Bringing together all the pieces of the Lassa fever vaccine puzzle

2024 saw great progress in developing the world’s first Lassa fever vaccine, with a CEPI-funded vaccine candidate entering Phase II trials in West Africa. Now, CEPI is working alongside regional partners to speed up the path to licensure and enable equitable access to future doses.

The month of January not only marks the start of a new year but also, for West Africa—and Nigeria in particular—the yearly peak of Lassa fever infections. 

First discovered in 1969 after two nurses fell sick in the Nigerian town of Lassa, the viral haemmorhagic illness – a member of the Arenavirus family – today affects hundreds of thousands across the region each year. 

The virus is typically spread after coming into contact with the excrement of the infected Mastomys rat found in West Africa, or an infected person. Sufferers can experience headache and fever as well as more serious symptoms like vomiting, swelling, chest and back pain, and bleeding from body parts including the eyes and nose. For pregnant women, the virus is particularly dangerous, capable of causing severe complications for both the mother and baby. 

Altogether, around 1% of Lassa fever cases prove fatal, although this can be as high as 20% for hospitalised cases. And for those that recover, a quarter experience hearing loss.

Long seen as a burden primarily affecting West Africa, scientists are now concerned that the virus’s outbreak hotspots could spread more widely because of climate change and population growth. One study predicts up to 600 million people could be at risk of Lassa virus infection by 2050. 

Despite the urgent need to develop tools to protect those most vulnerable, the world is still waiting on a Lassa fever vaccine. As a result, it’s been recognised by experts, like Professor Sylvanus Okogbenin, as one of the most ‘neglected of neglected diseases’.

Consistent with WHO’s recognition of Lassa fever as a priority pathogen target, the virus has been a core focus for CEPI since our launch in 2017. We are now one of the disease’s top research funders. Our aim is to advance a vaccine to licensure for routine immunisation and outbreak response in affected countries.

The impact of an approved Lassa fever vaccine could be striking. Based on assumptions, recent modelling data funded by CEPI and led by UK universities and research institutions estimated that a safe and effective Lassa vaccine could save nearly 3300 lives over ten years and $128 million in socioeconomic costs.

To date, three of the six CEPI-funded Lassa fever vaccines remain in development. 

One of these vaccine candidates, developed by IAVI, became the first Lassa fever vaccine to enter Phase II trials last year. Over 600 participants are being enrolled into the study in Nigeria, Ghana and Liberia to evaluate the candidate vaccine’s safety, tolerability and immunogenicity at different dosage levels in adults, including people living with HIV, adolescents and children over two. 

CEPI is also funding another vaccine candidate against Lassa fever built on the ChAdOx platform. This is the same technology used to create the Oxford-AstraZeneca COVID-19 vaccine. Researchers hope to progress this candidate into clinical trials this year.

But we can’t just develop a vaccine in isolation. If we are to get a vaccine to licensure, we need all parts of the puzzle to come together, with equitable access underpinning all of our work. 

First and foremost is capitalising on the strong regional leadership to help align across our collective disease response efforts. A major milestone was achieved this month by the West African Health Organisation (WAHO), working together with Ministers of Health from Economic Community of West African States (ECOWAS) Member States and CEPI, to host the first-ever Lassa fever Governing Entity. Partners at the meeting included the World Health Organization, IAVI and MRC Gambia. 

This pioneering initiative builds on the work of existing national Lassa coalitions and is tasked with ensuring regional leadership across Lassa fever vaccine development activities. Its aims for the year ahead include developing a policy research agenda, reviewing plans to enable equitable access to future vaccine doses, and continuing support for Lassa fever R&D.

Improved disease tracking is critical to effective vaccine development and later deployment. Since 2019, over 23,000 participants across West Africa have taken part inEnable, the largest-ever Lassa fever study led by CEPI and partners to better understand the true disease burden. This builds on the work of institutions like the Nigeria Centre for Disease Control who have established an incidence management system to monitor suspected and confirmed cases across its states.

Additional research, launched as an extension to Enable last year, is also now taking an in-depth look at Lassa fever symptoms, potential co-infection with other diseases like malaria and its impact in young children. 

The data generated from the study will fill major gaps in our knowledge of the disease. 

Such information is being used to guide the location of vaccine trials, while also helping to determine who should be prioritised to receive a future vaccine. It will additionally provide real-world evidence that further refine our measurements looking at the impact of a Lassa fever vaccine. Our investment in other tools, like high-quality diagnostic tests, will support these efforts.

Across the region, there are also broader initiatives being established by CEPI and others to strengthen local clinical trial capacity and build up locally led structures to ward off Lassa fever and other infectious diseases. 

By working with scientists, manufacturers, regulators, other leading public health bodies and civil society organisations in the region, we are jointly building the framework we need to move at speed. And it is this diversity in investment—away from just funding the core vaccine research—which has put us on track to making a vaccine against Lassa fever not just a theoretical possibility but an achievable reality.

Years in advance of a licensed product, we are already working with regional partners to design an end-to-end access roadmap which will outline what is needed, when and by whom to achieve timely availability of and access to a safe and effective vaccine. 

The roadmap aims to align the needs and roles across the vaccine chain—from developers to manufacturers and policymakers to procurers—to ensure a smooth transition in getting the future vaccine from the scientific bench into the arms of those that need it the most.

So, while January continues to be synonymous with Lassa fever’s annual season peak, from this year onwards it can also be seen as a major milestone moment with the launch of the new Lassa fever coalition and other advances putting us on the path to success.

As epidemic threats continue elsewhere, the lessons learned from this diverse set of Lassa fever activities will help shape CEPI’s work against other high-risk diseases. This includes other known troublesome pathogens in the same Arenavirus family or a novel or as-yet-identified Disease ‘X’ that could strike in the future.

The collaborations and knowledge we’re making today could therefore be instrumental in guiding our epidemic and pandemic response tomorrow.

• Katrin Ramsauer is CEPI's Lassa Disease Programme Lead
• Dr Virgil Lokossou is Acting Director, Directorate of Health Care Services, West African Health Organisation (WAHO)
• Oyeronke Oyebanji is CEPI's Head of Lassa Engagement