Women who got pregnant during a Zika outbreak in Tahiti two years ago had about a 1 percent chance of having a baby with an abnormally small head, according to a new study published Tuesday. It's a surprisingly low risk that experts say might not match the threat of the epidemic now spreading explosively in the Americas.
The World Health Organization declared Zika to be a global emergency last month, based on suspicions it is causing a spike in a worrying birth defect known as microcephaly as well as a rare condition that sometimes results in temporary paralysis.
Before reaching the Americas last year, the mosquito-spread Zika triggered epidemics in the South Pacific and in French Polynesia, including its biggest island, Tahiti. Most people who are infected don't get sick or only have mild symptoms.
After a surge in microcephaly was detected in Brazil, international scientists combed through records from the 2013-14 Tahiti outbreak to see if the same thing had happened there.
They found only eight such cases after tracking about 8,000 pregnant women. Seven occurred near the end of the outbreak, which researchers said suggested their mothers had been infected early in their pregnancies. Of those cases, five of the pregnancies were terminated.
The researchers created a mathematical model that estimated about 1 out of every 100 pregnant women infected with Zika during their first trimester might have a baby with an unusually small head. The paper was published online Tuesday in the journal, Lancet.
But another researcher not involved in the study said the figure seems too low given the birth defects seen in Brazil. Laura Rodrigues, a professor of infectious disease epidemiology at the London School of Hygiene and Tropical Medicine, said she expected the figure to be closer to 10 percent
"If 1 percent is right, then that would be great news," said Rodrigues, who has spent time in Brazil researching the outbreak. "But it just seems a bit implausible right now."
Other viruses that cause the birth defect carry higher risk — like rubella, which has a 40 to 100 percent risk of birth defects when women are infected during their first trimester of pregnancy.
Simon Cauchemez, the study's lead author, warned that the widespread Zika infections seen in French Polynesia — where two thirds of the population were sickened — could have serious consequences for the outbreak in Brazil.
"If you take 1 percent of the many pregnant women infected, clearly this is a big concern for public health," Cauchemez said.
Other experts said Zika might have mutated into a more dangerous form since it first appeared in French Polynesia.
"Something in the virus could have changed to make it more or less harmful to the fetus," said Dr. Ganeshwaran H. Mochida, a neurologist at Boston Children's Hospital.
Rodrigues said it would likely be several more months before any definitive conclusions can be reached.
"We will soon have more pieces of the puzzle," she said.