The region of the Americas has seen eight times the number of yellow fever cases this year, compared to the same period in 2024. The vaccine-preventable, viral, mosquito-borne disease has surged as cases “spillover” from animals, according to a Pan American Health Organization (PAHO) rapid risk assessment.

Dense jungles in Colombia and Brazil have seen the most sylvatic – or “jungle” – yellow fever cases originating from an animal host. In these areas, unvaccinated humans can be infected from mosquitoes that are carrying the virus after biting infected monkeys and other primates.

Brazil, Bolivia, Colombia, Ecuador, and Peru have reported 221 confirmed human cases of yellow fever, and 89 deaths. Brazil accounts for nearly half of these cases and deaths.

These countries typically see jungle cases each year, but the combination of increased spillover events and a persistent unvaccinated population has meant there is now a risk of urban outbreaks. Almost all cases and deaths reported this year and last were in unvaccinated people.

PAHO has denoted the public health risk of yellow fever as high, even though a single dose of the vaccine is enough to provide life-long protection.

Prior to the COVID-19 pandemic, vaccine rates in the 12 countries in the region prone to yellow fever were quite high, ranging between 57-100% in young children, according to PAHO. But rates have dropped to below the recommended 95% coverage in 10 out of the 12 countries with circulating yellow fever, leaving a “substantial proportion” of the population unprotected, according to PAHO. 

Symptoms like fever, muscle pain, headache, nausea, and vomiting occur three to four days after a mosquito bite. Roughly 15% of patients will develop a toxic, hemorrhagic phase. The mortality rate of this type of yellow fever is over 50%. 

While recent vaccination campaigns have increased the number of people protected, fewer vaccine suppliers, cost, and the long process to make the vaccine means that there’s a limited global supply. “Current supplies [are] unable to cover demand in the Americas and in Africa,” said PAHO in a statement.

This has led countries to consider splitting doses among multiple people, so that each jab is one-fifth the normal dose. This smaller dose provides immunity for up to 12 months.

Historically notorious, but preventable disease

Yellow fever has a long history in the Americas, with periodic epidemics crippling cities and rural areas alike in the 19th and 20th centuries. 

But following the introduction of an effective vaccine, most cases remained concentrated in the Amazon and mountainous regions in Brazil, Colombia, Ecuador, and Peru. But this year, Colombia and Brazil have detected cases outside the typical Amazon region, in the state of São Paulo in Brazil and the department (state) of Tolima in Colombia.

This increase in yellow fever is primarily due to the reactivation of the jungle transmission cycle in the Amazon region, according to a recent PAHO epidemiological alert.

The expansion of cases beyond the typical regions points to an increase in human-forest interaction in forest edges.

“These areas provide ideal conditions for canopy-dwelling mosquitoes to transmit the disease to human populations from non-human reservoirs,” said PAHO in its alert.

While this year marks a concerning surge, it still hasn’t reached the levels of epidemics of the past decade, such as in 2016, said Dr Andrea Vicari, unit chief of infectious hazards at the World Health Organization (WHO).

Vaccine hesitancy delays containment

In Colombia, all yellow fever deaths have been of unvaccinated people. 

“All mortality has been among people not vaccinated,” said Dr Diana Pava, director of the Colombian National Institute of Health, told a PAHO briefing. The populations most affected are older adult men who work in the agricultural sector. 

Few, if any children catch yellow fever because the vaccine is “obligatory,” Pava noted. Children receive the jab with their measles, mumps, and rubella (MMR) vaccine. The same isn’t true for older generations, hence the concentration of cases in adults 65 and older.

Colombia’s rising cases highlight the difficulties in reaching older, rural, and isolated communities for vaccination. 

“In the rural areas, the beliefs may be different, or they don’t want the vaccine,” Pava said.

Colombia’s health ministry is concentrating its outreach efforts on these areas, especially because any outbreak, even in a remote area, could lead to cases in more populated regions.

For Brazil, which has seen over one hundred cases this past year, challenges include high vector density, and outright vaccine refusal out of fear, said Dr Daniel Ramos, arbovirus coordinator for the Brazilian Ministry of Health. Despite this, Ramos praised Brazil’s high vaccination rate, which has protected Brazilians even with increased circulation of the disease in animals.

Need to monitor animal hosts

Changing ecological conditions have also made it easier for yellow fever to jump from non-human primates like monkeys, to people.

Urbanization, “edge” habitats, and ecosystem fragmentation are all connected to a greater risk of yellow fever spread, noted Vicari. 

As a result, PAHO and health ministries have flagged the need to monitor yellow fever not only in humans, but in animals too. Tracking monkey deaths serves “an early warning to identify yellow fever circulation,” said PAHO in its alert. This could then alert officials to vaccinate people nearby. 

“While vaccination is certainly a key action, it must be integrated within a balanced response,” said Vicari, such as animal surveillance, clinical management, risk communication, vaccine stock management, and even contingency planning for potential urban outbreaks.

Image Credits: PAHO, PAHO.