If a woman is infected with Zika virus in the first trimester of pregnancy, she has a 1 in 100 risk of her baby developing microcephaly, researchers estimate.
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 of microcephaly - 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.
- Microcephaly occurs when the baby's brain does not develop properly during pregnancy or stops growing after birth
- In the US, microcephaly is estimated to affect between 2-12 babies per 10,000 live births
- The causes of microcephaly are unknown for most babies.
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 their babies developing microcephaly.
Commenting on the results, the authors say:
"This study provides strong statistical support for the suspected association between infection with Zika virus and microcephaly.
Our findings support the need for a strong and prompt response to protect, inform and monitor pregnant women and to provide strong research agendas to clarify the causal link between Zika virus and microcephaly and develop effective treatments and vaccines."
Findings 'biologically plausible'
In an editorial linked to the study, Dr. Laura Ridrigues, of the UK's London School of Hygiene & Tropical Medicine, says the finding that the risk of microcephaly is highest for women infected with Zika in the first trimester of pregnancy is "biologically plausible, given the timing of brain development and the type and severity of the neurological abnormalities."
However, she notes that further research is warranted to gain a better understanding of the link between the two conditions.
"Further data will soon be available from Pernambuco, Colombia, Rio de Janeiro, and maybe other sites," she adds.
"The fast production of knowledge during this epidemic is an opportunity to observe science in the making: from formulation of new hypotheses and production of new results that will provide confirmations and contradictions to the refinement of methods and the gradual building of consensus."
Earlier this month, Medical News Today reported on a study suggesting that the possible link between Zika and microcephaly may be down to the spread of infection through neural stem cells.