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 In the study in the Lancet medical journal which analyzed a 2013-14
Zika outbreak in French Polynesia, researchers said the risk of
microcephaly is about 1 for every 100 women infected with the virus
during the first trimester of pregnancy.
While more research is needed to understand the biological
mechanisms by which Zika might cause microcephaly, the researchers
said, these findings suggest the World Health Organization's (WHO)
advice that pregnant women should protect themselves from mosquitoes
is a sound precaution.
"Our analysis strongly supports the hypothesis that Zika virus
infection during the first trimester of pregnancy is associated with
an increased risk of microcephaly," said Simon Cauchemez, an
infectious disease mathematical modeling expert at France's
Institute Pasteur who co-led the study.
The WHO declared on Feb. 1 that the suspected link between
microcephaly and an outbreak of Zika virus spreading from Brazil was
a public health emergency.
The WHO says the outbreak, which began in Brazil in 2014, is
spreading rapidly through the Americas, with transmission reported
in 31 countries and territories of the region.
Brazil has confirmed more than 580 cases of microcephaly, a disorder
in which a child is born with an abnormally small head and brain.
Authorities there say they think most of these cases are related to
Zika. The country is also is investigating another 4,100 suspected
microcephaly cases.
Cauchemez's team looked at a Zika outbreak in French Polynesia which
began in October 2013, peaked in December 2013 and ended in April
2014. Over the course of the outbreak, eight cases of microcephaly
were identified. Of these, five pregnancies were terminated and
three cases were born.
Using data on the number of cases of microcephaly, the weekly number
of consultations for suspected Zika, blood tests confirming Zika
antibodies, and the number of births during the outbreak, the
researchers used modeling to estimate expected numbers of
microcephaly cases under different risk scenarios.
By comparing the models to the number and timing of actual
microcephaly cases in the Polynesia outbreak, they found the
scenario in which the first trimester of pregnancy was linked with
an increased risk was most consistent with the data.
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The researchers were then able to estimate the risk of microcephaly
as 95 in 10,000, or around 1 percent, of pregnant women infected
with Zika in the first trimester.
Cauchemez stressed that since his study looked back at an outbreak
that is already over, it could only offer insights but not rock
solid predictions about what might happen elsewhere.
"It remains to be seen whether our findings apply to other countries
in the same way," he said.
Experts asked to comment on the findings said they were an important
development in international efforts to establish the potential
public health risk of Zika.
Derek Gatherer, a virus expert at Britain's Lancaster University,
said this was "the first published study that moves us in the
direction of being confident that Zika virus infection in pregnancy
can cause microcephaly".
Peter Openshaw, a professor of experimental medicine at Imperial
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 1 percent
in those infected in the first trimester of pregnancy is
surprising," he said, noting that a preliminary study by Brazilian
researchers published this month estimated the risk at more than 20
percent.
(Reporting by Kate Kelland; Editing by Mark Heinrich)
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