When Zika virus emerged in the Americas it was observed that the was an increase in the number of babies born with microcephaly, leading to a Public Health Emergency of International Concern being declared by the WHO. However the association was not noted during the previous outbreak of Zika virus in French Polynesia. The authors retrospectively analysed the data from this outbreak and used serological and surveillance data to estimate the probability of infection with Zika virus for each week of the epidemic. They compared this to medical record reports of microcephaly from 2013 to 2015. The Zika virus outbreak in French Polynesia began in October 2013 and ended in April 2014, with 66% (95%CI 62–70%) of the general population being infected. Eight microcephaly cases were identified during the 23-month study period, 88% of which occurred in the 4-month period March 1 to July 10, 2014. The timing of these cases is best explained by a period of risk in the first trimester of pregnancy. By generating a model of this risk the authors found that the baseline prevalence of microcephaly was two cases (95%CI 0–8 cases) per 10,000 neonates, with this risk increasing when associated with Zika virus infection to 95 cases (95%CI 34–191) per 10,000 women infected in the first trimester. Therefore there does appear to be a quantitative risk of microcephaly in neonates associated with Zika virus infection in the mother during the first trimester.
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