Global & Disaster Medicine

Archive for the ‘Yellow Fever’ Category

Nigeria has now seen at least 84 suspected yellow fever cases and 26 deaths in an ongoing outbreak


Yellow fever – Nigeria

Disease outbreak news
26 September 2019

On 16 July 2019, the Ebonyi State Ministry of Health received information about suspected yellow fever cases in Ndingele ward, Izzi Local Government Area (LGA), Nigeria. The cases had symptoms of fever and jaundice, reported with onset since May 2019. As of 30 August 2019, a total of 84 suspected yellow fever cases, including 26 deaths (case fatality ratio: 31 %), have been reported across nine LGAs within Ebonyi State. Of the suspected yellow fever cases, fifty-five per cent (46/84) are male. The most affected age group is 0-9 years (28 cases, 33%), followed by age groups 20-29 years and above 30 years each with 20 cases (24%). The 10-19 year age group has the least number of cases (16 cases, 19%). Seventy-nine per cent of suspected cases (66/84), including seven confirmed cases positive by real-time polymerase chain reaction (RT-PCR) are reported from Izzi LGA which is located in the north-eastern part of the State, bordered with Cross River State in the east, and Benue State in the north. One case has been confirmed in an international worker involved in the extractive industry in Izzi LGA, and residing in adjacent Abakiliki LGA (an urban centre in the State).

The assessment conducted by the LGA rapid response team (RRT) and national agencies found low vaccination coverage and poor routine immunization documentation. Community surveys were conducted and yellow fever vaccination coverage was estimated to be 56% (64% for children aged less than 5 years and 48% for those older than 5 years of age). Though Nigeria introduced routine vaccination for yellow fever into the immunization schedule in 2004, most adults remain susceptible and overall population immunity is low. Although no entomological studies were conducted at the time, the geography and vegetation of the affected state is compatible with the presence of the Aedes mosquitoes, as illustrated by the transmission patterns.

Since September 2017, when the Nigeria Centre for Disease Control (NCDC) informed WHO of a confirmed case of yellow fever in Kwara State, Nigeria has been responding to successive yellow fever outbreaks over a wide geographic area. Following the country’s official notification to WHO (via the International Health Regulations, 2005) on 15 September 2017 of the resurgence in yellow fever outbreaks, suspect cases have been reported from all States and the Federal Capital Territory (FCT), and outbreak responses to protect over 10 million people have occurred in select areas in 13 States. Efforts to strengthen surveillance are ongoing.

From 1 January through 31 July 2019, over 2,000 suspected yellow fever cases have been reported in 506 LGAs from all 36 States including the Federal Capital Territory (FCT) of Nigeria.

Public health response

The response to the ongoing outbreak is being coordinated by the Nigerian Centre for Disease Control (NCDC) at the national level through a multi-agency, multi-partner Incident Management System (IMS). The public health emergency operations centre (PHEOC) located at the Ebonyi State Ministry of Health has been activated to monitor the outbreak. Active case finding, case management, risk communication and community engagement have been implemented in affected LGAs. An International Coordination Group (ICG) request for reactive vaccination in the affected area has been approved covering three LGAs in Ebonyi, two LGAs in Benue State and one LGA in Cross River State targeting a population of about 1.64 million people aged 9 months to 44 years in Ebonyi and Benue States, and aged 1-6 years in Yala LGA of Cross River State.

Routine yellow fever vaccination was introduced to Nigeria’s Expanded Programme on Immunization (EPI) in 2004, but the overall population immunity in areas affected by the current outbreak remains below recognized herd immunity thresholds.

A four-year (2018-2021) national yellow fever Preventive Mass Vaccination Campaign (PMVC) plan supported by GAVI and partners is currently being implemented. By 2024, it is anticipated that all the States in Nigeria will have undergone campaign activities to protect at-risk populations against yellow fever.

WHO risk assessment

Yellow fever is an acute viral haemorrhagic disease transmitted by infected mosquitoes and has the potential to spread rapidly and cause serious public health impact. There is no specific treatment, although the disease is preventable using a single dose of yellow fever vaccine, which provides immunity for life. Supportive care is required to treat dehydration, respiratory failure, and fever; and antibiotics are recommended to treat associated bacterial infections.

