Hidden Virus May Have Infected 9.4 Million People – Scientists Say We’ve Missed Most Cases
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New research indicates the Oropouche virus has spread far more widely than reported, exposing major gaps in disease surveillance.
Until recently, few people had heard of the Oropouche virus. But after a massive 2023 outbreak swept across Brazil, infecting tens of thousands of people, causing the country’s first confirmed death, and spreading far beyond its traditional Amazonian range, the virus quickly became an international concern. The World Health Organization (WHO) has since urged the rapid development of new tools to improve surveillance, prevention, and control.
Two studies published in Nature Medicine and Nature Health suggest that Oropouche virus has affected far more people than official figures show. By combining mathematical modeling, historical data, and blood bank analysis, the researchers estimate that the virus has infected about 9.4 million people across Latin America and the Caribbean since 1960. Brazil alone may account for roughly 5.5 million infections.
The illness causes fever and symptoms that can resemble dengue. In some cases, it can trigger severe complications, including neurological conditions such as meningitis and meningoencephalitis, as well as microcephaly when the virus passes from a mother to a fetus. “We’re facing a disease of much greater magnitude than previously imagined, which requires greater attention.
We estimate that one in every thousand diagnosed cases progresses to serious complications such as neurological disorders, microcephaly, miscarriages, and liver complications, raising the priority level for public health,” says José Luiz Proença Módena, coordinator of the Laboratory for the Study of Emerging Viruses (LEVE) at the State University of Campinas (UNICAMP) and co-author of the studies, which were supported by FAPESP. In Manaus, the largest city in the Amazon region, an estimated 300,000 people were infected between 2023 and 2024. That figure is nearly 260 times higher than the confirmed case count.
According to the researchers, antibodies against the virus rose from 11.4% of the population in November 2023 to 25.7% in November 2024, a sign that transmission was widespread. “The capital of the state of Amazonas is a city with over two million inhabitants and is considered the gateway to the Amazon region. The striking underreporting occurred due to several factors, mainly because the virus circulated silently before reaching the outskirts of the urban center, with many cases being asymptomatic or mild and going undiagnosed,” says William de Souza, a professor at the University of Kentucky in the United States and co-author of the study.
That pattern helps explain how Oropouche virus reached all Brazilian states and spread into neighboring countries, leading the WHO to issue an international alert.
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