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The Horizon 2020 call by the European Commission’s Directorate-General Research and Innovation has funded the ZIKAlliance project to better characterise the clinical and fundamental aspects of infections by Zika virus (ZIKV), an emerging pathogen currently spreading in the Americas. As of now, the exact risk of microcephaly and other congenital malformations linked to Zika virus infection during pregnancy remains unknown.

A study published in the Bulletin of the World Health Organization describes a plausible range for the risk of microcephaly in women who were infected with Zika virus during pregnancy compared to those who were not infected.

Investigators obtained data on the number of notified and confirmed microcephaly cases in each Brazilian state between November 2015 and October 2016 from the health ministry. For Pernambuco State, one of the hardest hit, weekly data were available from August 2015 to October 2016 for different definitions of microcephaly.

The estimated absolute risk of a notified microcephaly case varied from 0.03 to 17.1% according to geographical area, the definition of microcephaly used and the infection rate. Assuming a 50% infection rate, there was an 18–127-fold higher probability of microcephaly in children born to mothers with infection during pregnancy compared with children born to mothers without infection during pregnancy in Pernambuco State. For a 10% infection rate, the probability was 88–635 folds higher.

The key message from this study is that the large uncertainty around the risk estimate needs to be further investigated because of a) the possible existence of co-factors that are yet to be validated, b) the assumptions that the authors needed to make in the absence of good data for the proportion of women who were infected during pregnancy.

This work was supported by the European Union’s Horizon 2020 Research and Innovation Programme (ZIKAlliance grant agreement no. 734548) and Seventh Framework Programme for Research and Technological Development (International Research Consortium on Dengue Risk Assessment, Management and Surveillance, IDAMS, grant agreement no. 281803).

For information about the scientific outputs of the paper, please contact Thomas Jaenisch at thomas.jaenisch@urz.uni-heidelberg.de. For more information about ZIKAlliance or ZIKAlliance’s activities, please contact Xavier de Lamballerie (Scientific Coordinator) at xavier.de-lamballerie@univ-amu.fr or phone: +33 491324420; or Flavia Mariani (Project Manager) at flavia.mariani@zikalliance.eu or phone: +33 491324412.