Yellow Fever

Primary reference(s)

WHO, 2023. Yellow fever. World Health Organization (WHO). Accessed 27 May 2025.

Annotations

Key relevant UN convention / multilateral treaty

International Health Regulations (2005), 3rd ed. (WHO, 2016).

Drivers

Major epidemics of yellow fever occur when infected people introduce the virus into heavily populated areas with high mosquito density and where most people have little or no immunity, due to lack of vaccination. In these conditions, infected mosquitoes of the Aedes aegypti species transmit the virus from person to person (WHO, 2023). 

Impacts

Yellow fever is an infectious disease transmitted by mosquitoes that bite mostly during the day. As of 2023, 34 countries in Africa and 13 countries in Central and South America are either endemic for, or have regions that are endemic for, yellow fever (WHO, 2023).

Multi-hazard context

Yellow fever is an epidemic-prone mosquito-borne vaccine-preventable disease that is transmitted to humans by the bites of infected mosquitoes. Yellow fever is caused by an arbovirus (a virus transmitted by vectors such as mosquitoes, ticks or other arthropods) transmitted to humans by the bites of infected Aedes and Haemagogus mosquitoes. These day-biting mosquitoes breed around houses (domestic), in forests or jungles (sylvatic), or in both habitats (semi-domestic). Yellow fever is a high-impact high-threat disease, with risk of international spread, which represents a potential threat to global health security (WHO, 2023).

Risk Management

Prompt detection of yellow fever and rapid response through emergency vaccination campaigns are essential for controlling outbreaks. However, underreporting is a concern; WHO estimates the true number of cases to be 10 to 250 times what is now being reported (WHO, 2023). The risk of yellow fever transmission in urban areas can be reduced by eliminating potential mosquito breeding sites, including by applying larvicides to water storage containers and other places where standing water collects. Both vector surveillance and control are components of the prevention and control of vector-borne diseases, especially for transmission control in epidemic situations (WHO, 2023).  

Yellow fever is prevented by an extremely effective vaccine, which is safe and affordable (WHO, 2023). However, there is no specific antiviral therapy, so if severe illness develops, only supportive care is available. Vaccination is the most important measure for preventing yellow fever (WHO, no date b). Yellow fever vaccine is safe, affordable, and a single dose provides life-long protection against yellow fever disease. The yellow fever vaccine provides immunity within one week in 95% of people vaccinated. A booster dose is not needed. Yellow fever vaccination is carried out for the following reasons: 

• to protect populations living in areas at high-risk or endemic for yellow fever disease;
• to protect travellers visiting these areas; and,
• to prevent international spread by minimizing the risk of importation of the virus to unaffected areas (WHO, no date b). 

All currently available yellow fever vaccines are live and attenuated formulations. Vaccines from four manufacturers are currently prequalified by WHO. Yellow fever control is based on the prevention of outbreaks, and this can only be achieved if most of the population (recommended ≥80% of eligible population) is immunised in all high risk and endemic areas. It is recommended that the yellow fever vaccine be given to infants at age 9–12 months at the same time as the measles vaccine in yellow fever-endemic countries. All countries with areas at risk of yellow fever disease should aim to reach and sustain high coverage through routine immunization services. High-risk countries are recommended to establish a timeline for introduction if the vaccine has not yet been introduced. Countries at risk of yellow fever that are establishing and strengthening immunization in the second year of life (2YL) and beyond should consider yellow fever vaccination of children missed in the first year of life (WHO, no date b). 

In addition, preventive mass vaccination campaigns are recommended for areas at risk of yellow fever where there is low vaccination coverage. Vaccination should be provided to everyone aged ≥9 months in any area with reported cases. Countries should aim to integrate yellow fever mass vaccination campaigns with other campaigns or interventions, where feasible (WHO, no date b). 

During yellow fever epidemics, reactive vaccination campaigns are carried out with minimal delay to limit the spread of the disease. There is a global emergency yellow fever vaccine stockpile of 6 million doses (rotating stock) per year funded through Gavi. The stockpile is managed through the International Coordinating Group (ICG) for vaccine provision for yellow fever. To request access to ICG stocks, national or international health authorities should submit an application form to the ICG Secretariat (WHO Geneva), or to an ICG member agency (IRFC, MSF, UNICEF) present in the country (WHO, no date b).  

The Eliminate Yellow Fever Epidemics (EYE) Strategy was developed in response to two urban yellow fever outbreaks – in Luanda (Angola) and Kinshasa (Democratic Republic of Congo), with international exportation to other countries, including China, showing that yellow fever poses a serious global threat requiring new strategic thinking. The EYE strategy is comprehensive, multi-component and multi-partner. In addition to recommending vaccination activities, it calls for building resilient urban centres, planning for urban readiness, and strengthening the application of the International Health Regulations (2005) (WHO, 2023). 

Occasionally, travellers who visit yellow fever-endemic countries may carry the disease to countries free from yellow fever. In order to prevent importation of the disease, many countries require proof of vaccination against yellow fever before they will issue a visa, particularly if travellers come from, or have visited yellow fever endemic areas (WHO, 2023).

Monitoring

WHO recommends that every at-risk country has at least one national laboratory where basic yellow fever blood tests can be performed.  A confirmed case of yellow fever in an unvaccinated population is considered an outbreak. A confirmed case in any context must be fully investigated. Investigation teams must assess and respond to the outbreak with both emergency measures and longer-term immunization plans (WHO, 2023).

WHO supports countries to conduct all-hazards strategic risk assessment in the contexts of health emergencies and disasters, which results in the development of a country risk profile. Empowered with the country risk profile, inclusive of a seasonal risk calendar, countries can anticipate potential emergencies before they occur to trigger early alerts and inform early actions (WHO, 2021).

WHO's Early Warning, Alert and Response System (EWARS) has been designed to improve disease outbreak detection in emergency settings, such as in countries in conflict or following a disaster from natural hazards. It is a simple and cost-effective way to rapidly set up a disease surveillance system. EWARS is deployed during an emergency as an adjunct to the national disease surveillance system. WHO works with Ministries of Health and health sector partners to train local health workers to use the system. After the emergency, EWARS should re-integrate back into the national system (WHO, no date a).