Protecting against life-threatening Lassa fever

A new study in The Lancet Global Health presents, for the first time, the health-economic burden of Lassa fever across various age and sex groups in West Africa, as well as the potential impacts of targeting specific groups for vaccination.

Led by Dr Koen Pouwels from the University of Oxford – recipient of the Medical Research Foundation’s Emerging Leaders Prize - and his colleague Dr David Smith, these findings provide the first evidence to guide which population groups should be prioritised for the rollout of forthcoming Lassa fever vaccines.

The study was commissioned by the Coalition for Epidemic Preparedness Innovations (CEPI).

What is Lassa fever?

Lassa fever is a viral, animal-borne infection spread by wild rodents. It is caused by the Lassa virus, a member of the Arenavirus family. These rodents are common in rural areas of West Africa, contributing to the disease’s high prevalence in the region.

Most human infections result from zoonotic transmission, or ‘spillover’, from rodents to humans. Although human infections are largely asymptomatic or cause only mild illness, a recent Foundation-funded study (also conducted by Dr Smith and Dr Pouwels) estimated that Lassa fever causes 24,000 hospitalisations and 4,000 deaths every year across West Africa.

Because of this, and the disease’s worrying potential to trigger epidemics and significantly affect public health, the World Health Organization classifies Lassa fever as a priority pathogen for research and development.

It is likely to impose a growing health and economic burden across West Africa, particularly as populations expand in endemic regions.

Protecting against Lassa fever

While it is unlikely to reduce the spread of infection, a Lassa vaccine could be transformative for controlling disease burden and preventing severe outcomes, including long-term hearing loss and even death.

Dr Pouwels explains, “Our modelling suggests that preventive vaccination - even if only moderately priced and partially effective - could prevent a substantial number of hospitalisations, deaths, and years lived with disability in endemic areas.”

Once a safe and powerful vaccine against Lassa fever has been licensed for human use, this study aims to support the implementation of a cost-effective immunisation programme.

He adds, “Right now, it is crucial to strengthen surveillance, diagnostics, and vaccination readiness, to prevent wider spread and protect both communities and front-line health workers.”

Challenges facing mass vaccination

In general, most vaccines in national immunisation programmes tend to focus on targeting high-risk groups.

However, with Lassa fever, there are varying risks specific to population subgroups; children are at the highest risk of infection, but most hospitalised patients are middle-aged adults, and rates of death are particularly high among pregnant women.

It is unclear who to target to ensure there is a maximum return on vaccine programme investment, and no studies to date have investigated how vaccine impact could vary across these groups.

How will this study help?

In this study, the University of Oxford team developed a mathematical model using a zoonosis risk map, identifying areas with a high probability of Lassa fever outbreaks.

Their aim was to predict the health-economic burden of Lassa fever across age and sex groups in regions where the disease is endemic, to help understand which groups bear the greatest burden of disease – and so may benefit most from vaccination.

They simulated vaccination campaigns targeting various risk groups, to examine how cost-effective various strategies for Lassa vaccine administration would be.

Dr Pouwels relays the results of the team’s findings, “We show that targeting teenagers and adults aged 15–49 years old is likely the most cost-effective strategy for a future Lassa vaccine.”

The researchers note that the optimal approach will depend on vaccine price, efficacy and potential limitations caused by an overall lack of infection data, but the greatest health-economic value per vaccine dose seems to come from targeting adolescents and adults in this age group.

Commenting on the significance of these findings, Dr Pouwels says, “Once a Lassa vaccine becomes available, our findings could help guide governments and global health partners in designing efficient, equitable vaccination programmes.”

Monitoring Lassa vaccine efficacy will be crucial to inform future decision-making around target groups to prioritise.

“Combined with ongoing surveillance and strong community engagement, vaccination could shift Lassa fever from an under-recognised endemic threat to a preventable disease.”

The impact of the Foundation’s Emerging Leaders Prize

Dr Koen Pouwels was the winner of our 2021 Emerging Leaders Prize (ELP) for his work in optimising the design of disease surveillance programmes.

The Foundation’s ELP funding has enabled him to hire post-doctoral researcher, Dr David Smith, and build the analytical capacity to respond rapidly to the Coalition for Epidemic Preparedness Innovations (CEPI)’s call for Lassa vaccination modelling.

He comments, “The ELP support created the space and capability to apply advanced modelling methods and make optimal use of infectious disease surveillance data to inform vaccine policy and preparedness. This timely investment allowed us to contribute evidence just as the first Lassa vaccine entered human trials, helping ensure that new vaccines can be deployed as effectively as possible once available.”

Read the full study here >>