New research, based on data from the 2013-14 Zika outbreak in French Polynesia, further supports the association between Zika virus and microcephaly.
The study, published in The Lancet, estimates that the risk of microcephaly is about 1 for every 100 women infected with Zika virus during the first trimester of pregnancy.
The authors say that quantifying the risk may help better inform the broader public health response.
Although the risk of microcephaly associated with Zika virus infection is relatively low compared to other maternal infections, the authors say that the association remains an important public health issue because the risk of Zika virus infection is particularly high during outbreaks, such as the current one in South America.
“Our analysis strongly supports the hypothesis that Zika virus infection during the first trimester of pregnancy is associated with an increased risk of microcephaly”, says Dr Simon Cauchemez, co-author from the Institut Pasteur in Paris, France. “We estimated that the risk of microcephaly was 1 in 100 women infected with Zika virus during the first trimester of pregnancy. The findings are from the 2013-14 outbreak in French Polynesia and it remains to be seen whether our findings apply to other countries in the same way.”
On 1 February 2016, the WHO declared the suspected link between Zika virus and microcephaly as a Public Health Emergency of International Concern. Microcephaly is a neurological abnormality that is present at birth. In Europe and Brazil, about 2 per 10000 babies are born with microcephaly. Babies are born with abnormally small heads, and the condition is associated with a reduction in brain volume, often leading to intellectual disabilities, speech impairment and behavioural issues. Causes include genetic and environmental factors, including prenatal viral infections (such as rubella or herpes), maternal alcohol use, and hypertensive disorders.
Although evidence of the association between microcephaly and Zika virus is growing, the risk has so far not been quantified. The outbreak in French Polynesia began in October 2013, peaked in December 2013 and ended in April 2014. Over that period, more than 31000 people saw their doctor with suspected Zika virus infection.
Over the course of the outbreak, 8 cases of microcephaly were identified. Of these, 5 pregnancies were terminated through medical abortion (average gestational age 30.1 weeks), and 3 cases were born. Nearly all of the cases of microcephaly (88%) occurred in a 4 month period around the end of the outbreak.