A yellow fever outbreak that began a year and a half ago in Brazil is spreading further, with additional cases since last July in the Brazilian states of Sáo Paulo, Minas Gerais and Rio de Janeiro, plus several cities in Bahia State.
The World Health Organization began recommending yellow fever vaccination for all travelers and residents of these areas in January, but since then, 10 travelers have contracted the disease—all unvaccinated—and four died.
Kacey Ernst, PhD, MPH, an associate professor and the program director at the University of Arizona’s Epidemiology and Biostatistics Department in Tucson, answers some key questions about the outbreak and what it means for the future. But first, here’s a primer on yellow fever and the Brazilian outbreak.
What’s Yellow Fever?
Yellow fever is a virus carried by mosquitos, usually (but not always) Aedes Aegypti, the same species that transmits dengue fever, chikungunya and Zika. It starts with typical viral symptoms, including fever, chills, headache, backache and muscle aches, about 3-6 days after infection.
For the 15% of people who develop severe illness, the disease can cause internal bleeding, organ failure, shock and death. For every five people with severe illness, one to three of them dies.
The CDC recommended last week that anyone at least 9 months old who is headed to the Brazilian states affected by the outbreak should get vaccinated against the virus at least 10 days before traveling there. Not all U.S. vaccination clinics carry yellow fever vaccine, so you can look up the nearest one to you here. However, not everyone should receive the vaccine, so be sure you are not among those people who have higher risk of a serious side effect.
Travelers should also take usual precautions to reduce the risk of mosquito bites, including wearing long pants and long sleeves, using insect repellent, staying in air conditioned buildings as much as possible and avoiding areas with lots of mosquitoes.
A Q&A with Dr. Ernst
Why is this outbreak so bad?
The geographic areas affected by this outbreak are different than where transmission typically occurs in Brazil. This means that people are less likely to have received yellow fever vaccination in the past and medical and public health professionals in the area have less experience managing these outbreaks.
Is this outbreak in Brazil surprising?
No, there are fairly regular outbreaks in Brazil and in other places in South American countries. This outbreak is somewhat different because the states affected are different than where yellow fever is typically transmitted in Brazil. It is important to review reputable information on all countries prior to travel to ensure that you are up to date on all the recommended travel vaccinations.
Will this outbreak affect production of the yellow fever vaccine?
Because yellow fever is endemic to Brazil, they produce their own vaccine in the country, and they have ramped up production of vaccine internally. Yet since this is a new area of transmission, coverage has been low in the past, so there is still a gap in vaccines available and the population that needs to receive it.
In the United States and other parts of the world, there have been vaccine shortages due to manufacturing problems of the only U.S.-licensed yellow fever vaccine, made by Sanofi Pasteur. This shortage started in 2017 and prompted the use of a fractional dose (one fifth of the vaccine), which was effective during yellow fever outbreaks in western Africa.
How does this outbreak compare to the Zika outbreak in Brazil?
The current transmission of yellow fever appears to be circulating in what we call the sylvatic cycle, which means transmission is occurring in rural forested areas where there are non-human primates probably also experiencing an epizootic (outbreak in animals). The mosquito implicated in this type of transmission is different than the Ae. aegypti, which was the primary mosquito implicated in widespread Zika transmission.
Transmission of yellow fever can occur in Ae. aegypti, however, and they are nicknamed the yellow fever mosquito because they can lead to widespread outbreaks in urban areas. If the Brazil outbreak does spill over into [cities], the mosquito control will be the same as it was during the Zika pandemic.
I think we all have our fingers crossed this will not happen because Ae. aegypti are notoriously difficult to control given their ability to exploit very small water-holding containers and their strong preference to feed on humans.
How has public information and misinformation differed between the two outbreaks?
Zika was a new pathogen to Brazil and, in effect, to the rest of the world. We knew so little about Zika at the beginning of the pandemic that there was an information vacuum. The fact that it caused severe birth defects and can be sexually transmitted mean the perception and control of the disease differ from yellow fever virus.
Yellow fever virus has been circulating in Brazil for centuries, so people are at least somewhat familiar with the disease. And the primary control measure being implemented, vaccination, differs from vector control measures for Zika. [Vectors are carriers of disease. Vector control refers to reducing populations of disease-carrying mosquitos or their opportunities to bite people.]
While vector control of Ae. aegypti to reduce the chance of urban transmission is important, most rumors are focusing on the vaccine. I don’t think specific Zika misconceptions are spilling over, but the social and political environment that allowed misconceptions to thrive during the Zika pandemic still exist.
What is contributing to fears about the yellow fever vaccine?
Everything really boils down to trust or lack of trust in the government and public health systems. Given recent corruption and scandals in Brazilian officials, people are more likely to trust the friend, relative or other contact they know personally that spreads them misinformation through social media or in person. What’sApp has been particularly implicated as a vehicle for the spread of misinformation in this outbreak.
