Abstract:
Background
The impacts of vaccination on the transmission of Rift Valley fever virus (RVFV) have not
been evaluated. We have developed a RVFV transmission model comprising two hostsÐ
cattle as a separate host and sheep and goats as one combined host (herein after referred
to as sheep)Ðand two vectorsÐAedes species (spp) and Culex sppÐand used it to predict
the impacts of: (1) reactive vaccination implemented at various levels of coverage at
pre-determined time points, (2) targeted vaccination involving either of the two host species,
and (3) a periodic vaccination implemented biannually or annually before an
outbreak.
Methodology/Principal Findings
The model comprises coupled vector and host modules where the dynamics of vectors
and hosts are described using a system of difference equations. Vector populations are
structured into egg, larva, pupa and adult stages and the latter stage is further categorized
into three infection categories: susceptible, exposed and infectious mosquitoes. The survival
rates of the immature stages (egg, larva and pupa) are dependent on rainfall densities
extracted from the Tropical Rainfall Measuring Mission (TRMM) for a Rift Valley fever
(RVF) endemic site in Kenya over a period of 1827 days. The host populations are structured
into four age classes comprising young, weaners, yearlings and adults and four
infection categories including susceptible, exposed, infectious, and immune categories.
The model reproduces the 2006/2007 RVF outbreak reported in empirical surveys in the
target area and other seasonal transmission events that are perceived to occur during the
wet seasons. Mass reactive vaccination strategies greatly reduce the potential for a major
outbreak. The results also suggest that the effectiveness of vaccination can be enhanced
by increasing the vaccination coverage, targeting vaccination on cattle given that this species
plays a major role in the transmission of the virus, and using both periodic and reactive
vaccination strategies.
