“It’s very early and limited but valuable information,” said Daniel Lucey, an infectious-disease specialist at Georgetown University.

Zika could even have been introduced to Brazil during the 2013 Confederations Cup, a soccer tournament that featured a team from Tahiti — which suffered a Zika epidemic that year.

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“We managed to trace the date of introduction between May and September 2013,” said Nuno Faria, a researcher at Oxford University and one of the authors of the paper, published Thursday in the journal Science.

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The research by scientists from Britain’s Oxford University and Brazil’s Evandro Chagas Institute is the first to have analyzed the Zika genome found in Brazil. Since reaching here, it has spread to another 33 countries across the Americas.

“This is the first study combing genetic and epidemiological data on the Zika virus in Brazil, so it is a good baseline for future research,” Faria said.

Researchers found that Brazil’s Zika virus is closely related to the Asian strain of Zika that was behind an outbreak in French Polynesia in 2013. The strain also circulates across Southeast Asia. Zika was first found in Uguanda in 1947, and there is also an African strain.

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As many as 1.5 million Brazilians may have caught Zika, the government estimates. It has confirmed nearly a thousand cases of the birth defect microcephaly – which causes babies to be born with abnormally small heads and can lead to motor and cognitive difficulties and problems with hearing and sight. It is investigating thousands more.

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But little is known about how Zika got to Brazil. And though Zika is increasingly blamed for the microcephaly outbreak, it unclear how the virus could cause brain damage in fetuses.

Faria said theories that co-infection with a related virus like dengue, or previous exposure to dengue, might be factors, are “very plausible.”

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“We are providing some first data to help us answer these questions,” he said.

The report analyzed seven genomes, including that of one adult with Zika who died and a newborn with microcephaly.

“They did a genetic profile of the Brazilian virus,” said Marcos Lago, an infectious disease specialist at the Federal University of Rio de Janeiro. “This is important.”

The authors said Zika circulated in Brazil for a year or more before being identified.

In June 2013, Tahiti’s national soccer team took part in the Confederations Cup – a trial run for the World Cup a year later – and played in Recife, capital of Brazil’s northeastern state of Pernambuco where around a third of microcephaly cases have been concentrated.

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But the study also noted a 50 percent rise from 2012 to 2014 in passengers flying to Brazil from countries where Zika circulated — New Caledonia, Indonesia, Malaysia, French Polynesia, Thailand and Cambodia — and said this was a more likely cause.

The 2014 World Cup and a canoeing tournament that year that featured a team from Tahiti have both previously been cited as events that could have led to Zika entering Brazil.

The study did not test the link between Zika and microcephaly, but did find that most suspected microcephaly cases happened at 17 weeks of pregnancy and more severe cases at 14 weeks.

As of March 19, Brazil had confirmed 907 cases of microcephaly and other alterations caused by a congenital disease since last October. Another 4,293 are still being investigated, and 1,471 have been discarded. The latest figures showed that Brazil confirmed 44 new cases in a week. In addition, 12 countries have reported an increase in Guillain-Barré Syndrome, a neurological condition that has been linked to Zika.

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In May the research team plans to visit Brazil and profile a thousand genomes using a portable new technology. “It’s going to be targeting genetic surveillance,” Faria said.