People think the vaccine is a way to extract money from the people. In reality, the vaccine is provided for free. The yellow fever vaccine does have slightly higher risks than other vaccines. It is a live virus vaccine, which means it is more likely to cause some reactions and adverse events.
The CDC estimates 1 in 4 will have a mild fever and some soreness near the site where the shot was given. More seriously, about 1 in 55,000 have a severe allergic reaction, 1 in 125,000 have a nervous system reaction and 1 in 250,000 have severe illness and organ failure that leads to death in about 1 in 500,000.
So if someone hears that their friend got a fever after getting vaccinated, they may automatically assume that the vaccine didn’t work and, worse yet, that it may be harmful. But if you get yellow fever, the chances of dying are about 1 in 20, and in some outbreaks, deaths have exceeded 1 in 10. That is about 50,000 times more fatal than the vaccine. But I think public health professionals don’t talk much about the rare but real potential side effects. This can exacerbate existing mistrust in the medical system and the government.
Another issue is that with the vaccine shortage, people are primarily being given the fractionalized dose, one fifth the regular dose. While this has been shown to be effective, people don’t trust that it works, leading them to refuse vaccination. Even the term “fractionalized” has been misunderstood. It is true that we don’t know how long immunity lasts from the fractionalized dose, but we do know that it has been effective during other outbreaks and is critical to keeping people safe during a vaccine shortage.
It is estimated that to prevent a possible urban outbreak, about 95% of people need to be vaccinated while the latest figures I found indicated vaccination rates hover around 80%.
What needs to be done to combat misinformation and vaccine fears?
The Brazilian Ministry of Health has been trying to combat the misconceptions through messaging in the media as well as posting information on the importance of vaccination on their websites and Facebook pages. There are even door-to-door campaigns.
Fighting misinformation spread through social media is a Herculean task, and I don’t think we are at a point yet where we can apply a standard model of intervention. Long-term initiatives need to be made that really work to build trust so that subsequent campaigns will be better received. This can be done now by making information about the current outbreak as transparent as possible.
To what extent might climate change be playing a role in these disease outbreaks?
If climate change and mosquito-borne diseases had a relationship status on Facebook, it would be “it’s complicated.” Climate change can both directly and indirectly influence transmission of mosquito-borne disease. Generally speaking, warmer and wetter conditions allow mosquitoes to thrive, and there is a correlation between having a lot of mosquitoes and having more mosquito-borne diseases. Warmer conditions also mean that the time between when a mosquito gets infected and when they become infectious (able to transmit to the next host) gets shorter.
But really hot and really cold temperatures are not good for mosquito survival, so the increasing extreme weather conditions may suppress mosquito populations. Under climate change conditions, most areas will see increasing minimum temperatures. This means mosquitoes may be able to inhabit new areas, and, in areas that are highly seasonal, the season when mosquitoes thrive may lengthen.
It could also mean, however, that some areas become drier. Depending on the mosquito, dry conditions may reduce mosquito populations. The Ae. aegypti, the mosquito that transmits Zika, dengue, yellow fever and chikungunya, however, lays its eggs in manually-filled containers. If people start storing more water during dry periods, this may sustain Ae. aegypti mosquito populations.
Other indirect impacts of climate change include increasing food insecurity, which may lead to higher levels of malnutrition. When people are malnourished, they are more susceptible to infectious diseases, including mosquito-borne illnesses. In addition, infrastructure decline that may result from more frequent and intense weather events (hurricanes, floods, etc.) may weaken existing public health response.
To what extent should we expect to see other yellow fever outbreaks or other vector-borne disease outbreaks going forward?
Evidence suggests our risk of outbreaks of yellow fever and other vector-borne diseases is increasing. This is not just due to climate change but also to globalization and trade, which means more rapid introduction of pathogens and vectors to new places, and to destruction of our natural environment, which increases human contact with new pathogens.
One of the first things that governments need to do is to build capacity to recognize outbreaks when they begin. This means enhancing surveillance capacity, not just for the pathogens but also for their vectors. We actually know quite a bit about the transmission dynamics of many vector-borne diseases, and our ability to model transmission is increasing. Yet most of these models still remain an academic exercise.
We need to invest in sustainable systems that can model periods of highest risk so that public health and communities can be on the alert. While these will always be uncertain, I think we sometimes underestimate the public’s ability to understand uncertainty. Weather forecasting is an example. I think many people understand what a 20% chance of rain means.
We also need to invest in developing strategies for disseminating this information quickly and accurately to the public. Many of these vector-borne diseases require significant community engagement to fight. Empowering people with knowledge—accurate knowledge—will aid in our fight.