Yellow Fever

Table of Contents

  1. Overview
  2. Epidemiology and Transmission Cycles
  3. Pathophysiology
  4. Clinical Phases
  5. Intoxication Phase and Severe Disease
  6. Diagnosis
  7. Treatment and Supportive Care
  8. Yellow Fever Vaccine
  9. Travel Requirements and Certificate
  10. Complications and Prognosis
  11. References
  12. Connections
  13. Featured Videos

1. Overview

Yellow fever (YF) is an acute hemorrhagic viral disease caused by the yellow fever virus, a member of the genus Flavivirus within the family Flaviviridae — the same genus as dengue, Zika, West Nile, and Japanese encephalitis viruses. The disease takes its name from the jaundice that develops in severe cases, turning the skin and sclerae yellow due to hepatic failure and profound hyperbilirubinemia. Before the availability of an effective vaccine, yellow fever caused devastating urban epidemics across Africa, the Americas, and even southern Europe.

Yellow fever remains one of the most important vaccine-preventable viral hemorrhagic fevers. An estimated 200,000 cases and 30,000 deaths occur annually, concentrated in sub-Saharan Africa and tropical South America. Despite the existence of the highly effective 17D live-attenuated vaccine — one of the most successful vaccines ever developed — large outbreaks continue to occur in under-vaccinated populations, including a major urban Angola-DRC outbreak in 2015–2016 that strained global vaccine supplies, and repeated outbreaks in Brazil between 2016 and 2019.

The virus is transmitted by the bite of infected female mosquitoes. The disease follows a characteristic biphasic course in a minority of patients: after an initial febrile illness and a brief period of apparent remission, approximately 15% of infected individuals enter a severe toxic phase characterized by jaundice, acute kidney injury, hemorrhage, and multi-organ failure, with a case fatality rate of 20–50% in this group.

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2. Epidemiology and Transmission Cycles

Yellow fever virus circulates in three distinct transmission cycles that define its epidemiology:

Sylvatic (jungle) cycle: In forest environments, the virus circulates between nonhuman primates (monkeys) and arboreal mosquitoes, principally Haemagogus and Sabethes species in South America, and Aedes africanus and related species in Africa. Monkeys are the primary vertebrate reservoir. Humans become incidental dead-end hosts when they enter the forest — typically hunters, loggers, agricultural workers, and eco-tourists. Jungle yellow fever outbreaks in Brazil from 2016 to 2019 caused over 2,000 confirmed human cases and mass die-offs of howler monkeys serving as sentinel events.

Intermediate (savannah) cycle: In Africa, semi-domestic Aedes mosquitoes infect both monkeys and humans in villages near forest margins. This is the most common transmission pattern in Africa and accounts for the majority of recent African outbreaks, including recurring outbreaks in Ethiopia, Nigeria, and the Democratic Republic of Congo.

Urban cycle: When a viremic individual enters a city with high Aedes aegypti density and low vaccination coverage, person-to-mosquito-to-person amplification can produce explosive urban epidemics. Aedes aegypti is the sole vector responsible for urban transmission. The 2015–2016 Angola-Luanda outbreak — the largest urban YF epidemic in decades — infected over 960 confirmed cases across Angola and spread to Kinshasa, DRC, via infected travelers.

The endemic zone spans approximately 47 countries: 34 in Africa and 13 in the Americas. Africa bears roughly 90% of the global yellow fever burden. In South America, Brazil, Bolivia, Peru, Colombia, and Ecuador are considered high-endemicity countries. Travelers to endemic areas who are not vaccinated face a real risk; travel-associated fatal yellow fever cases have been documented in unvaccinated visitors to Brazil and West Africa within recent decades.

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3. Pathophysiology

Following inoculation by an infected mosquito bite, yellow fever virus replicates initially in local dendritic cells and macrophages at the bite site, then migrates to draining lymph nodes and disseminates hematogenously. The primary target organs are the liver, kidneys, heart, and spleen.

Hepatic injury is the central pathological feature and responsible for the disease's defining characteristics. The hallmark histopathological finding is midzonal hepatic necrosis — a distinctive pattern in which hepatocytes in the middle zone of the liver lobule (zone 2) are preferentially destroyed, while cells in zones 1 (periportal) and 3 (centrilobular) are relatively spared. This midzonal distribution is pathognomonic and distinguishes yellow fever hepatitis histologically from other viral hepatitides and from acetaminophen toxicity (which preferentially destroys zone 3). Councilman bodies (also called Councilman-Rokitansky bodies) — eosinophilic acidophilic inclusions within or extruded from hepatocytes — are the result of apoptotic hepatocyte death and are the classic microscopic hallmark of yellow fever. They are identified at autopsy or, rarely, on liver biopsy obtained during the acute illness.

Severe hepatic necrosis impairs coagulation factor synthesis, producing a profound coagulopathy. Decreased production of factors II, V, VII, IX, and X combines with thrombocytopenia to create hemorrhagic diathesis. Disseminated intravascular coagulation (DIC) may supervene, exacerbating bleeding from multiple sites simultaneously.