The recent confirmation by the national and regional reference laboratories of the Yellow fever cases in Ebonyi State, with the probable epicentre in Izzi LGA, represents a concerning situation in Nigeria.

Given the rapid evolution of the situation in Ebonyi State, the national risk is assessed as high due to the high CFR (36%) and the potential for ongoing local transmission and amplification. Factors considered include the low vaccination coverage; probability of the presence of competent vectors including Aedes species; the proximity of a yellow fever case to Abakiliki LGA (an urban centre in the State); and the potential spread to new LGAs.

There is currently a moderate risk at regional level due to the possible movement of the individuals of affected States to adjacent areas and neighbouring countries, particularly if there is arrival of unvaccinated visitors to the State. There is risk to national and international workers in high-risk extractive industries like open pit mining and forestry, underscoring the importance of ensuring all international travelers and workers are vaccinated according to International Health Regulations (IHR) recommendations as emphasized in the Eliminate Yellow Fever Epidemics (EYE) strategy. The current overall risk is considered to be low at the global level.

Nigeria is facing several concurrent public health emergencies, including circulating Vaccine Derived Polio Virus (cVDPV), measles, cholera and Lassa fever outbreaks and a humanitarian crisis in the north-east of the country.

WHO advice

Nigeria is a high priority country for the EYE strategy. Vaccination is the primary intervention for prevention and control of yellow fever. In urban centres, targeted vector control measures are also helpful to interrupt transmission. WHO and partners will continue to support national & state authorities to implement these interventions to control the current outbreak.

WHO recommends vaccination against yellow fever for all international travelers more than nine months of age going to Nigeria, as there is evidence of persistent or periodic yellow fever virus transmission. This is of particular importance to temporary or international workers engaged in extractive industries. Nigeria also requires a yellow fever vaccination certificate for all arriving travelers over one year of age.

Yellow fever vaccines recommended by WHO are safe, highly effective and provide life-long protection against infection. In accordance with the IHR (2005), Third edition, the validity of the international certificate of vaccination against yellow fever extends to the life of the person vaccinated with a WHO approved vaccine. A booster dose of approved yellow fever vaccine cannot be required of international travelers as a condition of entry.

WHO encourages its Member States to take all actions necessary to keep travelers well informed of risks and preventive measures including vaccination. Travelers should also be made aware of yellow fever signs and symptoms and instructed to seek rapid medical advice should they develop signs of illness. Travelers returning to Nigeria who may be infected with possible high levels of the virus in the blood may pose a risk for the establishment of local cycles of yellow fever transmission in areas where a competent vector is present.

WHO does not recommend any restrictions on travel or trade to Nigeria on the basis of the information available on this outbreak.

For more information on yellow fever, please see:

August 20: World Mosquito Day

Malaria, Dengue, Yellow Fever, Japanese Encephalitis, West Nile Chickungunya, Kunjin Jamestown Canyon Virus, Zika, Venezuelan Equine Encephalitis, St Loius Encephalitis, Pogostra Disease, Ross River Virus, Lymphatic Filariasis, Eastern Equine Encephalitis, Murray Valley Encephalitis, Rift Valley Fever, Lacrosse Encephalitis.

And the list goes on.”

New study: What yellow fever patients are likely to die?


“……Therefore, 76 patients with confirmed yellow fever virus infection, based on detectable yellow fever virus RNA in blood (74 patients) or yellow fever virus confirmed only at the autopsy report (two patients), were included in our analysis. 27 (36%) of 76 patients died during the 60 day period after hospital admission. We generated 14 complete yellow fever virus genomes from the first 15 viral load-detectable samples. The genomes belonged to a single monophyletic clade of the South America I genotype, sub-genotype E. Older age, male sex, higher leukocyte and neutrophil counts, higher alanine aminotransferase, aspartate transaminase (AST), bilirubin, and creatinine, prolonged prothrombin time, and higher yellow fever virus RNA plasma viral load were associated with higher mortality. In a multivariate regression model, older age, elevated neutrophil count, increased AST, and higher viral load remained independently associated with death. All 11 (100%) patients with neutrophil counts of 4000 cells per mL or greater and viral loads of 5·1 log 10 copies/mL or greater died (95% CI 72–100), compared with only three (11%) of 27 (95% CI 2–29) among patients with neutrophil counts of less than 4000 cells per mL and viral loads of less than 5·1 log 10 copies/mL. ….”

