Theoretical risk of genetic reassortment should not impede development of live, attenuated Rift Valley fever (RVF) vaccines commentary on the draft WHO RVF Target Product Profile

Genomic Epidemiology of Rift Valley Fever Virus Involved in the 2018 and 2022 Outbreaks in Livestock in Rwanda

Rift Valley fever (RVF), a mosquito-borne transboundary zoonosis, was first confirmed in Rwanda's livestock in 2012 and since then sporadic cases have been reported almost every year. In 2018, the country experienced its first large outbreak, which was followed by a second one in 2022. To determine the circulating virus lineages and their ancestral origin, two genome sequences from the 2018 outbreak, and thirty-six, forty-one, and thirty-eight sequences of small (S), medium (M), and large (L) genome segments, respectively, from the 2022 outbreak were generated. All of the samples from the 2022 outbreak were collected from slaughterhouses. Both maximum likelihood and Bayesian-based phylogenetic analyses were performed. The findings showed that RVF viruses belonging to a single lineage, C, were circulating during the two outbreaks, and shared a recent common ancestor with RVF viruses isolated in Uganda between 2016 and 2019, and were also linked to the 2006/2007 largest East Africa RVF outbreak reported in Kenya, Tanzania, and Somalia. Alongside the wild-type viruses, genetic evidence of the RVFV Clone 13 vaccine strain was found in slaughterhouse animals, demonstrating a possible occupational risk of exposure with unknown outcome for people working in meat-related industry. These results provide additional evidence of the ongoing wide spread of RVFV lineage C in Africa and emphasize the need for an effective national and international One Health-based collaborative approach in responding to RVF emergencies.

Rift Valley Fever Phlebovirus Reassortment Study in Sheep

Rift Valley fever (RVF) in ungulates and humans is caused by a mosquito-borne RVF phlebovirus (RVFV). Live attenuated vaccines are used in livestock (sheep and cattle) to control RVF in endemic regions during outbreaks. The ability of two or more different RVFV strains to reassort when co-infecting a host cell is a significant veterinary and public health concern due to the potential emergence of newly reassorted viruses, since reassortment of RVFVs has been documented in nature and in experimental infection studies. Due to the very limited information regarding the frequency and dynamics of RVFV reassortment, we evaluated the efficiency of RVFV reassortment in sheep, a natural host for this zoonotic pathogen. Co-infection experiments were performed, first in vitro in sheep-derived cells, and subsequently in vivo in sheep. Two RVFV co-infection groups were evaluated: group I consisted of co-infection with two wild-type (WT) RVFV strains, Kenya 128B-15 (Ken06) and Saudi Arabia SA01-1322 (SA01), while group II consisted of co-infection with the live attenuated virus (LAV) vaccine strain MP-12 and a WT strain, Ken06. In the in vitro experiments, the virus supernatants were collected 24 h post-infection. In the in vivo experiments, clinical signs were monitored, and blood and tissues were collected at various time points up to nine days post-challenge for analyses. Cell culture supernatants and samples from sheep were processed, and plaque-isolated viruses were genotyped to determine reassortment frequency. Our results show that RVFV reassortment is more efficient in co-infected sheep-derived cells compared to co-infected sheep. In vitro, the reassortment frequencies reached 37.9% for the group I co-infected cells and 25.4% for the group II co-infected cells. In contrast, we detected just 1.7% reassortant viruses from group I sheep co-infected with the two WT strains, while no reassortants were detected from group II sheep co-infected with the WT and LAV strains. The results indicate that RVFV reassortment occurs at a lower frequency in vivo in sheep when compared to in vitro conditions in sheep-derived cells. Further studies are needed to better understand the implications of RVFV reassortment in relation to virulence and transmission dynamics in the host and the vector. The knowledge learned from these studies on reassortment is important for understanding the dynamics of RVFV evolution.

