Around 1 in 100 women infected with Zika virus in the first trimester of pregnancy are at risk of microcephaly, according to estimates of a new study published in The Lancet.

Study coauthor Dr. Simon Cauchemez, of the Institut Pasteur in Paris, France, and colleagues say their findings provide further evidence that Zika virus is associated with increased risk ofmicrocephaly – a neurological condition that can result in babies being born with smaller-than-normal heads.

Last month, the World Health Organization (WHO) declared Zika virus as a public health emergency of international concern, a decision spurred by the suspected link between Zika virus and microcephaly.

While the association is yet to be scientifically proven, there has been a surge in microcephaly cases in Brazil, where Zika virus transmission is ongoing; microcephaly incidence in the country is around 20 times higher than that of previous years.

‘Strong statistical support’ for link between Zika and microcephaly

For their study, Dr. Cauchemez and colleagues used serological and surveillance data to assess incidence of microcephaly during the Zika outbreak in French Polynesia, which took place between October 2013-April 2014.

During this period, 66% of the general population were infected with Zika virus, and eight cases of microcephaly were identified; seven of these occurred within a 4-month period near the end of the Zika outbreak.

Using mathematical and statistical modeling, the researchers estimated the risk of microcephaly with Zika virus infection during six different periods of pregnancy – trimesters one and two; trimesters one, two and three; trimester two; trimesters two and three; and trimester three – and compared their estimates with actual number and timing of Zika virus and microcephaly cases.

To reach their estimates, the team used data on the total number of microcephaly cases, the weekly number of consultations for suspected Zika infection, the total number of births that occurred during the outbreak, and blood tests taken prior to the outbreak that confirmed the presence of Zika virus antibodies.

The researchers found that women who were infected with Zika virus in the first trimester of pregnancy were at greatest risk for microcephaly, with this situation most closely matching the actual data.

The team calculated that 95 in 10,000 – or around 1 in 100 – women infected with Zika virus in the first trimester of pregnancy are at risk of microcephaly.