Illnesses on the rise from mosquito, tick, and flea bites



Almost everyone has been bitten by a mosquito, tick, or flea. These can be vectors for spreading pathogens (germs). A person who gets bitten by a vector and gets sick has a vector-borne disease, like dengue, Zika, Lyme, or plague. Between 2004 and 2016, more than 640,000 cases of these diseases were reported, and 9 new germs spread by bites from infected mosquitoes and ticks were discovered or introduced in the US. State and local health departments and vector control organizations are the nation’s main defense against this increasing threat. Yet, 84% of local vector control organizations lack at least 1 of 5 core vector control competencies. Better control of mosquitoes and ticks is needed to protect people from these costly and deadly diseases.

State and local public health agencies can

  • Build and sustain public health programs that test and track germs and the mosquitoes and ticks  that spread them.
  • Train vector control staff on 5 core competencies for conducting prevention and control activities.
  • Educate the public about how to prevent bites and control germs spread by mosquitoes, ticks,  and fleas in their communities.

Increasing threat, limited capacity to respond

More cases in the US (2004-2016)

  • The number of reported cases of disease from mosquito, tick, and flea bites has more than tripled.
  • More than 640,000 cases of these diseases were reported from 2004 to 2016.
  • Disease cases from ticks have doubled.
  • Mosquito-borne disease epidemics happen more frequently.

More germs (2004-2016)

  • Chikungunya and Zika viruses caused outbreaks in the US for the first time.
  • Seven new tickborne germs can infect people in the US.

More people at risk

  • Commerce moves mosquitoes, ticks, and fleas around the world.
  • Infected travelers can introduce and spread germs across the world.
  • Mosquitoes and ticks move germs into new areas of the US, causing more people to be at risk.

The US is not fully prepared

    • Local and state health departments and vector control organizations face increasing demands to respond to these threats.
    • More than 80% of vector control organizations report needing improvement in 1 or more of 5 core competencies, such as testing for pesticide resistance.
    • More proven and publicly accepted mosquito and tick control methods are needed to prevent and control these diseases.

Graphic: Disease cases from infected mosquitoes, ticks, and fleas have tripled in 13 years

Graphic: Disease cases from mosquitoes (2004-2016, reported)

Graphic: Disease cases from ticks (2004-2016, reported)

WHO: How YF spreads

Current WHO recommendation for yellow fever control


Yellow fever vaccine: a global partnership

Current WHO recommendation for yellow fever control

Vaccination is the single most important measure for preventing yellow fever. Yellow fever vaccine is safe and effective and provide immunity within one week in 95% of those vaccinated. Yellow fever control is based on the prevention of outbreaks, and this can only be achieved if the majority of the population is immunized. A two-pronged strategy is used to achieve this goal.

The first strategy is the inclusion of the yellow fever vaccine in national childhood routine immunization programmes, to be administered at 9 months of age, simultaneously with the measles first dose. However, reaching the desired level of population immunity takes more than 30 years with infant routine immunization only, even with a high coverage. Therefore this strategy needs to be combined with the implementation of mass preventive vaccination campaigns to rapidly increase the population immunity and to protect susceptible older age groups in selected high risk areas.

During YF epidemics, reactive vaccination campaigns are carried out with minimum delay in order to limit the spread of the disease.

The International Coordinating Group mechanism

In 2000, a global shortage of yellow fever vaccines for outbreak response arose from the long lead-time needed to produce the vaccine. Moreover, poor surveillance and reporting mask trends and make the forecasting of vaccine needs especially difficult.

WHO and its partners in the International Coordinating Group (ICG) for Vaccine Provision for Epidemic Meningitis Control are applying the ICG strategy and mechanism for the release and use of yellow fever vaccine.

With vaccine manufacturers as partners in the ICG, a stockpile of 2 million doses reserved for outbreaks response was established. The yellow fever ICG sub-group evaluates request for yellow fever vaccine based on agreed criteria for vaccine provision. If the criteria is fulfilled, doses are released from the stockpile for emergency control of the outbreak.

