Yellow fever: what is it, symptoms, vaccine (reactions), in Brazil

The number of yellow fever cases in Brazil is considerably low. According to the Ministry of Health, there are a maximum of 85 diagnoses per year, considering the last 15 years.

However, the disease comprises a dangerous, life-threatening condition if the correct interventions are not made.

Although there is a vaccine available in the vaccination schedule, immunization estimates are not always reached. Therefore, it is important that information about the disease is reinforced.


What is yellow fever?

Yellow fever is a serious viral infection, similar to the flu , normally transmitted by the Aedes aegypti mosquito . It is characterized by high fever and jaundice (yellowing in the body), which is why the disease is called that way.

It is considered a zoonosis (diseases that affect humans and animals), since it also affects some other primates.

Those infected by the virus do not always develop the disease, and may experience weak symptoms that pass within a few days.

However, those who develop the condition may suddenly experience symptoms such as high fever, chills, tiredness , headache , muscle pain, nausea and vomiting.

When transmitted to humans, the yellow fever virus can damage the liver and other internal organs, in addition to being potentially fatal. The disease occurs mainly in areas of southern Africa and South America, affecting travelers and residents.

Some studies have shown that the disease is more common in the months of December to May. As this is the rainy season, there is an increase in mosquito populations, which favors the circulation of the virus.

Yellow fever in Brazil

In 2018, the number of yellow fever cases grew by more than 400% in São Paulo, according to data from the State Department of Health. Of the 537 cases registered in the state, 185 died.

In the first half of 2017, Brazil had an outbreak of yellow fever and, as of October of that year, the city of São Paulo had several parks closed after confirmation of cases of infected monkeys, which could start a new cycle of transmission of the disease. disease.

In April 2018, these parks were reopened, as the risk had decreased, however, the disease is now considered to be “here to stay”, as yellow fever cases are recorded throughout the year in Brazil.

But in 2019, the disease manifested itself in the first days of the year. In Paraná, a man, who had not received the vaccine, was diagnosed as the 1st case of infection in the state since 2015.

The state of São Paulo, until January 22, recorded 12 confirmed cases and 6 deaths since the beginning of the year.

From the new occurrences, the Health Departments warn about the care with vaccination and inform about the intensification of the inspection in streets, houses and spaces that can accumulate standing water, which facilitates the proliferation of the transmitting mosquito.

Types of yellow fever

Yellow fever can be divided according to the mosquito and transmission area:

Yellow Urban Fever (FAU)

In the urban area, the main transmitter of yellow fever is Aedes aegypti , a vector responsible for several other endemic diseases, such as dengue , chikungunya and zika.

Yellow Wild Fever (FAS)

In rural areas, yellow fever is transmitted by mosquitoes of the Haemagogus and Sabethes species , which are present in the forests and river banks. The cases recorded in other primates are believed to come from these mosquitoes, but humans can also be bitten by them.

Stages of yellow fever

Yellow fever has two stages that can be considered as mild (acute) and as (toxic).

About 90% of diagnoses are acute and do not represent any risk to life. But the other 10% of patients, according to estimates, develop the toxic form, which is potentially fatal.


The acute phase is the initial stage of the disease, with symptoms such as fever, pain and indisposition, which manifest themselves suddenly. In up to 4 days, on average, the patient shows complete improvement of the condition.


About 10% of patients, after the acute phase, have only one period or stage of symptomatic remission. There is a decrease in symptoms and a feeling of improvement, which lasts for a few hours (usually between 24 and 48 hours).

Then, the patient gets worse. The period is called the intoxication phase, in which the virus lodges in the organs, causing damage to the body’s systems.

The symptoms, in this phase, are more accentuated and others appear that were not present in the acute phase, such as hemorrhages (mainly gastrointestinal ones), changes in the color and quantity of urine (oliguria), in addition to multiple organ failure.

In such cases, between 20% and 50% may die, according to the Ministry of Health.


Yellow fever is caused by a virus of the Flavivirus family , which is transmitted through the bite of Aedes aegypti mosquitoes (the same that transmits diseases such as dengue, chikungunya and zika), Sabethes and those belonging to the species Haemagogus , such as Haemagogus janthinomys and the Haemagogus leucocelaenus .

