Population dynamics of Rift Valley fever virus: effects of live and killed vaccines on epizootic outbreaks and enzootic maintenance

2 Rift Valley fever virus (RVFV) is an arthropod-borne viral pathogen that causes significant 3 morbidity and mortality among ruminants throughout Africa and the Middle East. Vaccination 4 is an important means to reduce the risk of RVFV infection in the ruminant population but has 5 proved to be challenging due to the sporadic and explosive nature of RVF outbreaks; currently, 6 two types of vaccines, live and killed, are available in endemic areas. Two modeling approaches 7 have been developed in this study to explore the impact of vaccination via live versus killed 8 vaccines on the transmission dynamics of RVFV. We demonstrate in general that vaccination 9 helps reduce the severity of RVF outbreaks and that less delay in implementation and more 10 effective vaccines can reduce the outbreak magnitude and the prevalence of RVFV. However, 11 an introduction of a number of ruminants vaccinated by live vaccines in RVFV-free areas may 12 cause an outbreak and RVFV may become endemic if there is sustained use of live vaccines. 13 Moreover, unsustained vaccination programs and the increase of susceptibility in the ruminant 14 population may lead to the recurrence of RVF outbreaks. The abundance of mosquitoes is one of 15 the important determinants of the onset of RVF outbreaks. We show that the higher the number 16 of mosquitoes, the more severe the outbreak, and in endemic areas annual RVF cases may 17 fluctuate according to seasonal abundance of mosquitoes. Furthermore, recruiting susceptible 18 ruminants during high mosquito activity may increase the risk of outbreaks and the risk is higher 19 in areas where live vaccines are used for preventive control. Our models predict that the risk of 20 outbreaks is also increased by vaccinating ruminants with live vaccines during periods of high 21 mosquito activity but vaccination is recommended if the period with high mosquito activity is 22 longer than the period with low mosquito activity. 23