The World Health Organization this week reported a three fold increase in the number of cases of Yellow Fever in Brazil.  This could pose a public health risk: Carnival is next week, and Brazil is about to be swarmed by happy revelers, out all night —  precisely when the mosquitos that spread yellow fever are feasting.

This combination of a fast spreading deadly virus plus Carnival is concerning public health professionals — but how worried should we be that Brazil is on the precipice of a major outbreak?

20 people have died of yellow fever in Brazil since July, out of 35 cases of diagnosed yellow fever. Of the 25 cases, 20 of them were in Sao Paulo state (not necessarily in the city.) Most of the Sao Paulo cases, according to WHO data, were 15 kilometers north of the city. So, the relative increase is sharp, but the absolute numbers aren’t that high.

Yellow Fever is a viral hemorrhagic fever that is transmitted by infected mosquitoes. The typical case of yellow fever causes symptoms like fever, headache, jaundice, muscle pain, nausea, vomiting and fatigue. It’s a very unpleasant illness, and it leads to severe pain to individuals and economic and productivity losses to communities and nations. There is no cure or specific treatment; patients are treated with rest, fever reduction, and hydration. Most people improve after a few days of care.

In some cases, though, it gets worse. Approximately 15% of yellow fever cases develop into a more severe form that includes high fevers, jaundice (that’s the “yellow” in yellow fever) and bleeding from the eyes, nose, and mouth. In this way, it resembles other hemorrhagic fevers, like Ebola. It appears the high death rate in this Brazil outbreak rate means that milder cases are going undetected, and only severe cases are being reported. About half of the people who get the more severe form of Yellow Fever die.

To make things more complicated, yellow fever also affects non-human primates; in Brazil, that means forest monkeys. That’s why yellow fever can’t simply be eradicated – the virus circulates in animals. When a mosquito bites an infected monkey, and then a person, that person can be infected. This means that when you look at outbreak numbers in Brazil, you need to look an animal outbreaks (known as epizootics) as well as human outbreaks. Yellow fever epizootics happen frequently among Brazilian forest monkeys; the monkeys are an ongoing virus reservoir.

There is a highly effective vaccine against Yellow Fever

There is one other highly important fact about yellow fever, though: It has a highly effective vaccine. The vaccine is one of our oldest; it was developed in the 1930s and it is the reason we no longer have yellow fever outbreaks in the US. A single vaccine protects against yellow fever for the patient’s entire lifetime; no booster is needed. It provides effective immunity within 30 days of vaccination in 99% of people who are vaccinated.

Another factor mitigating against panic is that Brazil has a strong public health system. Yellow fever is spread by the same mosquitoes that spread Zika; Brazil knows what to do– the country effectively stopped the Zika outbreak by declaring war on it

This demonstrated that Brazil was able to marshall an effective response to a fast spreading mosquito-born virus (for which there was no vaccine). It put surveillance systems in place and took immediate and intense steps to bring the outbreak to an end.

That being said, the Zika response was also criticized for putting too much emphasis on the household level – pushing families to clean their water storage, for example – and not enough on the infrastructure level, like improving water and sanitation infrastructure. In addition, the funding used for Zika response has been reprogrammed into other work. There is no immediate emergency money ready for use in stopping yellow fever.

Putting it all together, we’re probably at fear level: medium. The virus reservoir among forest monkeys is bad news, but Sao Paulo and Rio de Janeiro cities aren’t full of monkeys, so that risk isn’t strictly relevant to Carnival visitors, who stay in the major cities. The number of yellow fever cases has increased, but the total numbers are still low. Yellow fever is contagious, but it spreads through a mosquito vector that can be stopped. And the vaccine for yellow fever is highly effective. This is probably why the World Health Organization is not recommending any major precautions for visitors to Brazil, beyond that they get vaccinated for yellow fever.