Global emergency yellow fever vaccine stockpile

The GAVI (Global Alliance for Vaccine and Immunization) support

As demands for the use of the stockpile increased, a joint WHO-UNICEF proposal was successfully presented to the Global Alliance for Vaccine and Immunization (GAVI) Board in 2003 for financing an increased global stockpile of 6 million doses per year for a period of 3 years. In July 2005, GAVI support was extended for an additional year until 2006.

Use of the vaccine stockpile

This mechanism will ensure that all available vaccine stocks are directed to the areas of most urgent need and that adequate supplies are manufactured and stockpiled against future outbreaks. Since the accumulation of the first stockpile in August 2003, the GAVI global vaccine stockpile has been successfully used in the outbreaks of urban yellow fever in Cote d’Ivoire, in Colombia, Liberia, Senegal and Sierra Leone.

Côte d’Ivoire: First urban fever epidemic in over a decade

In September 2001, yellow fever cases were confirmed in five of the ten communes in Abidjan; suspected cases were also reported elsewhere. Urban yellow fever, although very rare, can spread extremely rapidly among a dense urban population, causing many thousands of deaths and very serious humanitarian consequences. Over 3.5 million people live in Abidjan – the commercial capital of Cote d’Ivoire.

WHO, working closely with UNICEF, coordinated the international response team with partners in the Global Outbreak Alert and Response Network. An Operations Centre was established at the WHO office in Abidjan to coordinate activities in epidemiology, the immunization campaign, and vector control. Global reserve stock and delivery of vaccines and supplies were through the yellow fever sub-group of the International Coordinating Group for Emergency Vaccine Provision.

Vaccines and safe injection equipment were provided. The mass campaign immunized 2.9 million persons over a 10 day period.


Yellow Fever in Brazil


Disease outbreak news
18 April 2019

In Brazil, seasonal increases of yellow fever have historically occurred between December and May. During the 2016-2017 and 2017-2018 seasons, the number of yellow fever cases was much larger than in previous years (Figures 1 & 2). The increase in cases was partly due to a geographical expansion of the areas affected by yellow fever to include areas previously considered risk-free (Figure 3).

In the current 2018- 2019 season (July 2018 to March 2019), a total of 75 confirmed human cases, including 17 deaths (case fatality rate = 23%), have been reported in Brazil in the states of São Paulo (62), Paraná (12), and Santa Catarina (1). Of these cases, 88% (66/75) are males, the median age is 43 years, and 71% (53/75) are rural workers.

In the state of São Paulo, the municipalities that reported confirmed cases are: Eldorado (16), Iporanga (15), Barra do Turbo (6), Cajati (5), Cananeia (4), Jacupiranga (4), Pariquera-açu (4), Juquia (1), Registro (1), Serra Negra (1), Sete Barras (1), Ribeira (1), Vargem (1) and for 2 cases, the municipalities were unknown. In the state of Paraná, the municipalities with confirmed cases are: Guaraqueçaba (2), Antonina (2), São José dos Pinhais (2), Morretes (1), Andrinópolis (3), Paranaguá (1) and in 1 case, the municipality is unknown. The state of Santa Catarina reported one fatal confirmed human case of yellow fever. The case is a 36-year-old male, without vaccination, whose municipality is unknown.

Likewise, in the same reporting period of July 2018 to March 2019, 33 confirmed epizootics were reported in five federal states: São Paulo (20), Rio de Janeiro (8), Minas Gerais (1), Mato Grosso (2), and Paraná (2). In the four weeks preceding this report, epizootics have been confirmed in São Paulo and Paraná states.

Figure 1. Distribution of confirmed human yellow fever cases by year. Brazil, 1980–2018.

2018 As of Epidemiological week 26 Source: Data published by the Brazil Ministry of Health and reproduced by PAHO/WHO

Figure 2. Distribution of confirmed human yellow fever cases by epidemiological week (EW). Brazil, 2016–2019.

Epidemic curve showing two prior waves of transmission, one during the 2016-2017 seasonal period, with 778 human cases, including 262 deaths, and another during the 2017-2018 seasonal period, with 1,376 human cases, including 483 deaths.

