Yellow Fever and the Traveller's Vaccine: What You Need to Know Before You Go
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Yellow fever is a mosquito-borne viral disease that has shaped human history across sub-
Saharan Africa and tropical South America for centuries. Despite the availability of one of the
most effective vaccines ever developed, yellow fever continues to cause outbreaks, and
unvaccinated travellers remain at real risk (1). In 2024 alone, 61 confirmed human cases were reported in Bolivia, Brazil, Colombia, Guyana, and Peru, with a case fatality rate of
approximately 50%. By May 2025, that number had already risen to 235 confirmed cases and 96 deaths in South America, a trend that public health authorities have flagged as a high-priority concern (2).
The Global Picture for Travellers
The risk for travellers is not merely theoretical. Between 2016 and mid-2021, more than 37
travel-associated yellow fever cases were recorded in unvaccinated individuals from non-
endemic countries, including at least 15 European travellers and one American visiting Peru
(3). The 2016 outbreak in Angola was a turning point: over 250,000 Chinese workers were
present in the country during the epidemic, and 11 confirmed cases were subsequently reported in China — the first time yellow fever had ever reached Asia (4). This episode illustrated how the movement of unvaccinated people can carry the virus far beyond its traditional boundaries. Surveillance limitations mean that confirmed case counts likely underestimate the true burden of disease. Many infected individuals, especially those with mild illness, never seek medical attention, and local surveillance systems in endemic regions are not always able to detect every case (3). In short, the absence of reported cases from a destination does not mean the risk is absent. The COVID-19 pandemic also disrupted routine vaccination programmes globally, leaving growing gaps in population immunity that have contributed to the upsurge in cases seen from 2024 onwards (2).
How the Virus Infects the Human Body
Yellow fever virus (YFV) belongs to the Flaviviridae family, the same group that includes
dengue, Zika, and West Nile virus. It is a single-stranded, positive-sense RNA virus, roughly
40–50 nanometres in diameter, with a lipid envelope bearing two key glycoproteins: the
envelope (E) protein and the membrane (M) protein. These surface proteins are responsible for attaching the virus to host cells and facilitating its entry (1).
Transmission occurs through the bite of infected mosquitoes, primarily Aedes aegypti in urban settings and Haemagogus and Sabethes species in forested areas. After inoculation, the virus undergoes an incubation period of typically 3 to 6 days, during which it replicates in local lymph nodes before entering the bloodstream (3). From there, it spreads to the liver, kidneys, heart, and spleen. The liver is the primary target organ: viral replication within hepatocytes triggers cell death, leading to the characteristic yellow color in eyes and skin (jaundice) that gives the disease its name. Kidney injury can follow, resulting in protein loss in the urine and in severe cases, kidney failure (1,5). Clinically, the disease unfolds in stages. The initial phase, lasting 3 to 4 days, resembles many other febrile illnesses: fever, chills, headache, muscle aches, nausea, and vomiting. Around 85% of patients recover after this stage. However, approximately 15% enter a "period of intoxication," characterised by the return of high fever, jaundice, bleeding tendencies (due to the virus attacking the blood clotting system), and multi-organ failure. Among those who reach this severe stage, the fatality rate can exceed 50% (1,3,5).
How the Virus Evades the Immune System
Like other flaviviruses, yellow fever virus has evolved sophisticated strategies to delay and
dampen the body's early defences. The innate immune system (the rapid, non-specific first line of defence) normally detects viral RNA through molecular sensors known as Toll-like
receptors (TLRs) and cytoplasmic receptors such as RIG-I and MDA5. These sensors trigger
the production of type I interferons (IFN-α and IFN-β), signalling proteins that alert neighbouring cells and initiate an antiviral state (6). YFV non-structural proteins, particularly NS4A, NS4B, and NS5, actively interfere with this interferon signalling pathway, blunting the cell's ability to mount a timely response (6). By suppressing the interferon cascade, the virus effectively buys itself time to replicate and spread before the adaptive immune system, can mobilise to create a long-term defense against the viral particles. In severe cases, this initial suppression gives way to an overactivated immune response characterised by high levels of pro-inflammatory cytokines such as TNF-α, IL-6, and IL-1β. This so-called "cytokine storm" contributes significantly to tissue damage in the liver and other organs, meaning that some of the most dangerous manifestations of yellow fever are partly the result of the body's own inflammatory response spiralling out of control (1,6,7).