There is no evidence that Aedes albopictus is capable of transmitting the yellow fever virus and, to date, no representative of this species has yet been found that has been infected by the virus in nature. However, the mosquito was susceptible to infection in the laboratory.

Therefore, the proliferation of this type of mosquito is worrying, especially because of its ability to adapt to different environments (urban, rural, among others), which can transform it into a bridge between the cycles of wild and urban transmission.

The virus incubation period is 3 to 7 days after the bite.


Yellow fever is transmitted by the bite of an infected female mosquito. The virus can infect humans, some primates (monkeys) and various species of mosquitoes, but contamination can only be done from the bite.

That is, the transmission can be done via monkey-mosquito-person or person-mosquito-person, but never between two people or directly from monkeys to humans .


When a mosquito bites an infected human or monkey, the virus enters your bloodstream and circulates through it before settling in the salivary glands. When biting another monkey or human being, the mosquito transmits the virus, which enters the host’s bloodstream, infecting it.

Mosquitoes reproduce more in tropical forests, humid and semi-humid environments, as well as around standing water. Contact between potential hosts and infected mosquitoes, particularly in areas where people do not get vaccines for fever, can create small-scale epidemics.

From the wild cycle to the urban cycle

As the main mosquito that transmits wild yellow fever is unable to fly far and reach the cities, the disease can only start the urban cycle when a human being goes to the forest, is bitten and infected and, when he returns to the city, is bitten by Aedes aegypti , which acquires the virus and transmits it to other people.

Vector and etiologic agent

The name given to the causative agent of a disease is “etiologic agent”. In the case of yellow fever, it is an arbovirus (a virus transmitted by arthropods) from the same group as the dengue virus. It belongs to the genus Flavivirus , in the family Flaviviridae.

In Brazil, the main vectors of the disease are Aedes aegypti , Sabethes and those belonging to the species Haemagogus.

What is the role of monkeys in the fight against yellow fever?

Because it is a zoonosis, that is, a disease that affects humans and other animals, many people believe that the transmission of yellow fever occurs directly between monkeys and humans. However, this is not the truth.

In March 2017, the Ministry of Health launched the campaign “The Fault is not the Monkey’s”, warning the population not to mistreat or kill these animals. That’s because they have a very important role in fighting the disease: they alert us when the virus is being transmitted somewhere.

This is what happened in São Paulo, with the death of monkeys in the parks of the capital of São Paulo. Knowing that these deaths were yellow fever, the authorities were able to take preventive measures, closing the parks to prevent anyone from being bitten by an infected mosquito.

Taking into account that the transmission of yellow fever among monkeys occurs mainly through the Haemagogus , it is unlikely that the disease will reach the entire city, as this species has a reduced flight range. Still, it is best to prevent mosquitoes of the Aedes aegypti species from becoming infected.

In short, monkeys are sentinels for this disease, since the infection of these animals near urban areas is a sign that the disease is arriving in the city.

What to do when you find an infected monkey?

Upon sighting a dead or sick primate, the individual must call 136 to notify the Health Service of the municipality or state where he found the monkey. Then, the health service evaluates some points before proceeding with the identification of the disease. For this, they need to check if:

  • It is possible to collect a sample for analysis in the laboratory;
  • There are other dead or infected animals besides the one found;
  • The primate populations in the region are still visible or integrated;
  • It is an isolated death.

If there are reasons to suspect yellow fever, the health service begins an investigation that consists of collecting a sample of the animal’s blood and sending it for analysis in search of the virus.

If the result is positive, the health service can then take measures to prevent the disease from reaching the human population.

Groups of risk

Some people are more susceptible to being infected and developing yellow fever. Are they:

People without vaccine

People who have never been vaccinated, or who have never come into contact with the disease, may be at risk of becoming infected with the virus if they reside or travel to regions where the disease is potentially active, such as southern Africa or South America.

Elderly people aged 60 or over who have never had the vaccine, should consult a specialist before, as it is necessary to evaluate the cost-benefit of the medication.

