colored nanoparticles appear—red for Ebola, orange for yellow fever, and green for dengue—based on the proteins in the blood. While this new tool is showing promise, it could be some years before testing is completed and regulatory approval is secured.
With a limited pipeline of countermeasures against yellow fever, the global community is scrambling to ramp up vaccine production, expand vaccination coverage, and scale up vector control efforts. However, R&D for rapid, point-of care diagnostics; lifesaving treatments; and a safe, affordable, easy-to-produce vaccine is critical to combat this virus after a centuries-long battle.