The Vaccine: A Live Attenuated Masterpiece
The yellow fever vaccine known as YF-17D is a live attenuated vaccine, meaning it contains a weakened but living version of the virus. It is not a DNA vaccine, an mRNA vaccine, or a
protein subunit vaccine, but rather a carefully modified whole virus that can replicate in the
recipient to a limited degree, generating a robust immune response without causing disease (8). The 17D strain was derived from the wild-type Asibi strain through 204 serial passages in
mouse embryo tissue and chicken embryo cells beginning in 1937. This prolonged process of
laboratory adaptation introduced 31 amino acid changes throughout the viral genome (8,9).
Crucially, the resulting vaccine strain has an unusually high-fidelity RNA replication complex,
meaning it makes far fewer copying errors than the original virus, and is therefore far less likely to mutate back towards virulence. This genetic stability is one of the cornerstones of the
vaccine's outstanding safety record (7).
Once injected, the attenuated virus replicates sufficiently to be detected by the immune system, triggering a cascade of responses across both innate and adaptive immunity. The vaccine activates multiple Toll-like receptors (TLR-2, 7, 8, and 9) on dendritic cells, the sentinels of the immune system, stimulating the production of interferon and pro-inflammatory cytokines (9,10). These early signals orchestrate a broad adaptive immune response: neutralising antibodies are generated that target the envelope protein of the virus, while cytotoxic CD8+ T cells are primed to destroy cells that become infected. The resulting immunity is characterised by a balanced Th1/Th2 helper T-cell profile, a hallmark of durable, multi-pronged protection (10).
A single dose induces seroconversion, meaning the development of measurable protective
antibodies, in approximately 95–99% of recipients within 10 days, with protection rising to
99% by 30 days after vaccination (11). Neutralising antibodies have been detected in
individuals more than 30 to 60 years after a single dose, and the World Health Organization
updated its guidance in 2016 to state that one dose provides lifelong protection for most
people(8). Globally, over 600 million doses of YF-17D have been administered, making it one
of the most widely used vaccines in history (8,11).
Two licensed substrains are currently in use: 17D-204 (marketed as YF-VAX® and Stamaril®) and 17DD (used primarily in Brazil). Both are manufactured in embryonated chicken eggs and are considered equally safe and immunogenic (8). For travellers, the vaccine is typically administered at an approved yellow fever vaccination centre, and a valid certificate under the International Health Regulations is issued, required for entry to many countries in endemic regions (3).
Research into next-generation alternatives is ongoing. mRNA-based yellow fever vaccine
candidates, which encode the pre-membrane and envelope proteins of the virus encapsulated in lipid nanoparticles, have shown promising results in animal studies — inducing humor and cellular immune responses comparable to the licensed 17D vaccine (9,11). Such platforms could eventually help address supply constraints, as egg-based production remains limited and vulnerable to disruption during large outbreaks (8).
Practical Advice for Travellers
Any person travelling to sub-Saharan Africa or tropical South America should consult a travel
health clinic well in advance of departure. Vaccination should ideally be completed at least 10
days before travel to allow full immunity to develop (11).Those who received the vaccine more than a decade ago under the old guidelines need not be revaccinated for most destinations, though a small number of countries still require proof of a dose received within 10 years. Mosquito bite prevention, including the use of repellents, long clothing, and permethrin-treated gear, remains essential, as no vaccine provides 100% certainty (3).
Yellow fever is a preventable disease. With one of the most effective vaccines in the history of
medicine available, the principal risk factor for travellers remains a simple and correctable one: being unvaccinated.
Author: Lina María García-Taboada
References
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