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THOUSANDS of Zika virus cases went unreported in Cuba in 2017, according to an analysis of data on travellers to the Caribbean island. Veiling them may have led to many other cases that year.

The analysis suggests that Zika infections peaked in Cuba in the second half of 2017, at a time when the virus was waning in mainland North and South America. Cuban authorities didn’t follow the agreed practice of notifying the  (PAHO) of the outbreak.

Cuba’s first case of Zika occurred in March 2016. A says the country stopped providing updates on Zika in January 2017. In press reports in May 2017, Cuba said that nearly 1900 infections had been detected up to that point. But Nathan Grubaugh at the Yale School of Public Health and his colleagues estimate that the total cases in 2017 alone would have been more than double that at 5700.

“Our results therefore suggest that the 2017 Zika outbreak in Cuba was similar in size to the known 2016 outbreaks in countries with similar population sizes,” the authors write. They declined to comment on the work because it is under review for publication in a journal.

The team looked at the travel logs of 184 people who had contracted Zika while abroad and found that 95 per cent had been to Cuba. Such “hidden” outbreaks can spread epidemics to other countries because travellers and health authorities are unaware of the heightened risk of infection, the authors write (bioRxiv, ).

The team also sequenced the genomes of Zika viruses retrieved from nine Floridians who travelled to Cuba. This showed that the infection was distinct from Zika infections that occurred in Florida. The travel cases revealed that the strains active in Cuba at the time were related to ones previously detected in other Latin American countries.

The team’s research suggests that the Cuban outbreak seems to have been caused by travellers from nearby countries bringing it in during 2016. It then persisted at low levels, before peaking late the following year.

This is an important discovery, says Duane Gubler at the Duke-NUS Medical School in Singapore. He says Cuba has a history of not reporting epidemics until they become obvious, and Zika is only mildly symptomatic in adults.

“One of the problems we have is that islands that depend on tourism are not forthcoming in immediate reporting,” he says.

Cuba’s Public Health Ministry hadn’t responded to requests for comment by the time this article went to press.

More visible consequences of the outbreak would be appearing now, says Peter Hotez at Baylor College of Medicine in Texas. “It should be possible to detect a blip or increase in birth defects starting around now,” he says. A fetus exposed to Zika during pregnancy risks being born with an abnormally small head, a condition known as microcephaly.

Elizabeth Brickley at the London School of Hygiene and Tropical Medicine says work like this could help fill in gaps in the global disease surveillance system. However, it will still be vital to confirm findings with research on the ground, she says.