Yellow fever, the returning epidemic

May 2022

When Cuban physician Carlos Finlay suggested in 1881 that the mosquito was the vector through which yellow fever was transmitted, he did not imagine that this ancient disease had its natural reservoir in monkeys.

The importance of controlling and preventing the spread of yellow fever from one country to another was the driving force behind the creation in 1902 of what would later become the Pan American Health Organization (PAHO). More than a century later, yellow fever continues to generate epidemics and spread throughout the region and the world.

In 1932, the American physician Fred Soper, who would become director of PAHO in 1947, indicated for the first time that this virus has its jungle reservoir in monkeys, so its eradication was not a possibility. However, since 1937 there has been an effective vaccine to protect people at risk and prevent its spread.

Since 1970, yellow fever has re-emerged as a public health threat in the Americas. The disease is endemic in territories and regions of 13 countries in Central and South America, causing outbreaks and deaths.

Virus spreads in Brazil

The largest outbreak of the last 50 years in the Americas took place in Brazil, with transmission outside the Amazon region generating 2,154 cases and 745 deaths between 2016 and 2018.

Marlúcio Mendes de Jesus, a young Brazilian farmer from Imbé de Minas, 300 kilometers from Belo Horizonte, got yellow fever in 2017. Marlúcio had to be hospitalized for nine days. "I had never heard of yellow fever or the vaccine. But I don't usually go to the doctor either," he said. The area where he lives was not considered at risk until then.

In 2014, the virus expanded past the boundaries of the Amazon, where it is considered endemic, and began to reach the center and south of the country. Some experts attribute this process to environmental degradation and climate change, and to the proximity between monkeys, mosquitoes, and humans in areas where the boundaries between the jungle and urbanizations have become blurred.

Dr. Pedro Vasconcelos, former director of the Evandro Chagas Institute in Brazil, and an expert on yellow fever, said that "the reappearance of yellow fever in Brazil was caused by low vaccination coverage".

Mass vaccination to reach everyone

The spread of yellow fever in Brazil led the country to implement a mass vaccination campaign to protect millions of people outside the Amazon region.

"Due to outbreaks in Africa, the global vaccine supply was very limited, so Brazil followed a split-dose strategy that had already been applied in Congo for emergencies," explained Alba Maria Ropero, PAHO's regional immunization advisor, who supported the country in carrying out the campaign.

The split dose contains one-fifth of the regular dose and provides immunity for at least 12 months.

Consequently, by September 2018, 13.3 million people had been vaccinated in São Paulo, 6.5 million in Rio de Janeiro and 1.85 million in Bahia, representing more than half of the respective populations.

Outbreak under control

Thanks to this vaccination campaign, Brazil managed to control the outbreak, recording only nine cases and three deaths between 2020 and 2021. Since 2020, the country expanded the yellow fever vaccination area to the entire country and adopted a single-dose schedule starting at 9 months of age to protect younger generations for life.

PAHO provides extensive support to the Brazilian government, including the supply of yellow fever vaccines, the purchase of millions of syringes through its Revolving Fund, and the expansion of laboratory capacity to detect cases.

It has also helped strengthen surveillance of the disease in primates and mosquitoes, which serves as an early warning tool.

Vaccination is the best option for keeping yellow fever at bay. To that end, a WHO strategy driven by PAHO in the Americas seeks to vaccinate nearly one billion people by 2026.

"As long as there are unvaccinated people and the virus continues to spread outside endemic areas, the risk of transmission and outbreaks persists," said Sylvain Aldighieri, deputy director of PAHO's Department of Health Emergencies. "The outbreak in Brazil will undoubtedly not be the last to affect a country in the region, but should another one happen, PAHO will be there to help control it, reduce suffering and save lives as it has been doing for 120 years."

What is yellow fever

Yellow fever is a viral disease transmitted by infected mosquitoes, which has no cure and can be fatal, but there is an effective, safe, and affordable vaccine to prevent it. The most common symptoms are fever, muscle aches with lower back pain, headache, loss of appetite, nausea, or vomiting.

In most cases, symptoms disappear within three to four days, but 15-25% of patients enter a second, more severe phase, where the risk of death is higher and people may have dark urine, abdominal pain with vomiting, bleeding, and yellowing of the skin and eyes, hence the name of the disease.