Live vaccines require a single injection but are known to cause birth defects and abortions in sheep and induce only low-level protection in cattle. The live-attenuated vaccine, MP-12, has demonstrated promising results in laboratory trials in domesticated animals, but more research is needed before the vaccine can be used in the field. The live-attenuated clone 13 vaccine was recently registered and used in South Africa. Alternative vaccines using molecular recombinant constructs are in development and show promising results. A vaccine has been conditionally approved for use in animals in the US. It has been shown that knockout of the NSs and NSm nonstructural proteins of this virus produces an effective vaccine in sheep as well. Epidemiology RVF outbreaks occur across sub-Saharan Africa, with outbreaks occurring elsewhere infrequently. Outbreaks of this disease usually correspond with the warm phases of the EI Niño/Southern Oscillation. During this time there is an increase in rainfall, flooding and greenness of vegetation index, which leads to an increase in mosquito vectors. RVFV can be transmitted vertically in mosquitos, meaning that the virus can be passed from the mother to her offspring. During dry conditions, the virus can remain viable for a number of years in the egg. Mosquitos lay their eggs in water, where they eventually hatch. As water is essential for mosquito eggs to hatch, rainfall and flooding cause an increase in the mosquito population and an increased potential for the virus. The first documented outbreak was identified in Kenya in 1931, in sheep, cattle and humans; another severe outbreak in the country in 1950–1951 involved 100,000 deaths in livestock and an unrecorded number of humans with fever. An outbreak occurred in South Africa in 1974–1976, with more than 500,000 infected animals and the first deaths in humans. In Egypt in 1977–78, an estimated 200,000 people were infected and there were at least 594 deaths. In Kenya in 1998, the virus killed more than 400 people. Since then, there have been outbreaks in Saudi Arabia and Yemen (2000), East Africa (2006–2007), Sudan (2007), South Africa (2010), Uganda (2016), Kenya (2018), and Mayotte (2018–2019). 2020–2021 in Kenya, in 2022 an outbreak is ongoing in Burundi. Biological weapon Rift Valley fever was one of more than a dozen agents that the United States researched as potential biological weapons before the nation suspended its biological weapons program in 1969. Research The disease is one of several identified by