FAQ
Learn more about Lassa virus, a zoonotic virus that causes severe hemorrhagic fever in humans
What is Lassa fever?
Lassa fever is caused by Lassa virus (LASV), an emerging zoonotic virus that can cause severe hemorrhagic fever in humans, with symptoms including hemorrhage, vomiting, swelling of the face, and pain in the chest, back, and abdomen, shock from blood loss, and even death. The disease was first identified in 1969, when four missionary nurses died from a hemorrhagic illness in the town of Lassa, in northeastern Nigeria. Today the disease is known to be endemic in Benin, Ghana, Guinea, Liberia, Mali, Nigeria and Sierra Leone, and probably exists in other west African countries too. It is often reported that there are up to 300,000 cases and 5,000 deaths worldwide each year. However, these figures are likely to be a significant underestimate. One recent study estimated that 2.1–3.4 million human LASV infections occur annually throughout West Africa, resulting in 15,000–35,000 hospitalizations and 1,300–8,300 deaths. Testing for blood markers also suggests the virus is widespread in countries like Mali, where no cases are currently reported. The World Health Organization (WHO) has identified Lassa virus as one of the top emerging pathogens likely to cause severe outbreaks in the near future in its Research and Development Blueprint for Action to Prevent Epidemics.
How is Lassa virus transmitted?
Lassa virus is most commonly transmitted to humans from an infected rodent known as the multimammate rat (Mastomys natalensis). This type of rat is found throughout Sub-Saharan Africa but currently, only West African multimammate rats are known to carry LASV. The rats carry LASV in their urine and excrement, which contaminates areas where food is prepared or stored. There is also some evidence that LASV can be carried by other rodent species. The virus can also spread from person to person via bodily fluids. Nigeria, Liberia, Sierra Leone, and Guinea are most affected, but increasingly, neighboring countries are experiencing their own emerging outbreaks, with travelers occasionally carrying infections to other regions. If patients are asymptomatic, they usually do not transmit the virus, and the virus is not spread through casual contact such as hugging or shaking hands. There is no recorded evidence of airborne transmission.
What are the symptoms of Lassa fever?
Symptoms can take three weeks after infection to appear, and about three-quarters of people with infections experience only mild symptoms including a light fever and headache and may not be diagnosed with Lassa fever. However, about one in five infections results in severe disease, where the virus affects organs including the liver, spleen and kidneys, and causes serious symptoms including bleeding, vomiting, facial swelling, and difficulty breathing. About one-third of patients also suffer from hearing loss, which can be permanent. Pregnant women are at particular risk from the disease. Among women who contract Lassa fever during the third trimester of pregnancy, more than 80% of cases result in the death of either mother or child. The overall fatality rate for adults is estimated to be 1% but among those who are hospitalized with severe disease this rises to at least 15%.
Who is most at risk?
People usually become infected with LASV by coming into contact with substances which have been contaminated by rodents – such as by eating food which rats have touched, by handling objects which rats have been in contact with or urinated on, or by cleaning animal droppings. This means almost anyone who handles food or lives in a contaminated area is at risk. The virus is not transmitted via casual contact such as hugging an infected person but can be spread via the bodily fluids of an infected person. This means health workers caring for people infected with Lassa fever are at a high risk of catching the virus themselves. Good Personal Protective Equipment (PPE) and strict isolation protocols are essential. There are also rare examples of travelers returning from endemic areas infected with LASV. Experts say the geographic area vulnerable to Lassa fever is growing due to climate change.
Have there been any recent outbreaks?
Lassa fever is endemic to countries in West Africa, notably Benin, Ghana, Guinea, Liberia, Mali, Nigeria and Sierra Leone, where annual outbreaks occur. Lassa fever often goes undiagnosed because outbreaks typically occur in locations where laboratory testing capacity is very limited. The disease also closely resembles other endemic illnesses such as malaria and yellow fever in its early stages. At the time of writing (April 2025) there is an ongoing outbreak in part of Nigeria, which has infected hundreds of people and killed at least ten. In 2024, Nigeria recorded over 1,300 confirmed cases and over 10,000 suspected cases, with more than 200 confirmed deaths. There have also been isolated reports of travelers returning to the US and Europe infected with LASV (although subsequent testing has sometimes proved negative).
How can the spread of Lassa fever be prevented?
Mastomys rats are so abundant in West Africa that eliminating them from residential areas is impossible. However, in domestic settings, Lassa fever can be prevented by rigorous rodent control and hygiene, including storing food in rat-proof containers, sealing garbage and setting rat traps. In healthcare settings, full PPE plus strict isolation and sterilization protocols are essential.
How is Lassa fever treated?
Some patients are treated with Ribavirin, an antiviral drug, but there is an ongoing debate about its efficacy and dosing. Many patients receive only supportive care, including rehydration, rest, and treatment of symptoms.
Is there a vaccine for Lassa fever
There is currently no licensed vaccine for Lassa fever, but several candidates are in development. One of the most promising is the candidate known as rVSVΔG-LASV-GPC, which is currently being developed by IAVI and partners. This candidate is a single-dose vaccine formulation, based on the modification of an attenuated or weakened strain of vesicular stomatitis virus (VSV), which is then re-engineered to display the LASV surface protein that plays an essential role in establishing a viral infection and stimulating immunity. The same VSV vector platform has already been used to create ERVEBO®, Merck’s single-dose Ebola virus vaccine, which has been widely licensed as safe to use in more than a dozen countries, and which has already been used to control Ebola outbreaks in the Democratic Republic of Congo. Should the candidate be found to be safe and efficacious in clinical testing, IAVI is committed to making its Lassa vaccine affordable and accessible to all populations in need. Development of the vaccine has been funded by CEPI and the European & Developing Countries Clinical Trials Partnership (EDCTP), with trials conducted in partnership with organizations including the Nigeria Centre for Disease Control and Prevention and the Nigeria Lassa Vaccine Taskforce. A recent study by the University of Oxford estimated that vaccinating high-risk populations could avert up to 4,400 deaths in West Africa and save societal costs, including labor losses and healthcare, of almost US$129 million a year.