In a study published in the US journal Science, scientists from Britain's University of Oxford and Brazil's Evandro Chagas Institute sequenced seven genomes of the Brazilian Zika virus, including one fatal adult case and one newborn with the birth condition microcephaly, Xinhua reported.

The results showed yesterday that the virus had a single entry into the Americas, likely occurred between May and December 2013.

By comparison, Brazil reported the country's first Zika case in May 2015. The researchers also looked at airline data and found this timing coincided with an increase in air passengers to Brazil from Zika-endemic areas, and also with reported outbreaks in the Pacific Islands.

One hypothesis discussed by the researchers involved virus introduction during the Confederations Cup soccer tournament, which involved French Polynesian participation from Tahiti.

"Although the American outbreak virus is most closely related to a strain from French Polynesia, it's also possible that Zika was introduced separately to the Americas and French Polynesia from South East Asia," professor Oliver Pybus, a biologist in the Oxford University's Department of Zoology, said in a statement.

The Zika virus is transmitted to people primarily through the bite of an infected Aedes species mosquito. The main concern is that the virus is linked to microcephaly in infants.

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