Lassa fever is a cryptic viral disease that is spread predominantly from rodents to humans in West Africa. We estimated the health-economic burden associated with Lassa fever, highlighting the gains to population health and economies that could be afforded by Lassa vaccines, several of which are currently in development.
The problem
Lassa fever is a poorly understood zoonotic disease that causes viral hemorrhagic fever and thousands of deaths annually throughout West Africa. It has also been identified as a leading pandemic threat by the World Health Organization (WHO) and other global health institutions. This has motivated considerable investment in the development of Lassa vaccines, several of which are currently undergoing assessment in clinical trials1,2. However, to date, no estimates of the health and economic burden of Lassa fever are available, impeding analysis of how Lassa vaccines could affect population health and economies. These impacts include pandemic preparedness because Lassa vaccines developed now could form a crucial part of future public health response efforts if, as is feared, a pandemic variant of Lassa virus were ever to emerge.
The discovery
Lassa fever is underdiagnosed and poorly reported through much of its endemic range. Modelling approaches are therefore particularly useful to estimate the largely hidden burden of this disease and evaluate the potential effects of public health interventions.
We leveraged the best available ecological, epidemiological and economic data from West Africa to develop a model that predicts at a high geospatial resolution the daily number of Lassa virus infections occurring in humans, and the health outcomes and economic costs associated with infection (Fig. 1). We then simulated a range of vaccination campaigns that reflect realistic assumptions about vaccine availability and efficacy, and projected the benefits that these campaigns could have on population health and economies. We also evaluated the health-economic effects of large-scale reactive vaccination under a pandemic scenario in which a hypothetical variant of Lassa virus, which we call ‘Lassa-X’, emerges and rapidly spreads throughout the region.
We estimated that Lassa fever results in 24,000 hospitalizations, 3,900 deaths and 200,000 disability-adjusted life years (DALYs) annually, a burden comparable to those of other infectious diseases in West Africa such as rabies and lymphatic filariasis (although, unlike Lassa fever, these diseases are not thought to have pandemic potential). Vaccination averted roughly tenfold more DALYs when administered preventively in known Lassa fever hotspots, as opposed to reactively in response to local outbreaks. Preventive campaigns that target regions thought to be at greatest risk for Lassa fever were most effective, averting 20,000–29,000 hospitalizations, 3,300–4,800 deaths and 164,00–240,000 DALYs over 10 years, in turn averting US$42–61 million in healthcare costs, $86–126 million in productivity losses and $20–30 million in monetized DALY value.
In the hypothetical event of Lassa-X emerging and spreading throughout West Africa within 2 years, achieving the vaccination targets of the 100 Days Mission — a globally endorsed pandemic response roadmap — averted just 22% of DALYs given a vaccine 70% effective against the disease, but up to 74% of DALYs given a vaccine 70% effective against both infection and disease.
The implications
This work describes the large health and economic toll that Lassa fever bears on West African societies and suggests how forthcoming Lassa vaccines could be rolled out to optimize their impact. Our results also provide insights into the value of pandemic preparedness, by highlighting the health-economic effects that Lassa vaccines could have, assuming a rapid vaccine response is achieved in the hypothetical event that Lassa-X were to emerge.
Although regions known to be at high risk of Lassa fever are an obvious starting point for Lassa vaccination campaigns, our model also predicts a substantial burden of disease in regions not widely classified as endemic. However, these predictions are difficult to validate owing to data limitations, highlighting a need for further epidemiological study throughout the region.
It is hoped that the ongoing ‘Enable’ cohort study will provide important data on characteristics of Lassa fever that remain poorly understood, such as its spectrum of illness, extent of seroreversion and risk factors for infection and disease. Ongoing Lassa vaccine trials will also help to describe the public health potential of specific vaccine products currently in development. Integrating these data into future analyses will be key to estimating the cost-effectiveness of real-world Lassa vaccination programmes. Such analyses will provide stakeholders and decision-makers with critical estimates of the return on Lassa vaccine investment in the context of competing public health priorities.
By David R. M. Smith (University of Oxford, Oxford, UK) & Emily J. Nixon (University of Liverpool, Liverpool, UK.)
