Yellow Fever - Vaccine, Outbreaks and Symptoms

Yellow fever is an infectious disease of viral origin, transmitted through the mosquito bite of the genus Haemagogus. It is a disease that currently affects only countries in central Africa and northern South America, including Brazil, where it is usually confined to wild areas.

Haemagogus mosquito
Haemagogus mosquito

Wild & urban yellow fever

Yellow fever is divided into two types:
  • 1. Wild yellow fever - when infection runs in forest and sawed regions. This form is transmitted by mosquitoes of the genus Haemagogus.
  • 2. Urban yellow fever - when infection occurs in large cities and urbanized areas. This form is transmitted by the mosquito Aedes aegypti, the same one that transmits dengue.

The two forms of yellow fever are identical, this division has only epidemiological character to facilitate the control of the disease in urbanized areas (I explain later).

Yellow fever in urban areas has not occurred since 1942. In recent decades, including the outbreak of 2017, yellow fever cases identified in large centers have occurred in unvaccinated people who have traveled to areas of forest and have returned to urban areas before symptoms appear . In these cases, we consider that the patient acquired the wild form, since the contamination occurred in forest areas.


Yellow fever is not transmitted from human to human. Only mosquitoes transmit the virus. As there are currently no cases of urban yellow fever, the only form of transmission is the wild. This means that an unvaccinated human must go to forest areas where there are sick monkeys. The transmission occurs when a mosquito of the genus Haemagogus stings a contaminated monkey, acquires the virus and days later it stings an unvaccinated human.

Wild yellow fever is a disease that occurs mainly in monkeys, and humans are hosts of the virus.

In order for the urban form to recur, it is enough for a person contaminated by the wild form to return to a non-endemic city, such as Rio de Janeiro, for example, and be stung by the Aedes aegypti mosquito within the first 5 days of infection.

The urban form has not yet returned thanks to an effective control of the Ministry of Health, which provides vaccination for the entire population in endemic areas and monitors cases of yellow fever in monkeys, increasing vaccination coverage in the population every time there are primate outbreaks.


Individuals residing in indene areas, ie without wild circulation, should also be vaccinated 10 days before traveling to endemic areas. Not only will the people who do ecotourism be vaccinated. Even if the trip is only for business, the individual always staying in urban areas, vaccination must be carried out. People who live in indonesian areas and who will not travel to endemic areas do not need to be vaccinated.

The vaccine for yellow fever lasts for 10 years, being necessary the reinforcement after this period.

Among the contraindications to the yellow fever vaccine are:
  • Children under 6 months of age
  • People with egg allergy
  • Immunosuppressed people
  • Pregnant women


The incubation period (time interval between contamination and the onset of the first symptoms) of yellow fever is 3 to 7 days. Because the disease can take up to a week to manifest itself, many patients who acquire the wild form only develop symptoms after they have returned from their journey to the endemic area.

Most infected people do not develop the disease, presenting at most some non-specific viral symptoms. In patients who develop symptoms of yellow fever, the initial manifestations are high fever with sweating and chills, malaise, headache, muscle pain and tiredness. Nausea, vomiting, or diarrhea may also occur. After three or four days, most patients recover completely, being immunized against the disease for the rest of their lives.

In about 15% of cases, however, yellow fever develops severely. The patient shows a slight improvement after two or three days of illness, giving the impression that he will recover, but the fever returns with full force, this time accompanied by severe abdominal pains, nausea and vomiting, purple spots on the skin, bleeding gums , nose or stomach, and yellowish skin, called jaundice. Noble organs such as liver, lungs and kidneys can go bankrupt leading the patient to death. Severe forms have lethality above 50%, even with proper medical treatment.

The diagnosis is made through blood tests.


There is no specific treatment for yellow fever. There is no drug that cures the disease, which makes vaccination even more important.

In severe cases the patient is hospitalized to control the complications and to monitor bleeding. Some patients who have kidney failure need hemodialysis. In cases of respiratory failure, mechanical ventilation may be required.

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