The Zika epidemic, which began as a mosquito-borne viral infection and led to severe birth defects, affected far more people than previously thought, new research shows.
For the study, researchers analyzed data from 15 countries and territories in South America, Central America, and the Caribbean with a combined population of 507 million, and concluded that they had over 132 million Zika infections between 2015 and 2018.
That's far more than the 800,000 infections estimated by the Pan American Health Organization, a regional office of the World Health Organization.
The study authors said that the results show the need to improve current infectious disease surveillance systems — especially for diseases that yield a high number of asymptomatic infections.
"Fewer than 1% of cases were actually reported and it shows our surveillance systems catch only a small percentage of actual infections," said study author Sean Moore. He's a research assistant professor of biology at the University of Notre Dame, in Indiana.
Moore pointed to some similarities with the current coronavirus pandemic.
"Between 20% and 50% of Zika infections are asymptomatic," he said. "Even when symptoms are present, they tend to be mild, so if the infection is not severe enough for an individual to seek medical attention, those cases can go undiagnosed."
While Zika cases have dropped substantially since 2018, potential future outbreaks remain a concern.
"Our research suggests a need for a better understanding of how much transmission is happening within a community," Moore said. "The risk of congenital birth defects in pregnant women infected with Zika virus required a separate surveillance system — testing both the mother and the baby — to capture a more accurate indicator of underlying transmission."
Without widespread testing and a comprehensive surveillance, Moore said experts can miss how large an outbreak is in the general population.
The report was recently published online in the journal PLOS Neglected Tropical Diseases.