Source: Data published by the Brazil Ministry of Health and the São Paulo and Paraná State Secretariats of Health and reproduced by PAHO/WHO

Figure 3. Distribution of epizootics and confirmed human cases in Brazil from July 2018 to March 2019.

Public health response

Given the geographical expansion in Brazil of the human cases and the epizootic wave in the last two seasonal periods, the country has had to adjust its immunization policies for yellow fever. The number of areas with recommended vaccination has increased from 3,526 municipalities in 2010 to 4,469 municipalities in 2018.

In line with the World Health Organization guidelines, Brazil has adopted a single dose vaccination scheme for yellow fever since April 2017. The use of fractional doses was also adopted to respond to outbreaks and the risk of urbanization of yellow fever, especially in large cities. This strategy was implemented in response to the 2018 yellow fever outbreak in 77 municipalities with the greatest risk for yellow fever in the states of São Paulo (54 municipalities), Rio de Janeiro (15 municipalities), and Bahía (8 municipalities).

Prior to the vaccination campaign, the states of Rio de Janeiro and São Paulo had already vaccinated about 13.2 million people. During the campaign, an additional 13.3 million people were vaccinated in São Paulo, 6.5 million in Rio de Janeiro and 1.85 million in Bahia. This resulted in a vaccination coverage of 53.6%, 55.6%, and 55.0% respectively and across all 77 municipalities with the greatest risk of yellow fever1. Furthermore, data from the Brazilian Ministry of Health indicate that vaccination coverage of at least 95% was achieved in 17.8% (71/399) of the municipalities of Paraná, 23.7% (118/497) of the municipalities of Rio Grande do Sul and 14.9% (44/295) of the municipalities of Santa Catarina.

WHO risk assessment

Further transmission is expected in the coming months based on seasonal patterns. Recent human cases of yellow fever during the current seasonal cycle have been reported in São Paulo, Paraná, and Santa Catarina states in Southeast Brazil.

The preliminary assessment of the vaccination coverage in municipalities from Paraná, Rio Grande do Sul, São Paulo, and Santa Catarina states suggests a high proportion of persons remaining susceptible and the necessity to intensify communication to encourage greater vaccine uptake among groups at risk.

The geographical distribution of human cases and epizootics from the current and previous two seasonal cycles (Figure 3) suggests southward movement of the virus, which presents further risk to the states of Paraná, Rio Grande do Sul, and Santa Catarina. Furthermore, these areas have ecosystems favourable for yellow fever transmission and borders with other countries such as Argentina, Paraguay, and Uruguay.

During the 2017-2018 yellow fever season, human cases of yellow fever acquired in Brazil were reported among travelers, most of whom arrived from countries where the vector is absent (or absent during winter).

To date, yellow fever transmission by Aedes aegypti has not been documented. An investigation conducted by the Evandro Chagas Institute and reported by the Brazil Ministry of Health revealed the detection of the yellow fever virus in Aedes albopictus mosquitoes captured in rural areas of 2 municipalities in Minas Gerais (Ituêta and Alvarenga) in 2017. The significance of this finding requires further investigation. The last documented outbreak of urban yellow fever in Brazil was recorded in 1942. The sylvatic yellow fever virus is transmitted to monkeys by forest dwelling mosquitoes such as Haemagogus and Sabethes spp. Humans who are exposed to these mosquitoes can become infected if they are not vaccinated. In entomological studies conducted in some of the affected states during the 2016-2017 outbreak, isolated Haemagogus mosquitoes were found to be positive for yellow fever, indicating predominantly sylvatic transmission.

WHO continues to monitor the epidemiological situation and review the risk assessment based on the latest available information. Currently, based on available information, WHO assesses the overall risk as High at the national level, Moderate at the regional level, and Low at the global level.

WHO advice

On 25 January 2019, PAHO/WHO alerted2 Member States about the beginning of the seasonal period for yellow fever and therefore, the highest risk of transmission to unvaccinated humans. Thus, PAHO/WHO advises Member States with areas at-risk for yellow fever to continue efforts to immunize susceptible populations and to take the necessary actions to keep travelers informed and vaccinated prior to traveling to areas where yellow fever vaccination is recommended.