Safety and immunogenicity of inactivated Rift Valley Fever Smithburn viral vaccine in sheep

BACKGROUND

The live-attenuated Rift Valley Fever Smithburn (SB) vaccine is one of the oldest products widely used in ruminants for control of RVF infections. Vaccinations with RVF Smithburn result in residual pathogenic effect and is limited for use in non-pregnant animals. Commercially available RVFV inactivated vaccines are considered safer options to control the disease. These products are prepared from virulent RVFV isolates and present occupational safety concerns. This research study evaluates the ability of an inactivated SB vaccine strain to elicit neutralising antibody response in sheep.

METHODS

The RVF Smithburn vaccine was inactivated with binary ethylenimine at 37 °C. Inactivated RVFV cultures were adjuvanted with Montande™ Gel-01 and aluminium hydroxide (Al (OH)3) gel for immunogenicity and safety determination in sheep. The commercial RVF inactivated vaccine and a placebo were included as positive and negative control groups, respectively.

RESULTS

Inactivated RVFV vaccine formulations were safe with all animals showing no clinical signs of RVFV infection and temperature reactions following prime-boost injections. The aluminium hydroxide formulated vaccine induced an immune response as early as 14 days post primary vaccination with neutralising antibody titre of 1:20 and a peak antibody titre of 1:83 was reached on day 56. A similar trend was observed in the animal group vaccinated with the commercial inactivated RVF vaccine obtaining the highest antibody titre of 1:128 on day 56. The neutralizing antibody levels remained within a threshold for the duration of the study. Merino sheep vaccinated with Montanide™ Gel-01-Smithburn were characterised with overall lower immune response when compared to aluminium hydroxide vaccine emulsions.

CONCLUSIONS

These finding suggests that the inactivated RVF Smithburn vaccine strain adjuvanted with aluminium-hydroxide can be used an alternative to the products prepared from virulent RVFV isolates for protection of ruminants against the disease. The vaccine can further be evaluated for safety in pregnant ewes.

Perspectives of Next-Generation Live-Attenuated Rift Valley Fever Vaccines for Animal and Human Use

Live-attenuated Rift Valley fever (RVF) vaccines transiently replicate in the vaccinated host, thereby effectively initiating an innate and adaptive immune response. Rift Valley fever virus (RVFV)-specific neutralizing antibodies are considered the main correlate of protection. Vaccination with classical live-attenuated RVF vaccines during gestation in livestock has been associated with fetal malformations, stillbirths, and fetal demise. Facilitated by an increased understanding of the RVFV infection and replication cycle and availability of reverse genetics systems, novel rationally-designed live-attenuated candidate RVF vaccines with improved safety profiles have been developed. Several of these experimental vaccines are currently advancing beyond the proof-of-concept phase and are being evaluated for application in both animals and humans. We here provide perspectives on some of these next-generation live-attenuated RVF vaccines and highlight the opportunities and challenges of these approaches to improve global health.

Evaluations of rationally designed rift valley fever vaccine candidate RVax-1 in mosquito and rodent models

Rift Valley fever (RVF) is a mosquito-borne zoonosis endemic to Africa and the Arabian Peninsula, which causes large outbreaks among humans and ruminants. Single dose vaccinations using live-attenuated RVF virus (RVFV) support effective prevention of viral spread in endemic countries. Due to the segmented nature of RVFV genomic RNA, segments of vaccine strain-derived genomic RNA could be incorporated into wild-type RVFV within co-infected mosquitoes or animals. Rationally designed vaccine candidate RVax-1 displays protective epitopes fully identical to the previously characterized MP-12 vaccine. Additionally, all genome segments of RVax-1 contribute to the attenuation phenotype, which prevents the formation of pathogenic reassortant strains. This study demonstrated that RVax-1 cannot replicate efficiently in orally fed Aedes aegypti mosquitoes, while retaining strong immunogenicity and protective efficacy in an inbred mouse model, which were indistinguishable from the MP-12 vaccine. These findings support further development of RVax-1 as the next generation MP-12-based vaccine for prevention of Rift Valley fever in humans and animals.