Low risk of dengue predicted for foreign visitors to Rio Olympics

With the Olympics fast approaching, it may be time to reassess the dengue risks of holding them in Brazil.

In 2014, before the FIFA World Cup opened in Brazil, there were fears that many of the 600,000 foreign visitors expected for the world's largest soccer tournament would acquire dengue fever. Their numbers could reach hundreds or even thousands, according to some predictions.

These fears were not unfounded, since two years ago infestations of the Aedes aegypti mosquito that transmits dengue were as widespread in Brazil as they are today. In May 2014, however, a month before the World Cup was due to open, an epidemiological study surprised the scientific community by contradicting all the gloom and doom: the number of dengue cases among foreign visitors would be negligible, it predicted. And the prediction proved correct.

According to the article 'Risk of symptomatic dengue for foreign visitors to the 2014 FIFA World Cup in Brazil' by Eduardo Massad and collaborators at the University of Sao Paulo's Medical School (FM-USP), sophisticated mathematical modeling techniques pointed to a likely minimum of three cases and a maximum of 59.

"Do you know how many cases there actually were?" Massad asks with a smile. "Just three: two visitors from the United States and one from Japan. That's the low end of our predicted range."

In 2016, three months before the opening of the Rio de Janeiro Summer Olympics, the same researchers used the same mathematical models to calculate the risk of dengue acquisition by the 400,000-odd foreign visitors expected to attend, according to the Ministry of Tourism.

The epidemiologists used data from the Ministry of Health's Notifiable Diseases Information System (SINAN). Once again a very low number of dengue cases among foreign visitors to the Olympics is likely, said lead author Raphael Ximenes.

The mathematical model predicts a worst-case scenario of 23 symptomatic cases among the 400,000 foreigners expected to visit the Olympics, assuming the 2016 dengue epidemic displays the pattern observed in August 2007, when the number of reported cases was the highest ever for the month. Symptomatic cases are defined as those in which patients present with fever and other symptoms, whether or not they are hospitalized.

The model also predicts a maximum of 206 asymptomatic infections, in which visitors will be bitten by the mosquito and infected with dengue virus but will not develop symptoms or fall sick.

"You can't predict the risk of infection without knowing all about the past of the disease," Ximenes said. "Fortunately in the case of dengue the SINAN database is one of the most comprehensive in the world."

The study appears in BMC Infectious Diseases.