A plan to protect lives and livelihoods from a mosquito-borne killer

Rift Valley fever, caused by a virus from the Phenuivirus family, is a disease whose epidemic potential lies in the intersection of global climate, trade and health security. CEPI invests in a range of research programmes aiming to address the threat of Rift Valley fever, including vaccine development, disease burden research and outbreak preparedness.

To find out more, CEPI spoke to researchers working on how to reduce the risks posed by Rift Valley fever in Africa and beyond. 

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For pastoral farmers in sub-Saharan Africa the threat of Rift Valley fever is a source of great fear. Not just fear of what the disease does to people, but also fear of what it does to animals—and how that brings the threat of hunger, malnutrition and starvation for their families. 

Because Rift Valley fever, or RVF, a mosquito-borne viral haemorrhagic disease, causes devastating abortion storms that rip through the herds of goats, cattle and sheep it affects. And losing those livestock and their offspring means farmers also lose vital sources of trading income, food and nutrition for their children and grandchildren. 

So for Professor George Warimwe, a Kenyan-born scientist who has been studying Rift Valley fever virus for more than a decade and witnessing its impact since he was a child, developing a new vaccine that could protect people and their animals is both a global and a personal ambition.  

Raised by his grandparents in the Kawangware slum area of Kenya’s capital city, Nairobi, Professor Warimwe was only able to get high school and further education because his grandparents had some cattle in local smallholdings. “For me to go to university, and even just to get to normal high school, my grandparents sold animals to pay the fees. They sold a cow so I could get into university.”
 

New vaccine holds promise

Now, armed with that education and experience, Professor Warimwe is a principal investigator of a first-of-its-kind Phase II clinical study, due to start in May 2025, to assess an experimental human vaccine against Rift Valley fever known as ChAdOx1 RVF.  

“It's really exciting to be providing a solution here in Kenya, in the country where I was raised. I like the idea of using one's own knowledge, talents and skills to help solve the problems we face in the world.”  

If the upcoming CEPI-supported trial proves successful, the shot could soon become the world’s first licensed human vaccine against Rift Valley fever—a disease that is causing more frequent, widespread, deadly outbreaks and has already spread to new countries and regions far beyond its East African origins.

Previous trials have already shown the ChAdOx1 RVF provides protection against Rift Valley fever in multiple livestock species, suggesting it may one day be used to protect both animals and people.

“This is the most advanced stage of testing a human Rift Valley fever vaccine has ever reached in an outbreak-prone area,” said Peter Hart, CEPI’s Rift Valley fever Programme Lead.  “Of course, CEPI's priority for RVF is developing a safe and effective human vaccine, but the fact this vaccine may be able to protect both humans and animals—in other words it could be a so-called “dual use” vaccine—is an important step in a One Health approach to RVF prevention and control.”  

It’s been nearly 100 years since the then novel virus that causes Rift Valley fever was first identified in Kenya, where it killed thousands of sheep and caused many pregnant ewes to suddenly miscarry.

Until the mid-1970s, Rift Valley fever was thought to be a disease confined to the African continent and one that only really affected animals. Human cases were very rare and usually only caused mild symptoms. But after a season of heavy rain and flooding in South Africa in 1974, a severe outbreak of RVF spread to more than 500,000 animals and into more than 100 people. It was also during this outbreak that the first human RVF deaths were reported, with seven cases proving fatal.

One of the most devastating human Rift Valley fever epidemics was in Kenya and East Africa in 1997 and 1998, infecting about 27,500 people and killing more than 400 of them. In 2000, the disease spread beyond the African continent for the first time and was detected in Saudi Arabia and Yemen. More than 1,500 people were infected in this outbreak and at least 215 of them died of the disease.

Recent outbreaks of Rift Valley fever in the Indian ocean islands of Comoros, Mayotte and Madagascar between 2007 and 2021 evoke worrying memories of the early spread beyond Africa of another mosquito-borne disease—Zika virus—before it exploded into a global epidemic in 2015 and 2016.

Risk of wider spread

As yet, no human or animal outbreaks of Rift Valley fever have been recorded in Europe, the Americas or elsewhere, but experts say that because RVF infects many different mosquito species— including Aedes and Culex—and can transmit from infected animals to people, it is simply a matter of time before the disease reaches new areas.  

Research published in the BMJ in 2024 found that the epidemiology of RVF is changing, with the virus widening its geographic range as the global climate also changes. The study concluded the likely impact of these changes would be “wider dispersal of [Rift Valley fever] virus to new areas of endemicity and future epidemics”. 

According to Sarah Cleaveland, a professor at the University of Glasgow's School of Biodiversity, One Health and Veterinary Medicine, developing an effective vaccine against Rift Valley fever—especially one that could be used across animal and human at-risk populations—would be both an invaluable outbreak-response tool in endemic countries and a great boost to global health security by getting the world ahead of the disease’s potential future spread.   

“Targeting control of disease in endemic areas is one way also to protect global health security,” she told CEPI in an interview. “And with RVF, it’s not as though it’s an unknown risk.  

“We know it's there. We know it can spread. We know it's got epidemic potential. We know the mosquitoes and other vectors are spreading. So an RVF vaccine is a really important part of a strategy to reduce that risk.”