Renal tubular necrosis results from a combination of direct viral cytopathic effect on tubular epithelial cells, hemodynamic instability and renal hypoperfusion, and potentially direct immune-mediated injury. Albuminuria is an early and sensitive marker of renal involvement, appearing even in mild cases.

Myocarditis contributes to cardiovascular instability and explains the clinically important sign of Faget's sign (sphygmothermic dissociation): despite high fever, the pulse rate remains inappropriately slow or fails to rise proportionally — a consequence of direct myocardial viral involvement slowing cardiac conduction. This bradycardia relative to temperature elevation is shared with typhoid fever and leptospirosis.

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4. Clinical Phases

Yellow fever follows a triphasic clinical course. Not all patients complete all three phases; the majority recover after the initial infection phase without progressing to the severe intoxication phase.

Phase 1 — Infection phase (acute phase, days 1–4): After an incubation period of 3 to 6 days following the infecting mosquito bite, the illness begins abruptly with:

During this phase, yellow fever virus is detectable in the bloodstream (viremia), and patients are capable of infecting biting mosquitoes. Standard CBC may show leukopenia; transaminases begin to rise. Albuminuria may be detected on urinalysis.

Phase 2 — Remission (12–48 hours): Fever abates, symptoms diminish, and patients often feel subjectively improved. The majority of patients — approximately 85% — recover fully at this point without entering the third phase. This period of apparent improvement can be misleading, and premature reassurance is clinically dangerous because 15% of patients will relapse.

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5. Intoxication Phase and Severe Disease

Approximately 15% of patients who reach the remission phase re-enter illness within 24–48 hours in what is termed the intoxication phase — a term capturing the systemic toxicity of severe, multi-organ involvement. This phase carries a case fatality rate of 20–50%.

Cardinal features of the intoxication phase:

Laboratory findings in the intoxication phase include markedly elevated AST (often disproportionately higher than ALT — a pattern called AST predominance that reflects myocardial as well as hepatic injury), hyperbilirubinemia, prolonged prothrombin time and aPTT, thrombocytopenia, rising creatinine, and metabolic acidosis. Hypoglycemia due to hepatic glycogen depletion may occur and requires monitoring.

Death, when it occurs, typically happens between days 7 and 10 from cardiovascular failure, renal failure, or hemorrhagic shock. Survivors of the intoxication phase generally recover fully, as the liver has regenerative capacity, though recovery may take weeks. Jaundice deepens for several days before beginning to resolve. Yellow fever does not establish chronicity or carrier states.

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6. Diagnosis

Clinical diagnosis of yellow fever in the early infection phase is difficult because the presentation is non-specific and overlaps extensively with dengue fever, malaria, leptospirosis, viral hepatitis, and other febrile illnesses endemic in the same geographic regions. A key diagnostic clue is the travel and vaccination history: an unvaccinated individual presenting with acute febrile illness after travel to or residence in an endemic region in Africa or South America should be evaluated for yellow fever.

Laboratory confirmation methods:

In endemic or epidemic settings, specimens must be shipped to WHO-accredited reference laboratories. In the United States, testing is performed at CDC's Special Pathogens Branch. Yellow fever is a nationally notifiable disease in the United States; clinicians who suspect the diagnosis should notify their state health department immediately.

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7. Treatment and Supportive Care

There is no approved antiviral therapy for yellow fever. No agent — including ribavirin, interferon alfa, or nucleoside analogs — has demonstrated clinical efficacy in controlled trials. Management is entirely supportive, aimed at maintaining homeostasis and preventing or managing complications while the immune response controls viral replication.

Supportive care principles:

Intensive care unit admission is required for severe disease. Mortality correlates with the severity of hepatic failure (bilirubin, coagulation), the degree of renal failure, and the extent of hemorrhage. Liver transplantation has been considered experimentally but is not a standard intervention given the self-limited nature of the illness in survivors.

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8. Yellow Fever Vaccine

The 17D live-attenuated yellow fever vaccine, developed in the 1930s by Max Theiler and colleagues at the Rockefeller Foundation, is widely regarded as one of the greatest achievements in vaccinology. Theiler was awarded the Nobel Prize in Physiology or Medicine in 1951 for its development. The vaccine is produced by passaging wild-type yellow fever virus through embryonated chicken eggs until attenuation is achieved. Three substrains are in current production: 17D-204 (used in most vaccines worldwide), 17DD (produced in Brazil), and 17D-213.

Efficacy and durability: A single dose produces neutralizing antibody responses in 80–100% of recipients within 10–14 days and in 99% within 28 days. Protective immunity is durable; WHO revised its recommendation in 2013, concluding that a single lifetime dose is sufficient for most individuals — boosters are no longer routinely recommended or required for international travel certificates. However, booster doses after 10 years may be considered for specific groups: travelers whose first dose was given in infancy, women vaccinated during pregnancy, HIV-infected individuals, and persons who received a previous dose following bone marrow transplantation.