WHO recommends vaccination of international travelers above 9 months of age going to Brazil. The updated areas at-risk for yellow fever transmission and the related recommendations for vaccination of international travelers were updated by WHO on 3 May 20183; the map of revised areas at risk and yellow fever vaccination recommendations is available on the WHO International Travel and Health website:

Yellow fever can easily be prevented through immunization, if vaccine is administered at least 10 days before travel. A single dose of yellow fever vaccine is sufficient to confer life-long protection against yellow fever infection: a booster dose of the vaccine is not needed and should not be required of international travelers as a condition of entry. The vaccine has been used for many decades and is safe and affordable.

WHO advises against the application of any general travel or trade restrictions to Brazil based on the information available for this event.


Information on the yellow fever situation in Brazil and other countries in the Americas is published regularly on the PAHO/WHO website and on the Brazil Ministry of Health website:

Information on the yellow fever situation in São Paulo, Paraná, and Santa Catarina states are available at:

For more information on yellow fever, please see:

WHO: All about YF


Key facts

  • Yellow fever is an acute viral haemorrhagic disease transmitted by infected mosquitoes. The “yellow” in the name refers to the jaundice that affects some patients.
  • Symptoms of yellow fever include fever, headache, jaundice, muscle pain, nausea, vomiting and fatigue.
  • A small proportion of patients who contract the virus develop severe symptoms and approximately half of those die within 7 to 10 days.
  • The virus is endemic in tropical areas of Africa and Central and South America.
  • Large 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 specie transmit the virus from person to person.
  • • Yellow fever is prevented by an extremely effective vaccine, which is safe and affordable. A single dose of yellow fever vaccine is sufficient to confer sustained immunity and life-long protection against yellow fever disease. A booster dose of the vaccine is not needed. The vaccine provides effective immunity within 10 days for 80-100% of people vaccinated, and within 30 days for more than 99% of people vaccinated.
  • Good supportive treatment in hospitals improves survival rates. There is currently no specific anti-viral drug for yellow fever.
  • The Eliminate Yellow fever Epidemics (EYE) Strategy launched in 2017 is an unprecedented initiative. With more than 50 partners involved, the EYE partnership supports 40 at-risk countries in Africa and the Americas to prevent, detect, and respond to yellow fever suspected cases and outbreaks. The partnership aims at protecting at-risk populations, preventing international spread, and containing outbreaks rapidly. By 2026, it is expected that more than 1 billion people will be protected against the disease.

Signs and symptoms

Once contracted, the yellow fever virus incubates in the body for 3 to 6 days. Many people do not experience symptoms, but when these do occur, the most common are fever, muscle pain with prominent backache, headache, loss of appetite, and nausea or vomiting. In most cases, symptoms disappear after 3 to 4 days.

A small percentage of patients, however, enter a second, more toxic phase within 24 hours of recovering from initial symptoms. High fever returns and several body systems are affected, usually the liver and the kidneys. In this phase people are likely to develop jaundice (yellowing of the skin and eyes, hence the name ‘yellow fever’), dark urine and abdominal pain with vomiting. Bleeding can occur from the mouth, nose, eyes or stomach. Half of the patients who enter the toxic phase die within 7 – 10 days.


Yellow fever is difficult to diagnose, especially during the early stages. A more severe case can be confused with severe malaria, leptospirosis, viral hepatitis (especially fulminant forms), other haemorrhagic fevers, infection with other flaviviruses (such as dengue haemorrhagic fever), and poisoning.


Polymerase chain reaction (PCR) testing in blood and urine can sometimes detect the virus in early stages of the disease. In later stages, testing to identify antibodies is needed (ELISA and PRNT).

Populations at risk

Forty seven countries in Africa (34) and Central and South America (13) are either endemic for, or have regions that are endemic for, yellow fever. A modelling study based on African data sources estimated the burden of yellow fever during 2013 was 84 000–170 000 severe cases and 29 000–60 000 deaths.

Occasionally travellers who visit yellow fever endemic countries may bring the disease to countries free from yellow fever. In order to prevent such 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.

