An alarming outbreak of yellow fever that threatened Brazil early this year appears to be over, according to data released this week by the Pan American Health Organization.
There were no new cases reported in Brazil in the last month, said officials of the agency, a regional branch of the World Health Organization. Of the neighboring countries to which the outbreak had spread, only Bolivia reported a case.
Concerns were first raised in January, when Brazil reported 712 suspected cases — a fivefold increase over normal levels. Most were in Minas Gerais, a rural state. About 40 of the state’s residents died, and the governor declared a state of emergency.
Over the next few months, the outbreak spread, even reaching the states that are home to the megacities Rio de Janeiro and São Paulo. Some panicked residents reacted by killing monkeys, mistakenly blaming them for the spread.
The government distributed 20 million doses of yellow fever vaccine, including more than 3 million from the W.H.O. emergency stockpile.
In retrospect, reports show that total cases peaked in January, midsummer in the Southern Hemisphere, then fell substantially by March and nearly disappeared by June.
Yellow fever normally circulates in forest monkeys and forest mosquitoes, and it occasionally kills people who live or work there, such as loggers, miners and small farmers.
Scientists were particularly worried that infected humans would carry the virus into cities, where it could be picked up by Aedes aegypti mosquitoes, which thrive in huge numbers in urban slums and are aggressive biters of humans.
Had that happened, experts said, there was a serious risk that it would spread to the Northern Hemisphere, where vaccine for yellow fever is in short supply, or to Asia, where hundreds of millions of people with no natural immunity or history of vaccination live in tropical latitudes favored by Aedes aegypti mosquitoes.