Zika and microcephaly: first evaluation of a statistical risk

Publié le 16 Mar, 2016

Yesterday the Pasteur Institute published an initial study focusing on the risk to pregnant women infected with the Zika virus of giving birth to a child with microcephaly. This risk is believed to be of the order of 1%, “i.e. 50 times greater than normal”. The risk is greatest during the first trimester of pregnancy. In non-epidemic situations, the risk is estimated to be 0.02%.


This statistical study is based on data collected in French Polynesia and following the  2013-2014 Zika epidemic and the “retrospective identification of microcephaly cases observed between September 2013 and July 2015”. It focuses “solely on eight cases of microcephaly, seven of which appeared during the four months following the Zika epidemic”, but “given the quality of the virtually exhaustive data according to scientists, the study provides a powerful and unprecedented argument highlighting the involvement of the Zika virus in the outbreak of microcephaly reported in Latin America and the Caribbean”.


Doctor Simon Cauchemez, leading author of the study, stated that “the level of risk to infected pregnant women is lower than that observed with other viral infections linked to brain lesions during pregnancy”. For example, infection with the rubella virus during the first trimester of pregnancy poses a serious risk of foetal complications ranging from 38 to 100%.  The only difference is that there is a vaccine against rubella.


However, according to some specialists, “the risk may not be the same in French Polynesia and South America given ethnic, virological or environmental differences”. Another study conducted in Brazil “with a considerable margin of uncertainty” estimates the risk to be of the order of 22%.


Jean Yves Nau, journalist and doctor of medicine “emphasises”  the point that the cause-effect relationship is not always scientifically evident. “Should we wait until it is? And, if not, what conclusions should be drawn?”

Jean Yves Nau (16/03/2016)

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