Vaccine administration and scheduling: Administered as a single 0.5 mL subcutaneous injection. International Certificate of Vaccination or Prophylaxis (ICVP, "yellow card") is valid for entry to many countries immediately after vaccination — no waiting period now applies under the 2016 International Health Regulations revision, reflecting the lifetime duration of protection.

Contraindications to yellow fever vaccine:

Serious adverse events (rare):

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9. Travel Requirements and the International Certificate

Yellow fever is the only vaccine for which proof of administration can be legally required for entry into certain countries under the International Health Regulations (IHR 2005). This requirement is enforced at entry points via the International Certificate of Vaccination or Prophylaxis (ICVP) — commonly called the "yellow card" — an official WHO document that must be completed by an authorized vaccination provider and stamped with an official stamp.

Two distinct entry-requirement categories exist:

For travelers with medical contraindications to vaccination, an official waiver letter can sometimes be obtained from an authorized yellow fever vaccination center, though not all countries accept waivers. The CDC Travel Health website maintains up-to-date country-specific requirements at wwwnc.cdc.gov/travel/destinations/list.

Vaccination must be administered at an authorized yellow fever vaccination center (designated by national health authorities) to produce a legally valid ICVP. In the United States, these centers are listed on the CDC website. Vaccines administered outside these centers — even by licensed physicians — do not produce an internationally valid certificate.

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10. Complications and Prognosis

The overall case fatality rate for yellow fever across all infections — including the majority of mild cases that never reach the intoxication phase — is estimated at 5–10%. However, this figure obscures a critically important bimodal distribution: patients who progress to the intoxication phase face a case fatality rate of 20–50%, while the 85% who recover at or before the remission phase almost universally survive.

Major complications:

Prognostic factors associated with mortality: peak bilirubin above 10 mg/dL, creatinine above 2.0 mg/dL, prothrombin time ratio above 1.5, thrombocytopenia below 50,000/mm³, and the presence of shock. Patients who survive the intoxication phase typically recover fully; yellow fever does not cause chronic hepatitis, cirrhosis, or long-term liver disease. Natural infection confers lifelong immunity, as does successful vaccination.

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11. References

  1. Monath TP, Vasconcelos PF. Yellow fever. J Clin Virol. 2015;64:160–173. PMID 25453327
  2. Tomori O. Yellow fever: the recurring plague. Crit Rev Clin Lab Sci. 2004;41(4):391–427. PMID 15487583
  3. Quaresma JA, Barros VL, Pagliari C, et al. Revisiting the liver in human yellow fever: virus-induced apoptosis in hepatocytes associated with TGF-beta, TNF-alpha and NK cells activity. Virology. 2006;345(1):22–30. PMID 16242173
  4. Barrett AD, Teuwen DE. Yellow fever vaccine — how does it work and why do rare cases of serious adverse events take place? Curr Opin Immunol. 2009;21(3):308–313. PMID 19520559
  5. Staples JE, Bocchini JA Jr, Rubin L, Fischer M; Centers for Disease Control and Prevention (CDC). Yellow fever vaccine booster doses: recommendations of the Advisory Committee on Immunization Practices, 2015. MMWR Morb Mortal Wkly Rep. 2015;64(23):647–650. PMID 26086635
  6. Johansson MA, Vasconcelos PF, Staples JE. The whole iceberg: estimating the incidence of yellow fever virus infection from the number of severe cases. Trans R Soc Trop Med Hyg. 2014;108(8):482–487. PMID 24980556
  7. Shearer FM, Moyes CL, Pigott DM, et al. Global yellow fever vaccination coverage from 1970 to 2016: an adjusted retrospective analysis. Lancet Infect Dis. 2017;17(11):1209–1217. PMID 28867505
  8. Vasconcelos PF. Yellow fever in Brazil: thoughts and hypotheses on the emergence in previously free areas. Rev Saude Publica. 2010;44(6):1144–1153. PMID 21107502
  9. Kallas EG, D'Elia Zanella LG, Moreira CH, et al. Predictors of mortality in patients with yellow fever: an observational cohort study. Lancet Infect Dis. 2019;19(7):750–758. PMID 31130284
  10. Lindsey NP, Rabe IB, Miller ER, Fischer M, Staples JE. Adverse event reports following yellow fever vaccination, 2007–13. J Travel Med. 2016;23(5):taw045. PMID 27520328
  11. de Oliveira Figueiredo P, Goncalves de Aguiar Marques B, Ladislau de Oliveira T, et al. Evidence-based review of the literature on dengue, Zika, chikungunya, and yellow fever arboviruses. Rev Soc Bras Med Trop. 2022;55(suppl 1):e0048. PMID 35019099
  12. World Health Organization. Yellow fever. Wkly Epidemiol Rec. 2013;88(27):269–284. PMID 23909013

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12. Connections

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