More than 100 million infections of Zika virus within Central and South America and the Caribbean went undetected between 2015 and 2018, according to a new study.

The researchers who conducted the study, published in ‘PLOS Neglected Tropical Diseases’, said the results showed a need for improvement in the current infectious disease surveillance systems.

The study also provides insight into the potential severity of future outbreaks and the current state of herd immunity of Zika in the West.

“Fewer than 1 per cent of cases were actually reported and it shows our surveillance systems catch only a small percentage of actual infections,” said the study’s lead author Sean Moore from the University of Notre Dame in the US.

“We need to think about how to improve surveillance systems to get a better sense of transmission, especially in cases of diseases that yield a high number of asymptomatic infections,” Moore said.

The first confirmed case of Zika in the Americas was reported in Brazil in 2015, with the infection spreading rapidly, reaching as far north as Florida and Puerto Rico. In 2016, the World Health Organization (WHO) declared the epidemic a Public Health Emergency of International Concern.

The Pan American Health Organization, which serves as an international agency for the Americas and as a regional office of the WHO, totalled symptomatic infections of Zika at more than 800,000 between 2015 and 2018 — a number far below the results of the Notre Dame study.

Moore’s team estimated 132.3 million infections across Latin America and the Caribbean, having collected data from 15 countries and territories in South America, Central America and the Caribbean with a combined total population of 507.1 million.

“There are some similarities to the current situation with Covid-19,” Moore said.

“Between 20 per cent and 50 per cent of Zika infections are asymptomatic. 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,” Moore added.

According to the researchers, understanding the scope of underreporting is particularly important to gain an accurate sense of herd immunity in the region. Once infected, individuals who recover from Zika virus are believed to have lifelong immunity.

While cases have dropped substantially since 2018, a remaining concern is the potential for future outbreaks.

“Our research suggests a need for a better understanding of how much transmission is happening within a community,” said Moore.

A recent study co-authored by Moore and his colleagues at Notre Dame found that gaps in surveillance and limited testing resulted in more than 100,000 coronavirus infections in the US that went undetected in the early months of the pandemic.

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