People with weakened immune systems

Patients who are seropositive ( HIV ) and immunosuppressed should avoid travel to places where the disease is potentially active. If there is even a need, an infectious disease physician should be consulted to assess the situation.

Risk factors

Some other potential factors must be considered regarding the re-urbanization of yellow fever in Brazil, such as:

Territorial expansion of vectors

One of the major risk factors is the territorial expansion of the Aedes aegypti infestation , already detected in all Brazilian states. There are many areas infested by Aedes aegypti and Aedes albopictus and which are close to the areas of circulation of the yellow fever virus.

Many urban areas are infested by mosquitoes and are close to areas at risk for wild yellow fever, increasing the chances of spreading the disease even further.


Another important factor is the intense migration process in the rural-urban environment, leading to the possibility of importing the disease virus from the wild to the urban environments.

Low vaccine coverage

There is a greater chance of being infected in areas where vaccine coverage is low and a large circulation of the virus occurs.

Risk areas

The places where there is a greater chance of being infected with the yellow fever virus are located, mainly, in Africa, South and Central America:


Most yellow fever cases occur in sub-Saharan Africa. The countries where the disease is most likely to contract are:

  • Angola;
  • Benin;
  • Burkina Faso;
  • Burundi;
  • Cameroon;
  • Chad;
  • Congo;
  • Costa do Marfim;
  • Ethiopia;
  • Gabon;
  • Gambia;
  • Ghana;
  • Guinea;
  • Equatorial Guinea;
  • Guinea Bissau;
  • Liberia;
  • Mali;
  • Mauritania;
  • Niger;
  • Nigeria;
  • Kenya;
  • Central African Republic;
  • Democratic Republic of Congo;
  • Central African Republic;
  • Rwanda;
  • Sao Tome and Principe;
  • Senegal;
  • Sierra Leone;
  • Somalia;
  • Sudan;
  • Tanzania;
  • Togo;
  • Uganda;
  • Zambia.

Central America

In Central America, the island of Trinidad in Trinidad and Tobago has a high risk of transmitting yellow fever.

South America

In South America, the yellow fever virus is present in:

  • Argentina;
  • Bolivia;
  • Brazil;
  • Colombia;
  • Ecuador;
  • Guyana;
  • French Guiana;
  • Panama;
  • Paraguay;
  • Peru;
  • Suriname;
  • Venezuela.

In Brazil

Some Brazilian states already had yellow fever vaccination due to the risks of the disease. Since 1997, the Ministry of Health has been working on the gradual extension of coverage, aiming to offer vaccination to the whole country.

However, the states that are classified as areas of risk are:

  • Acre;
  • Amazonas;
  • Amapá;
  • For;
  • Rondônia;
  • Roraima;
  • Tocantins;
  • Federal District;
  • Goiás;
  • Mato Grosso do Sul;
  • Mato Grosso;
  • Bahia;
  • Maranhão;
  • Piauí;
  • Minas Gerais;
  • Sao Paulo;
  • Paraná;
  • Rio Grande do Sul;
  • Santa Catarina.

Symptoms of yellow fever

Sometimes, the patient may not show symptoms (asymptomatic), but when they do, they occur suddenly and can last between 2 and 4 days, usually. The characteristic symptoms of the disease are:


The fever appears suddenly and intensely, usually above 39ºC, which can be accompanied by chills.

Dore s

Headaches are severe and quite uncomfortable, which may be accompanied by high sensitivity to light.

With them, muscle pain can occur all over the body, but especially in the back and knees.

Nausea and vomiting

Yellow fever can cause stomach discomfort of varying intensities. Nausea is quite common and frequent vomiting can occur.

Loss of appetite

In general, the patient feels a reduction in appetite that can be caused or intensified by nausea, nausea and vomiting.

Fatigue and dizziness

The patient may experience fatigue , extreme tiredness, muscle weakness and dizziness . Conditions are especially aggravated when vomiting occurs, as the organism is further weakened.

Redness and bleeding

Reddish areas may appear on the skin or mucous membranes, especially on the face. The tongue and eyes can also be affected.

