yellow fever vaccine in 1937, for which he was subsequently awarded the nobel prize in physiology or medicine. that vaccine, 17d, is still in use, although newer vaccines, based on vero cells, are in development (as of 2018). current status using vector control and strict vaccination programs, the urban cycle of yellow fever was nearly eradicated from south america. since 1943, only a single urban outbreak in santa cruz de la sierra, bolivia, has occurred. since the 1980s, however, the number of yellow fever cases has been increasing again, and a. aegypti has returned to the urban centers of south america. this is partly due to limitations on available insecticides, as well as habitat dislocations caused by climate change. it is also because the vector control program was abandoned. although no new urban cycle has yet been established, scientists believe this could happen again at any point. an outbreak in paraguay in 2008 was thought to be urban in nature, but this ultimately proved not to be the case. in africa, virus eradication programs have mostly relied upon vaccination. these programs have largely been unsuccessful because they were unable to break the sylvatic cycle involving wild primates. with few countries establishing regular vaccination programs, measures to fight yellow fever have been neglected, making the future spread of the virus more likely. research in the hamster model of yellow fever, early administration of the antiviral ribavirin is an effective treatment of many pathological features of the disease. ribavirin treatment during the first five days after virus infection improved survival rates, reduced tissue damage in the liver and spleen, prevented hepatocellular steatosis, and normalised levels of alanine aminotransferase, a liver damage marker. the mechanism of action of ribavirin in reducing liver pathology in yellow fever virus infection may be similar to its activity in treatment of hepatitis c, a related virus. because ribavirin had failed to improve survival in a virulent rhesus model of yellow fever infection, it had been previously discounted as a possible therapy. infection was reduced in mosquitoes with the wmel strain of wolbachia. yellow fever has been researched by several countries as a potential biological weapon. notes also required for travellers having transited (more than 12 hours) through a risk country's airport. not required for travellers having transited through a risk country's airport. the who has designated (parts of) argentina, brazil and peru as risk countries, but these countries do not require incoming travellers to vaccinate against yellow fever.