A mother comforts her 5-year-old daughter as a health worker prepares to inject her with a yellow fever vaccine in Casimiro de Abreu, Brazil.

Brazil, in the grips of an unusually large yellow fever outbreak, has asked for millions of doses of vaccine from an international emergency stockpile.

The body that maintains and manages the stockpile, the International Coordinating Group on Vaccine Provision, has approved the release of more than 3.5 million doses of the vaccine, according to the Brazilian office of the Pan American Health Organization. The office said the vaccine should arrive in Brazil over the next few days.

The request signals public health officials’ increasing concern over the scale of Brazil’s outbreak. The country is already home to one of four yellow fever vaccine manufacturers in the world, and it normally produces enough to meet its own needs.

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But since last December, Brazil has recorded a larger-than-normal number of cases of yellow fever. And as cases have crept ever closer to two of the country’s largest cities — Rio de Janeiro and São Paulo — there have been fears that the disease could start to spread in these cities and, from them, to other parts of the Americas.

As of March 13, there had been 1,538 cases of yellow fever reported, according to Brazil’s health ministry. Of those, 396 have been confirmed and 184 discarded. More than 950 suspected cases remain under investigation.

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There have been 255 deaths. The confirmed cases have been recorded in three states: Minas Gerais, Espírito Santo, and São Paulo.

The health ministry recently announced it would vaccinate everyone in Rio de Janeiro state, and would also vaccinate people in parts of São Paulo state that have been deemed at risk.

The yellow fever virus, like the related dengue and Zika viruses, is spread by mosquitoes. It is deadlier, though, than its cousins, with a significant fatality rate.

The virus normally spreads in what is known as a jungle or sylvatic cycle, with transmission between Haemagogus mosquitoes and monkeys. Occasionally a person becomes infected, but human cases are generally rare in the jungle cycle

But if the virus finds its way into cities, it can infect Aedes mosquitoes, which live in close proximity to people, and that can trigger a cycle of hard-to-contain urban yellow fever. Given the severity of the disease, the specter of urban yellow fever alarms public health officials. Still, the World Health Organization said last week that to date there is no evidence that Aedes mosquitoes are spreading the virus.

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People who contract yellow fever can experience fever, headache, backache, muscle pains, loss of appetite, and nausea or vomiting. Those symptoms typically last three or four days. But about 15 percent of people then suffer a relapse, entering what is called a toxic phase in which they develop jaundice and more severe illness. Roughly half of the people who reach the toxic phase die.

In the Rio de Janeiro state, there have been two confirmed cases in a city located about 62 miles from the city of Rio de Janeiro, and four cases have been confirmed in cities located in São Paulo state between 160 miles and 220 miles from the city of São Paulo. The urban centers of Rio and São Paulo have populations of more than 6 million and 12 million people, respectively.

Given the situation, the WHO announced last week that international travelers who are traveling to Rio de Janeiro and São Paulo states should be vaccinated against yellow fever, though it exempted those whose travels are restricted to the cities of Rio de Janeiro and São Paulo as well as the cities of Niterói and Campinas.

The international emergency yellow fever stockpile, which normally contains 6 million doses, is managed by the International Coordinating Group on Vaccine Provision, which is a coalition of four agencies: the International Federation of Red Cross and Red Crescent Societies, Doctors Without Borders, the United Nations Children’s Fund (UNICEF), and the WHO.

Last year that stockpile distributed nearly 30 million doses of yellow fever vaccine to help extinguish a dangerous outbreak in Angola that spilled into the Democratic Republic of Congo, as well as an unrelated outbreak in Uganda.