Researchers who have been trying to reverse-engineer Zika’s “molecular clock” to find out how and when the viral outbreak jumped to the Americas have established a rough sketch of patient zero. The virus seems to have made its Western Hemisphere debut in the blood of a single traveler who went from French Polynesia or Southeast Asia to Brazil between May and December of 2013, according to a study published in Science on Thursday.
The disease was first identified in the West in March 2015 in northeast Brazil. Since then, there’s been lots of speculation about where it came from, much of it connecting Zika to various sports events, principally the FIFA World Cup in June and July 2014, and a canoe-racing meet that September. This new study, which puts Zika’s journey westward over a year earlier, suggests that it came over in June 2013, when the Confederations Cup soccer tournament was held in Rio, bringing in a lot of people from South Asia.
This study, unlike the others that study only people’s travel patterns, focuses on the virus itself. Researchers were able to locate similarities among 23 viral genomes from Thailand, French Polynesia, Haiti, Colombia, Martinique, Guatemala, Suriname, Puerto Rico, Brazil, and the Americas. Then, by creating a “molecular clock” that tracks the speed of mutations in blood samples from the past three years, they could work their way back to establish a date for its arrival.
Critics of the study have said the researchers worked with a small number of samples; Peter Palese, chairman of the microbiology department at the Mount Sinai School of Medicine, called the result “ridiculous.” But others have more faith in the work, saying the genomes studied were long and thus supplied a lot of information, and noting that the researchers also took into account air-travel traffic. In the summer of 2013, travel from French Polynesia to Brazil increased by 50 percent.