The Zika virus outbreak that has been ongoing for about a year may have started even earlier than previously believed, according to a new study. The study, published in Science, uses genetic data to claim that the Zika virus may have arrived in the Americas as far back as 2013 — following an increase in air travel during that time.

Until now, reports have stated that the outbreak originated in Brazil in April 2015. Afterward, it spread rather quickly throughout South America, Central America, and Mexico — and now, cases have even been confirmed in the United States, Bangladesh, South Africa, the Czech Republic, France, and dozens of other countries. Most of these cases are likely due to travelers returning from areas in Brazil where the virus has hit, although scientists are also beginning to observe that sexual transmission may also be a factor.

The Zika virus is especially dangerous to pregnant women in their first trimester; it has been linked to microcephaly, a disorder in infants that impairs brain and head development. In addition to microcephaly, scientists are also beginning to look into the virus’ link to Guillain-Barré Disease and temporary paralysis.

The researchers of the newest study, led by Nuno Faria of the University of Oxford, analyzed several Zika virus genomes that were linked to the Brazil outbreak. They received samples from a blood donor, an adult who had died from the virus, and an infected infant born with microcephaly. They used next-generation sequencing to generate seven Brazilian Zika genomes, and found that there was not much genetic variability among them.

They then compared these genomes to current Zika virus genomes, and found that there must have been some introduction of Zika into the Americas somewhere between May and December 2013. This discovery coincided with data on airlines which showed that the number of travelers to Brazil from areas in which Zika is endemic had increased during this time period. It’s possible, then, that travelers from the Pacific Islands or other areas that had had recent outbreaks arrived in Brazil with the virus far earlier than originally reported.

The Zika virus was first discovered in the Zika Forest in Uganda in 1947. It was initially detected in a rhesus monkey, as well as Aedes africanus mosquitos, which are members of the Aedes aegypti family. However, no major Zika outbreaks were reported until 2007 when the Yap Island of Micronesia had the first human outbreak. Several years later, the virus had traveled to French Polynesia, which is located in the South Pacific. It’s possible that from here, corresponding with the increase in airline travel to Brazil, the virus made its way to South America in 2013.

Research into the Zika virus, however, is still ongoing. There is no real way to prove that the virus had made its way to the Americas much earlier, as many cases may have gone undiagnosed. In addition, researchers are still working to confirm the link between Zika and microcephaly, as well as to develop a vaccine.

Source: Faria N, Azevedo S, Rodrigues S, Cruz A, Diniz B, Medeiros D. Zika Virus in the Americas: Early Epidemiological and Genetic Findings. Science, 2016.