The Zika virus now spreading widely throughout the hemisphere probably arrived in the Americas in a single traveler in the second half of 2013 — almost a year earlier than previous estimates — according to a new study of the virus’s genome led by Brazilian and British researchers.
Experts were divided in their opinions of the new study, published Thursday in the journal Science. Some praised the work, while others said it was too limited to draw such a specific conclusion.
By counting mutations in the viral genomes in different blood samples over time, the scientists created a “molecular clock” that estimates how fast the virus mutated.
The researchers then compared new samples with earlier ones from Asia, where the Zika virus had circulated for decades, and from the South Pacific, where it began circulating in 2007. The team calculated that the Zika virus arrived in the Americas between May and December 2013.
They also concluded that it probably — but not necessarily — arrived in Brazil first.
The virus was not positively identified in Brazil until May 2015. But by then it had clearly been circulating in the country’s northeast for many months, because cities there were experiencing large outbreaks of a mysterious disease causing rash, fever and bloodshot eyes.
(The virus did not become headline news around the world until December, when health officials in Brazil, alarmed over a surge in infants born with tiny heads, warned women not to become pregnant. The World Health Organization declared a global health emergency on Feb. 1.)
The new study relies on just 23 viral genomes. They include samples of Zika virus obtained in Thailand, French Polynesia and the Cook Islands and 20 in the Americas, including nine from Brazil and the rest from Colombia, Martinique, Haiti, Guatemala, Suriname and Puerto Rico.
The Haitian virus was noted as being particularly unusual, because it was collected in December 2014 — more than a year before the virus’s presence in Haiti was confirmed by the W.H.O. this past January.
The gene sequences from the Americas were all closely related, and most resembled one collected in French Polynesia in November 2013.
Two earlier studies have suggested that the virus reached Brazil either with the influx of athletes and tourists arriving for the 2014 soccer World Cup, which was played in host cities all over Brazil in June to July that year, or for the Va’a World Sprints, a set of outrigger canoe races held in Rio de Janeiro in mid-August of that year.
Those assumptions were based on air traffic patterns, not viral sequencing. A large Zika outbreak in French Polynesia began in 2013 and spread to New Caledonia, the Cook Islands and Easter Island.
No soccer team from the South Pacific played in the 2014 World Cup, but many teams from the affected South Pacific nations were among the 2,000 paddlers competing in the Va’a.
The new study notes, however, that French Polynesia’s outbreak peaked and crashed in only five months, and was nearly over by February 2014, making it unlikely that the virus left there for Brazil in July or August.
“I think we can now discount the whole World Cup connection,” said Kristian G. Andersen, a disease geneticist at the Scripps Research Institute, who said he felt the number of sequences used in the study was “fairly small but enough so that the main conclusions are likely to be valid.”
The new study’s researchers also noted that, in June 2013, the FIFA Confederations Cup, a prelude to the World Cup, was played in Brazil. It included a team from Tahiti, which is part of French Polynesia; the team played one game in Recife, one of the northeastern cities where the outbreak was first seen.
Oliver G. Pybus, a disease geneticist at Oxford University and one of the paper’s authors, said the virus’s arrival during the Confederations Cup was “within the range of possibilities but not something you can capture scientifically.”
That tournament took place before the first cases in French Polynesia were reported. Until this year, however, few doctors had even heard of Zika, and it was often misdiagnosed as dengue or other diseases.
Peter Palese, the chairman of the microbiology department at the Mount Sinai School of Medicine, who has calculated molecular clocks for flu viruses, said he felt that calculating a virus’s geographical movements based on so few genome sequences was “to be quite harsh, ridiculous.”
“You need hundreds or even thousands of strains to really say anything,” he said.
But Michael Worobey, a disease geneticist at the University of Arizona, said he found the new study’s findings “robust” because the genetic sequences were long and had what he called “a clear, clocklike signal.”
“People have a tendency to not imagine that viruses can circulate under the radar for a long time,” Dr. Worobey added. “But H.I.V. circulated for decades in Africa before it was noticed.”
“Yes, the tip of the Zika iceberg rose up in Brazil,” he added. “But the base of the iceberg might be a little more complicated than some people getting on a plane for a sporting event.”