In past centuries (17th to 19th), yellow fever was transported to North America and Europe, causing large outbreaks that disrupted economies, development and in some cases decimated populations.


The yellow fever virus is an arbovirus of the flavivirus genus and is transmitted by mosquitoes, belonging to the Aedes and Haemogogus species. The different mosquito species live in different habitats – some breed around houses (domestic), others in the jungle (wild), and some in both habitats (semi-domestic). There are 3 types of transmission cycles:

  • Sylvatic (or jungle) yellow fever: In tropical rainforests, monkeys, which are the primary reservoir of yellow fever, are bitten by wild mosquitoes of the Aedes and Haemogogus species, which pass the virus on to other monkeys. Occasionally humans working or travelling in the forest are bitten by infected mosquitoes and develop yellow fever.
  • Intermediate yellow fever: In this type of transmission, semi-domestic mosquitoes (those that breed both in the wild and around households) infect both monkeys and people. Increased contact between people and infected mosquitoes leads to increased transmission and many separate villages in an area can develop outbreaks at the same time. This is the most common type of outbreak in Africa.
  • Urban yellow fever: Large epidemics occur when infected people introduce the virus into heavily populated areas with high density of Aedes aegypti mosquitoes and where most people have little or no immunity, due to lack of vaccination or prior exposure to yellow fever. In these conditions, infected mosquitoes transmit the virus from person to person.


Good and early supportive treatment in hospitals improves survival rates. There is currently no specific anti-viral drug for yellow fever but specific care to treat dehydration, liver and kidney failure, and fever improves outcomes. Associated bacterial infections can be treated with antibiotics.


1. Vaccination

Vaccination is the most important means of preventing yellow fever.

The yellow fever vaccine is safe, affordable and a single dose provides life-long protection against yellow fever disease. A booster dose of yellow fever vaccine is not needed.

Several vaccination strategies are used to prevent yellow fever disease and transmission: routine infant immunization; mass vaccination campaigns designed to increase coverage in countries at risk; and vaccination of travellers going to yellow fever endemic areas.

In high-risk areas where vaccination coverage is low, prompt recognition and control of outbreaks using mass immunization is critical. It is important to vaccinate most (80% or more) of the population at risk to prevent transmission in a region with a yellow fever outbreak.

There have been rare reports of serious side-effects from the yellow fever vaccine. The rates for these severe ‘adverse events following immunization’ (AEFI), when the vaccine provokes an attack on the liver, the kidneys or on the nervous system are between 0 and 0.21 cases per 10 000 doses in regions where yellow fever is endemic, and from 0.09 to 0.4 cases per 10 000 doses in populations not exposed to the virus (1).

The risk of AEFI is higher for people over 60 years of age and anyone with severe immunodeficiency due to symptomatic HIV/AIDS or other causes, or who have a thymus disorder. People over 60 years of age should be given the vaccine after a careful risk-benefit assessment.

People who are usually excluded from vaccination include:

  • infants aged less than 9 months;
  • pregnant women – except during a yellow fever outbreak when the risk of infection is high;
  • people with severe allergies to egg protein; and
  • people with severe immunodeficiency due to symptomatic HIV/AIDS or other causes, or who have a thymus disorder.

In accordance with the International Health Regulations (IHR), countries have the right to require travellers to provide a certificate of yellow fever vaccination. If there are medical grounds for not getting vaccinated, this must be certified by the appropriate authorities. The IHR are a legally binding framework to stop the spread of infectious diseases and other health threats. Requiring the certificate of vaccination from travellers is at the discretion of each State Party, and it is not currently required by all countries.

2. Vector control

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. For yellow fever, vector surveillance targeting Aedes aegypti and other Aedes species will help inform where there is a risk of an urban outbreak.

Understanding the distribution of these mosquitoes within a country can allow a country to prioritize areas to strengthen their human disease surveillance and testing, and consider vector control activities. There is currently a limited public health arsenal of safe, efficient and cost-effective insecticides that can be used against adult vectors. This is mainly due to the resistance of major vectors to common insecticides and the withdrawal or abandonment of certain pesticides for reasons of safety or the high cost of re-registration.