Bleeding usually occurs in severe cases of the disease, after the stage of remission and new manifestation (toxic phase).


The second phase of the disease, the toxic phase, is marked by the yellow coloration of the skin, eyes and mucous membranes, due to hyperbilirubinemia, which is the high concentration of bilirubins in the blood.

The excess of the substance is due to the high destruction of hemoglobin (red blood cells), an action resulting from liver dysfunction.

The name of the disease is even based on this symptom.

Diagnosis: how to identify yellow fever?

Diagnosing yellow fever based on the initial symptoms can be difficult, because it can be easily confused with other diseases, such as malaria , hemorrhagic dengue fever, hepatitis and leptospirosis .

However, in the midst of outbreaks or when there are warning signs such as the death of monkeys in the parks of São Paulo, the suspicion is quickly raised and, with that, the diagnosis is faster.

To diagnose the condition, the doctor (who may be a general practitioner, travel physician, emergency physician or infectious disease ) will possibly ask about your travel history and will request a blood collection.

The blood test can show a reduction in the number of white blood cells that fight infection, being a sign of the disease because the yellow fever virus affects the bone marrow.

Other tests done for the diagnosis of yellow fever are:

ELISA serology

In ELISA serology, blood serum is analyzed for antibodies and antigens. When a person is infected with a virus, his or her body begins to produce antibodies specific to that virus.

With this, doctors can detect the specific antibody for the yellow fever virus, making the diagnosis possible.

Polymerase chain reaction (PCR)

The polymerase chain reaction is an examination done to identify the microorganism that causes the disease. For this, he uses the patient’s blood sample and copies the microorganism’s DNA, which is multiplied several times until the virus can be identified.

Can yellow fever be cured?

Yes, yellow fever can be cured . A few days after infection, the immune system itself is in charge of expelling the virus. In some cases, the disease progresses to the toxic phase, which can also be cured, but its mortality rate is high.

As the immune system has a “memory”, the individual who has already had the disease is immunized for the rest of his life, as the body already has antibodies against this virus, guaranteeing lifelong protection.

Yellow fever treatment

There are no drugs available to destroy the virus. Therefore, treatment consists mainly of care and amelioration of symptoms resulting from infection.

This includes supplying fluids and oxygen, maintaining blood pressure, replacing blood loss, providing dialysis for kidney failure and adequate treatment for any other infections that may develop.

Blood transfusion

One of the procedures to treat yellow fever is blood transfusion, especially in cases where there is excessive blood loss.

Some people may receive plasma transfusions to replace blood proteins that improve clotting.


In order to avoid dehydration, the patient must drink plenty of fluids. However, when this is not possible due to vomiting, an intravenous solution can be given to maintain hydration.


When the patient has kidney failure, dialysis is necessary. This procedure consists of filtering the blood through machines, preventing toxins from accumulating in the individual’s bloodstream.

Yellow fever medications

To alleviate the symptoms of yellow fever, your doctor may recommend the following medications:

  • Paracetamol , to reduce fever and headaches;
  • Ibuprofen and nimesulide , to relieve muscle pain;
  • Cimetidine and omeprazole , to protect the stomach, avoiding gastritis, ulcers and decreasing the risk of bleeding;
  • Metoclopramide , to control vomiting;
  • Aspirin ( acetylsalicylic acid ) should not be taken , as this medicine dissolves the blood, increasing the risk of bleeding.


NEVER self-medicate or stop using a medication without first consulting a doctor. Only he will be able to tell which medication, dosage and duration of treatment is the most suitable for his specific case. The information contained on this site is only intended to inform, not in any way intended to replace the guidance of a specialist or serve as a recommendation for any type of treatment. Always follow the instructions on the package insert and, if symptoms persist, seek medical or pharmaceutical advice.

