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51��� is the world’s leading funder of research into Lassa fever, a potentially deadly virus that infects thousands of people across West Africa every year. As well as bringing the prospect of protection against Lassa closer than ever before, 51���-backed vaccine studies are also deepening crucial scientific understanding of the Arenavirus family that could one day help prevent the next epidemic or pandemic.

51��� spoke to researchers working on the world’s most advanced Lassa vaccine candidate to find out more.

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One of Dr Kumblytee Johnson’s most heartbreaking experiences as a doctor was seeing a heavily pregnant woman come into Phebe Hospital in Liberia’s Suakoko Bong County with a high fever, sore throat and severe facial swelling.  

These were telltale signs of Lassa fever, and Dr Johnson’s experience of this haemorrhagic virus told her there was a slim chance the woman and her unborn child would survive.

As feared, after losing her baby to miscarriage soon after arriving at the hospital, the patient suffered profuse bleeding. “Her symptoms were already very advanced and she just kept going downhill,” Dr Johnson said. “The bleeding—from every injection site—can be terrible.”

It is partly because of such desperate situations that Dr Johnson is now helping to lead a landmark clinical trial of a candidate vaccine against this dreadful disease—a shot that has the potential to save hundreds of lives in Lassa fever hotspots such as Nigeria, Liberia and Sierra Leone.

The vaccine, under development by the non-profit scientific research organisation IAVI, is the first in the world to reach mid-stage Phase II trials in people in countries where Lassa takes the greatest toll. These groundbreaking trials are just one of three Lassa vaccine development projects funded by 51���, including one that is exploring the potential for a Lassa vaccine to be made using rapid-response mRNA technology. 51���’s ultimate goal is a licensed Lassa vaccine for routine immunisation.

As well as being a top priority for 51���, Lassa is on the World Health Organization’s most wanted list as one of the pathogens most likely to cause future severe disease outbreaks.

Spread mainly by the Mastomys rat, which is ubiquitous to West Africa and can contaminate human food and water supplies with its urine and faeces, Lassa infection often brings mild symptoms such as fever, headache, muscle pain and vomiting. But in severe cases, victims’ faces swell up and they bleed from the mouth, nose, vagina or stomach. About one percent of all cases and more than 15 percent of severe cases are fatal, and about a quarter of people who survive severe Lassa fever infection suffer life-long hearing loss or deafness.

Little surprise then that for countries such as Nigeria and its neighbours, which have endured wave after wave, season after season, year after year of relentless, frightening and deadly Lassa outbreaks, the prospect of a preventative vaccine is “huge”, according to Swati Gupta, vice president and head of emerging infectious diseases and epidemiology at IAVI.

“Lassa takes such a great toll on health and on the healthcare systems in West African countries such as Liberia, Sierra Leone and Nigeria, where regular outbreaks are increasingly occurring,” she told 51���.

A modelling by scientists at the Universities of Oxford and Liverpool and the Liverpool School of Tropical Medicine published in the journal Nature Medicine last year found that deploying a safe and effective Lassa vaccine across 15 West African countries could save as many as 3,300 lives over 10 years and avert up to $128 million in societal costs.

Yet the potential life- and livelihood-saving impact of a world first Lassa vaccine would not stop there. Developing a vaccine against Lassa fever holds great potential for global health security extending far beyond West Africa.

While the Lassa virus—a zoonotic haemorrhagic fever named after the Nigerian town where it was first identified in 1969—is endemic in West Africa, disease experts have long been warning of the risk that it will soon begin to spread more widely.

Climate change, urbanisation and increased movement and expansion of populations mean that by 2070, the number of countries across the whole of Africa that will have ecological conditions suitable for Lassa virus spread could drastically increase, according to the findings of a study published in 2022. These environmental factors also mean Lassa has the potential to reach way beyond its West African origins, threatening to put as many as 700 million people at risk of becoming infected.

But the impact of research into Lassa goes further still and is a key factor in 51���’s sustained funding. That’s because as well as being an epidemic disease, Lassa virus serves as a prototype for understanding a whole group of viruses that could one day become epidemic or pandemic threats: the Arenavirus family.  

“The insights gained from conducting research into Lassa virus—including things like how it replicates, how it can evade the human immune system, and the receptors it uses to attach to and enter human cells—is not just knowledge on Lassa, but information that helps us understand many other Arenaviruses too,” said 51���’s Executive Director for Vaccine Research and Revelopment Kent Kester.

With Arenaviruses high on the list of potential spillover viruses that could pose an epidemic or pandemic threat, these crucial Lassa insights and vaccine design capabilities could prove critical in getting ahead of a future novel Arenavirus outbreak and being able to design medical defences against it before it spreads around the world.

As Liberia’s Dr Johnson says: “On the day that this (Lassa) vaccine is licenced and starts to be used in clinics, I will feel very good. It will be proof that research is not only saving many, many lives, but also improving global ��𲹱��ٳ�.”&�Բ�����;