Historically, mosquito control campaigns successfully eliminated Aedes aegypti, the urban yellow fever vector, from most of Central and South America. However, Aedes aegypti has re-colonized urban areas in the region, raising a renewed risk of urban yellow fever. Mosquito control programmes targeting wild mosquitoes in forested areas are not practical for preventing jungle (or sylvatic) yellow fever transmission.

Personal preventive measures such as clothing minimizing skin exposure and repellents are recommended to avoid mosquito bites. The use of insecticide-treated bed nets is limited by the fact that Aedes mosquitos bite during the daytime.

3. Epidemic preparedness and response

Prompt detection of yellow fever and rapid response through emergency vaccination campaigns are essential for controlling outbreaks. However, underreporting is a concern – the true number of cases is estimated to be 10 to 250 times what is now being reported.

WHO recommends that every at-risk country have 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 response

In 2016, two linked urban yellow fever outbreaks – in Luanda (Angola) and Kinshasa (Democratic Republic of the Congo), with wider international exportation from Angola to other countries, including China – have shown that yellow fever poses a serious global threat requiring new strategic thinking. The Eliminate Yellow Fever Epidemics (EYE) Strategy was developed to respond to the increased threat of yellow fever urban outbreaks with international spread. Steered by WHO, UNICEF, and Gavi, the Vaccine Alliance, EYE supports 40 countries and involves more than 50 partners.

The global EYE Strategy is guided by three strategic objectives:

  1. protect at-risk populations
  2. prevent international spread of yellow fever
  3. contain outbreaks rapidly.

These objectives are underpinned by five competencies of success:

  1. affordable vaccines and sustained vaccine market
  2. strong political commitment at global, regional and country levels
  3. high-level governance with long-term partnerships
  4. synergies with other health programmes and sectors
  5. research and development for better tools and practices.

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).


The EYE partnership supports yellow fever high and moderate risk countries in Africa and the Americas by strengthening their surveillance and laboratory capacity to respond to yellow fever cases and outbreaks. EYE partners also support the implementation and sustainability of routine immunization programmes and vaccination campaigns (preventive, pre-emptive, reactive) whenever and wherever needed.


To guarantee a rapid and effective response to outbreaks, an emergency stockpile of 6 million doses of yellow fever vaccine, funded by Gavi, is continually replenished. This emergency stockpile is managed by the International Coordinating Group for Vaccine Provision, for which WHO serves as secretariat.


It is expected that by the end of 2026, more than 1 billion people will be protected against yellow fever through vaccination.

(1) WHO. Detection and investigation of serious adverse events following yellow fever vaccination, Geneva, World Health Organization (WHO); 2008.

“Scary Monsters and Nice Sprites” in the fight against Aedes Aegyptii

Acta Tropica

“…..The observation that such music can delay host attack, reduce blood feeding, and disrupt mating provides new avenues for the development of music-based personal protective and control measures against Aedes-borne diseases…..”

Acta Tropica

Volume 194, June 2019, Pages 93-99

Acta Tropica

The electronic song “Scary Monsters and Nice Sprites” reduces host attack and mating success in the dengue vector Aedes aegypti

Sudan set to protect over 8 million people with its largest ever yellow fever vaccination drive


WHO monitors the adminidtration of yellow fever vaccine by a mobile vaccination team in Blue Nile state

“…..The campaign forms a critical part of Sudan’s ongoing work to protect all populations against yellow fever epidemics, in alignment with the global Eliminate Yellow fever Epidemics (EYE) Strategy. The country plans to complement these yellow fever mass campaigns and ensure long-term protection through the introduction of yellow fever vaccination into routine immunization in the coming months.

“We acknowledge the commitment of the health authorities in Sudan to avail cash and fuel during this economic crisis to ensure that their people, especially children, are protected with a quality vaccine which will contribute to health security and making the world safer,” said Dr Naeema Al-Gasseer, WHO Representative in Sudan.

“Yellow fever vaccination is the most important tool we have to prevent yellow fever outbreaks. The vaccine will be freely available to any eligible person and will provide life-long protection against the disease. While protecting yourself against mosquito bites is important to reduce the risk of many diseases, only vaccination can eliminate the risk of yellow fever outbreaks,” she added……”


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