Natural Remedies

Along with clinical treatment, there are some natural remedies (herbal medicine) used to relieve yellow fever symptoms. Are they:

  • Angelicó ( Aristolochia trilobata L ): Helps in the control of gastric symptoms of yellow fever;
  • Passion fruit ( Passiflora edulis Sims ): Improves symptoms by reducing fever;
  • Alfavaca, lemongrass, stone breaking and nettle: Considered diuretic herbs, these four help in the treatment of anuria (decreased urine production);
  • Bitter stick and Lieutenant stick: Indicated for stomach upset, intestinal fevers, diarrhea and gas caused by the disease.

Attention:  It is important to emphasize that, before making any treatment, the patient should consult a doctor.


About 15% of patients progress, within a period of 48 hours, to a more severe phase of the disease, known as the toxic phase.

At this stage, a series of complications can develop:

Internal bleeding and scattered clots

As the disease progresses, several organs may experience bleeding (bleeding). At this stage, the patient may experience bleeding from the nose and gums. In addition, it can develop clots scattered throughout the body, causing blue or green patches under the skin.

Multiple organ failure

Clots and lack of blood impede blood circulation, leading to multiple organ failure. The main organs affected are the kidneys, liver and heart.

Blood in the vomit and feces

Gastrointestinal bleeding leads to the release of blood in vomit and feces.

Liver and kidney failure

With bleeding and clots, blood supply to the liver and kidneys is impaired, which can lead to insufficiency in these two organs.

Hypovolemic shock

When there is a lack of blood circulating in the veins and arteries – as in the case of hemorrhages -, the body may suffer a hypovolemic shock, that is, a shock caused by the low volume of blood circulating through the blood vessels.

This can cause the heart to beat faster, which causes an overload. Therefore, hypovolemic shock is a highly deadly condition, which should be treated as soon as possible.


Liver damage can cause an increase in the levels of bilirubin in the blood, giving a yellowish color to the patient’s skin and sclera (white part of the eye). This symptom is called jaundice.

Bilirubin is a pigment in bile juice, a substance in the liver that assists in the digestion of fats.

Delusions and convulsions

When the disease causes brain damage, the patient may experience symptoms such as seizures and delusions.

With the

Due to brain damage and a lack of blood supply to this organ, the patient may lose consciousness and fall into a coma: a state in which he cannot return to alertness, but is still not considered brain death.


The mortality rate in the toxic phase is quite high. It is estimated that between 20% and 40% of patients end up not resisting, being, in the great majority, children and elderly.

How to prevent yellow fever?

The prevention of yellow fever is accomplished through vaccination and fighting the mosquito. Some tips for this are:

  • Do not leave tires or containers that can accumulate water exposed to rain;
  • Put sand in the pots of the plants and use water treated with bleach to water the plants;
  • Leave the roof gutters uncovered so that there is no water accumulation;
  • Keep water tanks, cisterns, barrels and filters covered;
  • Let the spouts of the bottles down;
  • Put disinfectant in the drains;
  • If you have a swimming pool in your residence, clean it frequently;
  • Put your household waste in closed plastic bags and in cans with lids;
  • Use mosquito nets and screens to keep mosquitoes out of the house;
  • Use repellents, especially when traveling or in places where the number of mosquitoes is high;
  • If you need to travel to a place with a high incidence of the disease, get the vaccine at least 10 days before. If you are already vaccinated, there is no need for a second dose;
  • Always apply sunscreen before repellent. Otherwise, it may lose its effectiveness;
  • Avoid perfumes in forest areas, as the sweet smell of the fragrances attracts mosquitoes;
  • Do not use repellent on children under 2 months;
  • Wear long, light clothing, avoiding leaving too much skin exposed;
  • Do not apply repellent under clothing.

Yellow fever vaccine

According to the World Health Organization (WHO), vaccination is considered the main and most effective way to fight yellow fever. The yellow fever vaccine consists of the attenuated virus and, according to the WHO, immunity is acquired in 30 days in 99% of the people vaccinated.

In May 2013, WHO announced that a single dose of the vaccine guarantees lifelong immunity. Therefore, the application of the booster dose is not considered necessary.

However, in Brazil, the Ministry of Health chose to maintain the scheme of applying two doses of the vaccine until the beginning of 2017. In April of the same year, the country decided to adopt the WHO recommendation for a single dose.

