Zika French Polynesia study supports hypothesis virus linked to birth defect microcephaly

Thousands of pregnant womencaught in an ongoing outbreak of the mosquito-borne virusspreading from Brazil risk having a baby with the birth defectmicrocephaly, according to the results of a new study.

In the study in the Lancet medical journal which analysed a2013-14 Zika outbreak in French Polynesia, researchers said therisk of microcephaly is about 1 for every 100 women infectedwith the virus during the first trimester of pregnancy.

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While more research is needed to understand the biologicalmechanisms by which Zika might cause microcephaly, the
researchers said, these findings suggest the World HealthOrganization's (WHO) advice that pregnant women should protectthemselves from mosquitoes is a sound precaution.

"Our analysis strongly supports the hypothesis that Zikavirus infection during the first trimester of pregnancy isassociated with an increased risk of microcephaly," said Simon Cauchemez, an infectious disease mathematical modelling expertat France's Institute Pasteur who co-led the study.

The WHO declared on Feb. 1 that the suspected link betweenmicrocephaly and an outbreak of Zika virus spreading from Brazilwas a public health emergency.

The WHO says the outbreak, which began in Brazil in 2014, isspreading rapidly through the Americas, with transmissionreported in 31 countries and territories of the region.

Brazil has confirmed more than 580 cases of microcephaly, adisorder in which a child is born with an abnormally small headand brain. Authorities there say they think most of these casesare related to Zika. The country is also is investigatinganother 4,100 suspected microcephaly cases.

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Cauchemez's team looked at a Zika outbreak in FrenchPolynesia which began in October 2013, peaked in December 2013and ended in April 2014. Over the course of the outbreak, eightcases of microcephaly were identified. Of these, fivepregnancies were terminated and three cases were born.

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Using data on the number of cases of microcephaly, theweekly number of consultations for suspected Zika, blood testsconfirming Zika antibodies, and the number of births during theoutbreak, the researchers used modelling to estimate expectednumbers of microcephaly cases under different risk scenarios.

By comparing the models to the number and timing of actualmicrocephaly cases in the Polynesia outbreak, they found thescenario in which the first trimester of pregnancy was linkedwith an increased risk was most consistent with the data.

The researchers were then able to estimate the risk ofmicrocephaly as 95 in 10,000, or around 1 percent, of pregnantwomen infected with Zika in the first trimester.

Cauchemez stressed that since his study looked back at anoutbreak that is already over, it could only offer insights butnot rock solid predictions about what might happen elsewhere.

"It remains to be seen whether our findings apply to othercountries in the same way," he said.

Experts asked to comment on the findings said they were animportant development in international efforts to establish thepotential public health risk of Zika.

Derek Gatherer, a virus expert at Britain's LancasterUniversity, said this was "the first published study that movesus in the direction of being confident that Zika virus infectionin pregnancy can cause microcephaly."

Peter Openshaw, a professor of experimental medicine atImperial College London, said the new evidence was, "on the
surface, reassuring" and also somewhat unexpected.

"The finding that the risk of microcephaly is only about 1percent in those infected in the first trimester of pregnancy issurprising," he said, noting that a preliminary study byBrazilian researchers published this month estimated the risk atmore than 20 percent.