It is worth remembering that the vaccine itself has not changed. Therefore, those who have already had the vaccine and are waiting to take the second dose, no longer need to worry, as they are already immunized.

The vaccine is administered subcutaneously and is distributed free of charge at health clinics. Currently, in several Brazilian states, this vaccine is already part of the National Vaccination Calendar.

Who should get the vaccine?

Children in high-risk areas, from 9 months of age onwards, should receive the first dose of the yellow fever vaccine. The booster dose should be applied when they are 4 years old.

The vaccine is also indicated for all people, whether adults or children, who live in the vicinity of areas that may have an epidemic of the disease or who intend to travel to these places, inside or outside Brazil.

If you intend to travel, the first dose of the vaccine must be applied 10 days in advance, so that the body has time to produce the antibodies necessary to fight the virus.

Who should not get the vaccine?

Babies under 6 months should not get the vaccine, as they are more vulnerable to possible dose complications, including viral encephalitis .

Pregnant and lactating women should also not get the vaccine, as there is no evidence that the infection passes into the fetus or that the attenuated virus passes into breast milk.

Nor should the vaccine be given to immunosuppressed persons, HIV carriers, malignant tumors (including leukemia and lymphomas), who use cortisone-derived drugs, are undergoing chemotherapy or radiation therapy or are carriers of diseases that alter the functioning of the thymus (organ of the immune system).

Allergic to any vaccine substance, such as egg protein, gelatin or the antibiotic erythromycin, should avoid the vaccine.

Who depends on medical advice?

Adults aged 60 and over can only get the vaccine after consulting a specialist, who will assess the cost-benefit of the medication. In this age group, the risk of adverse reactions increases.

People with HIV or immunocompromised people in general should avoid traveling to places where the risk of yellow fever exists, however, if there is an absolute need, these patients should seek medical advice on the subject.

Guidelines for vaccination against yellow fever

Between 6 months and 9 months of age, the vaccine is indicated only in the event of an epidemiological emergency or travel to the risk area.

9 months until before the age of 5: 1 dose at 9 months of age and 1 booster dose at 4 years *.

* If the child is not vaccinated at 9 months, the vaccine and the booster must be given with a minimum interval of 30 days between doses.

From 5 years

  • They received two doses of the vaccine: they no longer need to be vaccinated, as they are immunized;
  • They received a dose of the vaccine: they need to take the booster even as adults;
  • They were never vaccinated or without proof of vaccination: 1st dose of vaccine + 1 booster dose after 10 years.

Pregnant women and infants

Vaccination is contraindicated. If it is impossible to postpone vaccination, as in situations of epidemiological emergency or travel to the area at risk of contracting the disease, the doctor must assess the benefit / risk of the vaccination.

For children, vaccination is contraindicated up to 6 months of age. If you have received the vaccine, breastfeeding should be discontinued for 28 days after the vaccine

People aged 60 or over who have not been vaccinated

They should talk to a health professional and be aware of health agency warnings about the risks of an outbreak.


International travel: follow the recommendations of the International Health Regulations.

Travel to areas with vaccine recommendation in Brazil: vaccinate, at least 10 days before the trip, in the case of the 1st vaccination. The 10-day period does not apply in case of revaccination.

Vaccine exemption letters

In cases where the yellow fever vaccine is not advisable, the doctor may transmit a letter of exemption.

The letter should be written on letterhead and contain all the practical details of why the vaccine is not recommended for that person. This letter can be accepted by some immigration authorities.

Yellow fever vaccine reactions

The vaccine is considered extremely safe. A single dose provides lifelong protection, however, some adverse reactions may appear. Are they:

  • Mild headache;
  • Muscle pain;
  • Pain at the injection site;
  • Fatigue;
  • Low grade fever.

If you get the vaccine and have adverse reactions, consult your doctor, but remember that the reactions are not the disease . That is, the vaccine does not cause yellow fever.

Yellow fever is known to be a highly dangerous disease and, if not treated properly, can be fatal! This article will help you understand the risks of the disease and how to prevent it. Share with your friends so they understand too!